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Iron adsorption onto soil and aquatic bacteria: XAS structural study
... Alternatively, abundant Fe sorption and/or precipitation at the cell surface at higher Fe(II) concentrations ( Table 2) could have blocked cellular transporters for Fe uptake. Fe sorbed to cyanobacteria surfaces was present in Fe(oxyhydr)oxide phases and bound to surface functional groups when Fe(II) was the Fe source at neutral pH (González et al., 2014), phases that might be less labile for uptake or block surface uptake sites. ...
... This inference is supported by the localization of reflective minerals away from cell surfaces, and the detection of a soluble or ligand-bound Fe(III) phase at the cell surfaces ( Figure 5). Previous experiments that looked at Fe sorption to cyanobacterial cell surfaces when Fe(II) was added found that only Fe(III) was bound (González et al., 2014). The phosphoryl groups of capsular EPS preferentially bind Fe(III) under neutral conditions, preventing growth of Fe(III) mineral polymers (González et al., 2014). ...
... Previous experiments that looked at Fe sorption to cyanobacterial cell surfaces when Fe(II) was added found that only Fe(III) was bound (González et al., 2014). The phosphoryl groups of capsular EPS preferentially bind Fe(III) under neutral conditions, preventing growth of Fe(III) mineral polymers (González et al., 2014). Although EPS production has not been reported in Synechococcus PCC 7002 (De Philippis and Vincenzini, 1998), but this strain produces both hydroxamate and catechol-type siderophores (Wilhelm and Trick, 1994). ...
Evidence for Fe(II) oxidation and deposition of Fe(III)-bearing minerals from anoxic or redox-stratified Precambrian oceans has received support from decades of sedimentological and geochemical investigation of Banded Iron Formations (BIF). While the exact mechanisms of Fe(II) oxidation remains equivocal, reaction with O2 in the marine water column, produced by cyanobacteria or early oxygenic phototrophs, was likely. In order to understand the role of cyanobacteria in the deposition of Fe(III) minerals to BIF, we must first know how planktonic marine cyanobacteria respond to ferruginous (anoxic and Fe(II)-rich) waters in terms of growth, Fe uptake and homeostasis, and Fe mineral formation. We therefore grew the common marine cyanobacterium Synechococcus PCC 7002 in closed bottles that began anoxic, and contained Fe(II) concentrations that span the range of possible concentrations in Precambrian seawater. These results, along with cell suspension experiments, indicate that Fe(II) is likely oxidized by this strain via chemical oxidation with oxygen produced during photosynthesis, and not via any direct enzymatic or photosynthetic pathway. Imaging of the cell-mineral aggregates with scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) are consistent with extracellular precipitation of Fe(III) (oxyhydr)oxide minerals, but that >10% of Fe(III) sorbs to cell surfaces rather than precipitating. Proteomic experiments support the role of reactive oxygen species (ROS) in Fe(II) toxicity to Synechococcus PCC 7002. The proteome expressed under low Fe conditions included multiple siderophore biosynthesis and siderophore and Fe transporter proteins, but most siderophores are not expressed during growth with Fe(II). These results provide a mechanistic and quantitative framework for evaluating the geochemical consequences of perhaps life's greatest metabolic innovation, i.e., the evolution and activity of oxygenic photosynthesis, in ferruginous Precambrian oceans.
... Because these data approached closed system equilibrium isotopic fractionation lines rather than Rayleigh curves, the most likely mechanism is a steady state isotopic fractionation, linked with short-term reversible Fe adsorption on cells. In agreement with X-ray Absorption Spectroscopy structural data obtained on the same adsorption experiment samples (González et al., 2014), the preferential enrichment of heavy Fe isotopes on the cell surfaces is attributed to the stronger covalent metal-ligand bonding (Fe\O\C/P) of octahedrally coordinated Fe with phosphoryl or carboxyl groups on the cell walls when compared with the Fe aquacomplexes (O\Fe\O) in solution. The larger isotopic fractionation factor in the experiments starting with Fe(II) results from the iron oxidation to Fe(III) which is itself accompanied by a strong isotopic effect. ...
... The morphology of the three cyanobacterial species is distinctly different and covers three dominant cyanobacteria groups found in natural settings: capsular, unicellular, and filamentous for Gloeocapsa sp., Synehococcus sp., and Planthothrix sp., respectively. In these cultures, the structural status of adsorbed Fe has been characterized in a recent in-situ X-ray Absorption Spectroscopy (XAS) study (González et al., 2014). We aimed to test the hypothesis that, similar to other metals and in inorganic systems, Fe adsorption on bacterial phytoplankton cell surfaces is capable of producing significant isotopic fractionation with preferential adsorption of the heavy isotope on the biomass. ...
... Trivalent iron was added to the cultures maintained at pH 3. The interaction time varied between 30 min and 3 h. Fe(III) adsorption was studied on live bacteria as a function of pH after 1 h and 24 h of exposure time (González et al., 2014). The similarity between adsorption curves at these short but nevertheless different exposure times allowed the longer exposure time to set to 3 h for the XAS and iron isotope study, which is consistent with the typical duration of adsorption experiments with other metals (Daughney et al., 1998;Wightman and Fein, 2005;Pokrovsky et al., 2008aPokrovsky et al., , 2008bPokrovsky et al., , 2012González et al., 2010). ...
... At pH 2.5, the plentiful amount of H + may block the active site for sorption, causing the lower adsorption efficiency than pH 3.5. Furthermore, Fe(III) was present in the form of individual octahedral Fe-O 6 linked to the carboxyl or phosphoryl groups of cyanobacterial cell wall under acidic conditions (González et al. 2014). When the pH was increased higher than 4.0, the concentration of hydroxide ions in the solution increased which caused the Fe(III) to precipitate (Fe(OH) 3 ). ...
... Fe(III) adsorption from the aqueous solution was stronger for cyanobacteria compared to soil bacteria. This difference might be partially linked to the major role of organic functional groups (González et al. 2014). Moreover, the optimum pH for adsorption of Fe(III) and Fe(II) onto the husk of Cicer arientinum and the rice husk ash was observed at pH 2.5 and 5.0 respectively (Ahalya et al. 2006;Zhang et al. 2014). ...
In this study, the dried biomass of Synechocystis sp. PCC 6803 was used as biosorbent for removing Fe(III)) ions from aqueous solution. The effects of exposure time, the initial metal concentration, biosorbent dose, and solution pH on the biosorption efficiency of Fe(III) from synthetic solutions were investigated. The Fe(III) adsorption was relatively fast and the equilibrium time was 60 min with the maximum biosorption capacity (qm) of 10.98 mg Fe(III) g⁻¹ biosorbent (85%) at pH 3.5, 10 g L⁻¹ biomass dosage, and 30 °C. Among four biosorption isotherms, the Redlich-Peterson and the Langmuir isotherm models described better the adsorption of Fe(III) onto dried biomass than did the Freundlich and the Temkin isotherm models. The biosorption of Fe(III) using dried biomass of Synechocystis sp. PCC 6803 followed the second-order kinetics. Thermodynamic studies established the biosorption process to be energetically favorable with negative free energy change. FTIR and SEM-EDX analyses revealed the presence of functional groups of negative valences on the biosorbent surface responsible for the Fe(III) binding. Desorption of Fe(III) was attained up to 79% using 0.1 M HNO3; however, the capacity of biomass as biosorbent was decreased after the first adsorption-desorption cycle. Moreover, the biosorption efficiency of the algal biosorbent for the removal of Fe(III) from groundwater was 65%. Overall, this finding suggested an eco-friendly strategy for remediation of Fe(III)-polluted wastewater by biosorption onto the Synechocystis sp. PCC 6803 biomass.
... XAFS spectra of YN and Fe-containing organic and inorganic compounds are shown in Fig. 8. Iron K-edge XAFS spectra vary in the position of the pre-edge peaks in the range of 7.11-7.12 keV, depending on the Fe oxidation state (Okudera et al. 2012;González et al. 2014;Boubnov et al. 2015;Fukuda et al. 2016). A characteristic pre-edge peak at 7.115 keV, which corresponds to Fe(III), was observed for YN as well as Fe(III)-BSA, ferritin, Fe(OH) 3 , and Fe(III) citrate (pre-edge peaks in the range of 7.114-7.117 ...
... Furthermore, these Fe(II)-containing compounds had a single near-edge peak at 7.126 keV, which shifted to the lower energy side compared to those of the Fe(III)-containing compounds. Generally, the peak position at around the absorption edge in the XAFS spectrum of metallic elements shifts to the lower energy side with the lower valence (Okudera et al. 2012;González et al. 2014;Boubnov et al. 2015;Fukuda et al. 2016). These results suggest that YC1 exists as a form of Fe(III) in the nacreous organic matrix in association with yellow color development in the nacre of Akoya pearl oyster shells. ...
The gold and cream colors of cultured Akoya pearls, as well as natural yellow nacre of pearl oyster shells, are thought to arise from intrinsic yellow pigments. While the isolation of the yellow pigments has been attempted using a large amount of gold pearls, the substance concerned is still unknown. We report here on the purification and characterization of yellow pigments from the nacre of Akoya pearl oyster shells. Two yellow components, YC1 and YC2, were isolated from the HCl-methanol (HCl-MeOH) extract from nacreous organic matrices obtained by decalcification of the shells with ethylenediaminetetraacetic acid (EDTA). Energy-dispersive X-ray and infrared spectroscopy analyses suggested that YC1 and YC2 precipitated under basic conditions are composed of Fe-containing inorganic and polyamide-containing organic compounds, respectively. YC1 solubilized under acidic conditions exhibited positive reactions to KSCN and K4[Fe(CN)6] reagents, showing the same ultraviolet-visible absorption spectrum as those of Fe(III)-containing compounds. In addition, X-ray absorption fine structure analysis supported the compound in the form of Fe(III). The total amount of Fe was approximately 2.6 times higher in the yellow than white nacre, and most Fe was fractionated into the EDTA-decalcifying and HCl-MeOH extracts. These results suggest that Fe(III) coordinated to EDTA-soluble and insoluble matrix compounds are mainly associated with yellow color development not only in the Akoya pearl oyster shells but also in the cultured Akoya pearls.
... This result is remarkable given that the divalent cations are present as both organic and free complexes and thus should exhibit strong difference in the dependence of their adsorption on pH. It is possible that, at relatively low concentration of free cations in natural DOM-rich substrates, the pH-dependent adsorption edge of these TE is located at pH < 5 (Gonzalez et al., 2010(Gonzalez et al., , 2014bPokrovsky et al., , 2013bOleinikova et al., 2017). Another explanation is that most TEs are > 98% complexed with DOM and thus their free concentration is very low. ...
... The EPS which are produced by heterotrophic bacteria are also capable to stabilize the metals in the vicinity of the cells (i.e., Tourney and Ngwenya, 2014). Former studies of Fe (II, III) interaction with heterotrophs demonstrated that, in the presence of surface organic ligands not screened by EPS layers, the polymerization of Fe(III) ions and formation of Fe-oxy(hydr)oxides is partially inhibited (Gonzalez et al., 2014b, Fakih et al., 2008. As a result, the adsorbed Fe(III) remains on the cell surface in the form of individual Fe atoms attached to organic moieties, conferring several advantages to the cells (Châtellier et al., 2004;Chan et al., 2009). ...
La matière organique dissoute (MOD) est un composant important des eaux naturelles, déterminant la forme des éléments et les émissions de gaz à effet de serre (CO2, CH4), et influence la biodiversité des milieux des eaux. Les eaux boréales sont connues pour être généralement riches en fer, et cela est vrai en particulier pour les eaux de la région étudiée (Carélie du Nord, Russie). Le fer est associée aux MOD dans les complexes de bas poids moléculaire et les colloïdes organo-ferreux de haut poids moléculaire, qui agissent comme les principaux transporteurs d'éléments traces métalliques dans le continuum hydrologique typiques de cette région : sol - marais - rivière - lac. La transformation des colloïdes organo-ferreux se fait sous l'influence de deux facteurs principaux: la dégradation bactérienne et la dégradation photochimique. Cette étude est consacrée à l'analyse du comportement du carbone organique dissous (COD) et des éléments traces (ET) des eaux de surface en zone boréale sous l'influence de l'activité métabolique des bactéries hétérotrophes et de l'oxydation photolytique des MOD sous l’effet de la lumière du soleil. Dans les expériences, un lixiviat de tourbière, un échantillon d’eau de marais, un échantillons d’eau de ruisseaux, un échantillon de rivière, un échantillon d’eau de lac humique, un échantillon d’eau de lac oligotrophique et un pluvio-lessivat collecté sous un pin, échantillons dans lesquelles dominent les MOD de nature humique, ont été utilisés comme substrats.Des expériences en laboratoire utilisant des communautés monosouches de bactéries aérobies hétérotrophes Pseudomonas aureofaciens et Pseudomonas saponiphila (prélevées sur le territoire de Carélie) ont permis d'établir que la MOD allochtone des milieux aquatiques en zone boréale possède une forte résistance à l'activité des bactéries étudiées. Le taux de minéralisation bactérienne du COD était de 0 à 4.3 mgC.L-1.jour-1 selon le substrat, et la fraction de faible poids moléculaire (<1 kDa) de la MOD est plus susceptible de destruction que celle de poids moléculaire élevé (de 1 kDa à 0.22 μm). L'interaction des bactéries avec divers substrats aqueux a montré pour une large gamme d'éléments (Al, Fe, Mn, Ni, Co, Cu, Cd, REE, U) une prédominance significative d’une adsorption rapide sur la surface cellulaire observée lors de la première heure d’expérience (t < 1h) par rapport à une diminution lente observée lors de la suite de l’expérience (1h < t < 96h), diminution lente lié à l’assimilation intracellulaire des métaux et à la coprécipitation extracellulaire des éléments avec des hydroxydes de Fe et Al. L'analyse de divers substrats a montré une augmentation de l'adsorption des métaux avec le rapport DOC/Fe dans la solution initiale, ce qui peut être due à la compétition entre Fe et d'autres cations métalliques pour les sites d'adsorption anioniques des surfaces des cellules bactériennes. Le suivi des dynamiques d'élimination des métaux des solutions expérimentales pendant 96 heures n'a pas mis en évidence de lien entre l’élimination des métaux et le pH et les valeurs absolues de concentration de DOC et de Fe. Une relation a été établie entre la diminution lente (96 h) des éléments et le degré initial de sursaturation de la solution en oxydes et hydroxydes de fer et d'aluminium.Des expériences en conditions naturelles avec des échantillons d'eau de rivière et de marais a montré que la destruction photolytique de la MOD est beaucoup plus efficace pour l'eau de marais. Le taux de minéralisation du COD était de 0.3 mgC.L-1.jour-1 dans la rivière et de 2.5 mgC.L-1.jour-1 dans l'eau de marais. Il existe trois processus principaux responsables de l'élimination et de la redistribution des microéléments entre les différentes fractions colloïdales dans les eaux de tourbières riches en MOD et en Fe sous l’action de la lumière du soleil: 1) la dégradation de la partie organique de la fraction colloïdale (1 kD – 0.22µm) avec la formation de ligands organiques à faible poids moléculaire (<1 kDa); 2) la coagulation et la précipitation des oxydes et des hydroxydes de Fe et Al avec les éléments-hydrolysat trivalents et tétravalents comme V, P, Cr et As, après l'élimination de la partie organique par rayonnement solaire; 3) libération de métaux alcalins et alcalino-terreux, Mn, Co, Ni, Zn et SO42- à partir de complexes organiques avec les MOD. Ainsi, la dégradation des MOD en zone boréale provoquée par la lumière du soleil peut conduire à la formation des formes ioniques, donc potentiellement biodisponibles, d'éléments traces tels que K, V, Cr, Mn, Co, Ni, Zn, Ba; à une réduction de la concentration dans l'eau les éléments toxiques (Cr, As); et enfin à une diminution de la concentration de l'élément limitant le développement des microorganismes – le phosphore.
... Such acids are generally forming dimers such as ellagic acid. ( ± ) -catechin, sinapic acid and gallic acid contain eOH groups in their molecules which at pH ~ 8 (natural seawater pH) should be deprotonated forming anionic ligands capable of complexing metals, as the organic ligands at the cell surface of microorganisms ( González et al., 2014). Accordingly, polyphenols at natural pH com- plex dFe in seawater, as it was previously reported at acidic pH and low ionic strength solutions ( Andjelković et al., 2006;Brown et al., 1998;Hynes and O'Coinceanainn, 2004;Khokhar and Apenten, 2003;Mira et al., 2002). ...
... The complexation of dFe with polyphenols can be considered as a first step in the process of internalization of Fe by phytoplankton, re- ducing Fe(III) to Fe(II) both in seawater (Santana-Casiano et al., 2010) and at cell surfaces ( González et al., 2014). The same mechanism was also studied for catechol (Santana-Casiano et al., 2010), ( ± ) - catechin and sinapic acid ) as a function of pH. ...
Iron is an essential micronutrient for phytoplankton and can limit primary production in the ocean. Fe chemistry is highly controlled by its interaction with organic complexes (>99%). It is still unknown which organic compounds produced by cells have the ability to bind Fe. Within the pool of organic ligands, polyphenols are known to be exudated by marine diatoms and, in this study, the role of three polyphenols ((±) – catechin, sinapic acid and gallic acid) was studied in terms of dissolved Fe complexation via kinetic and titration approaches, and also their role as a source of Fe(II) in seawater. The results demonstrated that these three polyphenols are weak L2-type Fe-binding ligands according to the conditional stability constant, computed by using the kinetic approach (log K′Fe′L = 8.86–9.2), where the formation rate constant (kf) was 3.1·10⁵–4.2·10⁵ M⁻¹ s⁻¹ and the dissociation rate constant (kd) was 2.43·10⁻⁴–4.4·10⁻⁴ s⁻¹. The conditional stability was also computed from the titration approach with log K′Fe′L from 8.6 to 9.5. These studied ligands also regenerated Fe(II) in seawater from 0.05% to 11.92%. The results obtained in this study suggest that polyphenols increase the persistence of dissolved Fe and should be considered as an important Fe-binding ligands in seawater to better understand the global biogeochemical cycles. This article is part of a special issue entitled: “ConwayGEOTRACES” - edited by Catherine Chauvel.
... The adsorption/precipitation of Fe on the cell walls have been thought to be a critical step for Fe uptake by microorganisms (González et al. 2014;Mulholland et al. 2015;Sutak et al. 2012 (2015) and Swanner et al. (2017) showed that the adsorbed Fe on the cell surface of different cyanobacteria species were heavier than the coexisting Fe II solutions by > 2‰ in δ 56 Fe, due to a combination of Fe adsorption and oxidation reactions. Laboratory experiments also indicated that heavier Fe isotopes are preferentially adsorbed onto the solid surfaces during abiotic equilibrium isotope fractionation processes (Crosby et al. 2005;Icopini et al. 2004). ...
... The contrasting isotope fractionation patterns of extracellular Fe (Fig. 5) and complex relationship between Δ 56 Fe extra and f extra (Fig. 6B) imply that multiple Fe redox processes occurred on the cell surface. In analogy to previous studies on Fe II /Fe III adsorption on the algae (González et al. 2014;Mulholland et al. 2015), the isotopically heavier extracellular Fe that was mainly observed at high initial FeSO 4 concentrations (500-1000 μM, Fig. 5A-B) might relate to Fe II adsorption/oxidation on the cell surface. Following processes might account for the isotopically lighter extracellular Fe observed in low initial FeSO 4 concentrations (≤100 μM, Fig. 5): 1) oxidation of adsorbed Fe II through a phototrophic reaction within the cytoplasmic membrane (Ehrenreich and Widdel 1994); 2) transport of isotopically heavier Fe III isotopes into the cells. ...
The uptake of iron (Fe) by phytoplankton is an important pathway that drives the global Fe biogeochemical cycle. However, limited information is available regarding the resulting Fe isotope signatures during this metabolic processes. Here, two algal species Chlorella pyrenoidosa and Chlamydomonas reinhardtii were cultured in a medium spiked with FeSO4 to study their ability to fractionate Fe isotopes. We quantified the total cellular and intracellular Fe, and measured their isotope compositions. The amounts and isotope compositions of extracellular Fe were estimated by mass balance. We found that the intracellular Fe of algae in concentration-gradient experiments was enriched in the heavier isotopes relative to the FeSO4 solutions, up to 3‰ in δ⁵⁶Fe, suggesting the heavier Fe isotopes are preferably taken up by the algae. However, the intracellular Fe of algae in time-course experiments showed inconsistent fractionation patterns, either enriching or depleting heavier Fe isotopes. Extracellular Fe was isotopically variable from -2.5‰ to 1.9‰ in δ⁵⁶Fe relative to the FeSO4 solutions, likely representing a mixture of FeII and FeIII adsorbed on the cell surface. Additionally, the variation of intracellular δ⁵⁶Fe values appears to be dependent of the intracellular Fe fractions, enriching heavier Fe isotopes at the lower intracellular Fe fractions. Our observations not only highlight the potential of using Fe isotopes as the tracer of biological Fe cycles, but also have important implications on the Fe metabolic pathways of algae.
... The soil aerobic heterotrophic gram-negative bacteria Pseudomonas aureofaciens (CNMN PsB-03) used in these experiments was isolated from the rhizosphere of organic-rich soils (Institute of Plant Genetics and Physiology, Moldova; see detailed description in González et al., 2014). This strain has been extensively used in several previous studies of bacterial degradation of natural DOM (Shirokova et al., 2017;Oleinikova et al., 2018). ...
... 27 In contrast, adsorbed Fe(II) increased first and then decreased over time in the Fh-FA-0Cr and Fh-FA-20Cr treatments (Figure 1b), owing to the rapid formation of Fe(II)-bearing minerals. 1 Moreover, a portion of the produced Fe(II) may be adsorbed by carboxylate or phosphorylate groups of microbes, 56 leading to a decrease in Fe(II) concentrations. For the Fh-FA-40Cr and Fh-FA-80Cr treatments, adsorbed Fe(II) concentrations remained low until 96 h, after which their concentrations gradually increased (Figure 1b). ...
Cr(VI) detoxification and organic matter (OM) stabilization are usually influenced by the biological transformation of iron (Fe) minerals; however, the underlying mechanisms of metal-reducing bacteria on the coupled kinetics of Fe minerals, Cr, and OM remain unclear. Here, the reductive sequestration of Cr(VI) and immobilization of fulvic acid (FA) during the microbially mediated phase transformation of ferrihydrite with varying Cr/Fe ratios were investigated. No phase transformation occurred until Cr(VI) was completely reduced, and the ferrihydrite transformation rate decreased as the Cr/Fe ratio increased. Microscopic analysis was uncovered, which revealed that the resulting Cr(III) was incorporated into the lattice structure of magnetite and goethite, whereas OM was mainly adsorbed on goethite and magnetite surfaces and located within pore spaces. Fine line scan profiles showed that OM adsorbed on the Fe mineral surface had a lower oxidation state than that within nanopores, and C adsorbed on the magnetite surface had the highest oxidation state. During reductive transformation, the immobilization of FA by Fe minerals was predominantly via surface complexation, and OM with highly aromatic and unsaturated structures and low H/C ratios was easily adsorbed by Fe minerals or decomposed by bacteria, whereas Cr/Fe ratios had little effect on the binding of Fe minerals and OM and the variations in OM components. Owing to the inhibition of crystalline Fe minerals and nanopore formation in the presence of Cr, Cr sequestration and C immobilization can be synchronously favored at low Cr/Fe ratios. These findings provide a profound theoretical basis for Cr detoxification and synchronous sequestration of Cr and C in anoxic soils and sediments.
... The short-range structure and coordination of iron within the cellular environment were monitored over time using XAS technique. 40 XAS spectra at the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regimes were measured at the Fe K-edge (7112 eV). Spectra of iron-loaded cells along their culture were determined in fluorescence mode and compared with initial iron-based precursors in solution and with reference samples, γ-Fe2O3 (maghemite), Fe2O3·0.5H2O ...
Iron is one of the most common metals in the human body, with an intrinsic metabolism including proteins involved in its transport, storage, and redox mechanisms. A less explored singularity is the presence of magnetic iron in the organism, especially in the brain. The capacity of human stem cells to biosynthesize magnetic nanoparticles was recently demonstrated, using iron released by the degradation of synthetic magnetic nanoparticles. To evidence a magnetic biomineralization in mammalian cells, it is required to address the biosynthesis of magnetic nanoparticles in cells supplied exclusively with non-magnetic iron salt precursors. Herein, mouse and human mesenchymal stem cells were incubated with ferric quinate for up to 36 days. By optimizing the concentration and culture time, and by measuring both total intracellular iron content and cellular magnetic signals, the biosynthesis of magnetic nanoparticles was found to occur from 14 days of continuous iron incubation and was correlated with important doses of intracellular iron. The local electronic structure and chemical environment of intracellular iron were further characterized by XAS spectroscopy at the Fe K-edge, showing a total conversion of Fe2+ to Fe3+ when using ferrous salts (ascorbate and sulfate), and a transformation towards ferrihydrite as well as a small proportion of a magnetic phase.
... The solubility of most heavy metals is highly pH dependent (48). High alkaline pH and low electrical conductivity reduce the solubility of certain metals like zinc, cadmium, and copper because they may be precipitated as hydroxides or carbonates (49)(50)(51)(52). ...
Marsh grasses have been used as efficient tools for phytoremediation and are known to play key roles in maintaining ecosystem functions by reducing the contamination of coastlines. This study was initiated to understand how human activities in wetlands can impact ion-heavy metal concentrations in relation to native and invasive marsh grasses. The study site, Blackbird Creek (BBC) is a tidal wetland that experiences agricultural, fishing, recreational, residential and other anthropogenic activities throughout the year. Heavy metals cadmium, arsenic, and lead in the soils and marsh grasses were monitored along with the ion compositions of soils. The main objective of this study was to understand if the marsh soils containing monotypic stands of native (Spartina) and non-native (Phragmites) vegetation display similar levels of heavy metals. Differences were observed in the concentrations of heavy metals at study sites with varying marsh vegetation types, and in soils containing vegetation and no vegetation. The soils with dense Spartina and Phragmites stands were anaerobic whereas soil at the boat ramp site was comparatively less anaerobic and also had increased levels of cadmium. Heavy metal concentrations in soil and Phragmites leaves were inversely correlated whereas they were positively correlated in Spartina sites. Electrical conductivity and pH levels in soil also showed increased cadmium and arsenic concentrations. These findings collectively infer that human activities and seasonal changes can increase soil complexities affecting the bioavailability of metals.
... Meanwhile, complexation is a function of deprotonated reactive sites (for example the carboxylate groups) that are responsible for metal ion-polymer structuring (González et al. 2014). The carboxylic groups have negative charges at neutral pH and easily bind to cations to form organometal complexes. ...
Organic and synthetic flocculants are conventional flocculants used in wastewater treatment and tap water purification. The challenges of toxic residue and the resultant secondary pollutants generated from organic and synthetic flocculants have attracted research efforts towards microbial extracellular polymers as nontoxic and biodegradable substitutes. However, the bioflocculants themselves have been associated with high production costs and low efficiency. To address these challenges, tremendous efforts have been made to hybridise cations with the bioflocculants. The contradictory reports on the role of cations in bioflocculation have necessitated this review. This paper reviews the relevant and recent literature on EPS-cation structuring, cationization of bioflocculants, the efficiency of the cationization of bioflocculants, the factors affecting cation induced bioflocculation, and the mechanisms of cation induced bioflocculation. Variations in experimental procedures microbial species and growth medium composition yield bioflocculants with positive or negative functional sites and may be responsible for the contradictory effect of the cations. Establishment of a standard fermentation system that could elucidate the mechanism of cation induced bioflocculation and of the standard techniques for evaluation of the cationic content of wastewater treated with cationized bioflocculants is needded for better understanding of the cation stimulated bioflocculation.
... At the same time, heavier isotopes are generally more prone to enrichment in species with heavier binding energy. Therefore, with the increasing of the biomass, organic adsorbed iron increases, and the iron isotope composition becomes heavier [59]. In addition, Hongfeng Lake belongs to the blue-green algae type moderate trophic seasonal layered lake, so this study also verified the experimental conclusion of Mulholland et al. [60], which is that blue-green algae produces iron isotope fractionation during iron adsorption and enriches heavy iron isotopes on the surface of blue-green algae. ...
The geochemical study of iron isotopes is of great significance to deeply understand the surface material circulation process and its environmental effects in surface and subsurface environments. Eutrophication lakes are an important part of the surface and subsurface environment; however, knowledge of the geochemical behavior and fractionation mechanism of iron isotopes in the biogeochemical cycling of eutrophication lakes is still scarce. In this study, a eutrophic lake with seasonal anaerobic characteristics (Hongfeng Lake) was selected as the study object to systematically analyze the iron isotope composition of suspended particles in lake water in different seasons as well as examining suspended particles in the main tributaries, sediments, pore water, planktonic algae, and other samples. The results show that the value of δ56Fe in Hongfeng Lake is between −0.85‰ and +0.14‰, and the value of δ56Fe has a high linear correlation with Fe/Al, indicating that the continental source material carried by the main inflow tributaries of the lake has an important influence on the source of iron in the lake. At the same time, Hongfeng lake is a medium eutrophication lake. Algal bloom and the content of chlorophyll a (Chl-a) is high, combined with the high correlation between Chl-a and the value of δ56Fe, which indicate that the growth of algae has an important influence on the change of iron isotope composition of suspended particles matter (SPM) in lake water and the adsorption and growth absorption of Fe by algae is the main reason for the change of the value of δ56Fe, so Fe isotope can be used to trace the lake’s biological action. For the lake and its inflow tributaries, δ56Fe values are higher in summer than those in winter. And the δ56Fe value of SPM in lake that varies with depth is more obvious in summer than in winter. In addition, there is an obvious thermocline in summer, which leads to hydrochemical stratification. Moreover, according to a linear correlation analysis, the content of DOC (dissolved organic matter) in Hongfeng Lake’s upper and lower waters, respectively, has a high correlation with the value of δ56Fe. Additionally, in the upper water, it is positively correlated, while on the bottom, there is a negative correlation relationship, which indicates that the difference in algae metabolism patterns between the upper and lower water bodies of Hongfeng Lake plays an important role in the iron isotope composition of suspended particulate matters (SPM). The composition of the Fe isotope in SPM is changed by organic adsorption and growth absorption of algae in upper water. With an increase in depth, the degradation process becomes the main one. In addition, the value of δ56Fe is low and Fe/Al is high in the water bottom, which indicates that “ferrous-wheel” cycle form at the bottom of the water.
... The EPS which are produced by heterotrophic bacteria are also capable to stabilize the metals in the vicinity of the cells (i.e., Tourney and Ngwenya, 2014). Former studies of Fe (II, III) interaction with heterotrophs demonstrated that, in the presence of surface organic ligands not screened by EPS layers, the polymerization of Fe(III) ions and formation of Fe-oxy(hydr)oxides is partially inhibited (Gonzalez et al., 2014b, Fakih et al., 2008. As a result, the adsorbed Fe(III) remains on the cell surface in the form of individual Fe atoms attached to organic moieties, conferring several advantages to the cells (Châtellier et al., 2004;Chan et al., 2009). ...
Dissolved organic matter (DOM) is an important component of natural waters, determining the form of elements and the emission of greenhouse gases (CO2, CH4), as well as affecting water biodiversity. A feature of boreal waters, and in particular the study area waters (North. Karelia, Russia), is high concentration of Fe (III), associated with DOM in low-molecular complexes and in high-molecular organo-ferric colloids, which act as the main carriers of metallic trace elements in the most typical hydrological continuum soil - bog - river - lake. The transformation of organo-ferric colloids occurs under the influence of two main factors: bacterial and photochemical degradation. The present study is devoted to the analysis of dissolved organic carbon (DOC) and trace elements (TE) behavior in surface waters of the boreal zone under the influence of the heterotrophic bacteria metabolic activity and photolytic oxidation of DOM under sunlight exposure. For the purpose of experiments there were used substrates with predominate allochthonous DOM of humic nature, including peat leachate, pine crown throughfall, fen, humic lake, stream, river and oligotrophic lake. An experiment carried out in laboratory conditions using monocultures of heterotrophic aerobic bacteria Pseudomonas aureofaciens and Pseudomonas saponiphila (isolated on the territory of Karelia) allowed to establish that allochthonous DOM of the boreal zone reservoirs possesses high resistance to the activity of the explored bacteria. The rate of bacterial mineralization of DOC was observed in range from 0 to 4.3 mgC L-1day-1, depending on the substrate, and the low molecular weight fraction (<1 kDa) of DOM was found to be more prone to destruction than high molecular weight (from 1 kDa to 0.22 µm). The interaction of bacteria with various aqueous substrates showed a significant predominance of short-term (< 1 h) adsorption on the cell surface for a wide range of elements (Al, Fe, Mn, Ni, Co, Cu, Cd, REE, U) over a long term (1 h - 96 h) intracellular uptake of metals and extracellular coprecipitation of elements with Fe and Al hydroxides. Among different substrates, there was an increase in the adsorption with the increase of DOC/Fe ratio in solution, which can be linked to a competition between Fe and metal cations for anionic adsorption sites on cell surface. The long-term removal of dissolved metals did not show any link to pH, DOC, and Fe concentration.
... Several natural biomasses have been used as biosorbent including algae (macro-and microalgae), bacteria, fungi, and agricultural wastes (Gonzalez et al. 2014;Kamarudzaman et al. 2015;Schiewer and Patil 2008). ...
The immobilized algae Sargassum vulgare was used as biosorbent for Fe3+ removal through a batch and continuous system in order to study the biosorption capacity and to establish a new method of the valorization of this waste. The kinetic data could be described by the pseudo first-order and pseudo second-order kinetic models. The batch equilibrium was fitted by the Langmuir model with a value of correlation coefficient (R2 = 0.98) higher than that of the Freundlich (R2 = 0.89). The process was exothermic and spontaneous and the biomass was successfully desorbed using 0.1 M HCl. Furthermore, the Thomas model, Bohart-Adams model, and Yoon-Nelson model were successfully applied to evaluate the dynamic behavior of Fe3+ biosorption in a fixed-bed column. The lower flow rate of 1.04 ml/min showed the greater performance of the process. Fourier transform infrared spectroscopy revealed the presence of several active binding sites, and scanning electron microscopy micrograph confirmed the metal adsorption on the surface. The results reveal that the immobilized algae have a potential removal for Fe3+ in a batch and continuous system.
... For Fe K-edge XANES, absorption edge and white line peak for co-precipitates with C/Fe/Ca ratio of 10/1/0 and 10/1/6 shifted slightly to lower energy compared to pure Fh and other co-precipitates with low OC content and the shift was even greater after the reduction ( Fig. 3; SI, Fig. S2). The metal-ligand association (between Fe and OC) and increase in Fe(II) fraction can shift XANES absorption edge and white line peak to lower energy (Shulman et al., 1976;Prietzel et al., 2007;Yano and Yachandra, 2009;Hohmann et al., 2011;Sarangi, 2013;Gonzalez et al., 2014). Similarly, the Fe-EXAFS spectra for co-precipitates with different Ca or OC content also showed some variations. ...
Complexation with minerals plays a critical role in regulating the stability of organic matter. The presence of cations is assumed to be important for the complexation between organic matter and minerals, but there is still limited direct analysis for the formation and reactivity of mineral-organic matter-cation ternary complexes, as well as governing factors for the fate of minerals and organic matter in the complexes. In order to close this knowledge gap, we investigated the formation and reactivity of ferrihydrite (Fh)-organic carbon (OC)-calcium (Ca) ternary co-precipitates. We performed microbial anaerobic Fe reduction using Shewanella putrefaciens CN32 on synthesized Fh-OC-Ca co-precipitates and characterized OC and Fe minerals using various spectroscopic and wet chemistry techniques. We found that Ca incorporated into the co-precipitate was a function of OC/iron (Fe) ratio, but OC incorporation was not impacted by the Ca content. During the reduction, the presence of Ca favored the formation of green rust but decreased the formation of magnetite and siderite in co-precipitates with high OC content. The reduction of Fe and reductive release of Fe-bound OC were controlled primarily by the C/Fe ratio, rather than Ca/Fe ratio. Phenolic OC was preferentially released or degraded during the reduction compared to aromatic and carboxylic OC. Collectively, C/Ca incorporation data, Fe K-edge extended X-ray absorption fine structure (EXAFS) analysis for co-precipitates before and after reduction, and the reductive release of Ca and OC suggest the formation of Fh-OC-Ca ternary co-precipitates, likely with OC as bridges. The reduction of Fe and reductive release of OC were primarily controlled by the C/Fe ratio, whereas the presence of Ca affected the mineral phase transformation for Fh during the reduction of Fe. Hence, our results provide novel understanding for the formation and reactivity of Ca-based ternary co-precipitates, which can be valuable for building up process-based models for cycles of carbon and metals.
... At the pH of seawater, O 2 is the most important oxidant when H 2 O 2 concentration is below 200 nM and [Fe (II)] is at nanomolar levels . The complexation of iron by organic ligands often alters the Fe(II) oxidation kinetics by either accelerating or retarding the oxidation rate, depending on the source and characteristics of the Fe-binding organic ligands (Emmenegger et al., 1998;González et al., 2014;Pham and Waite, 2008;Waite, 2002, 2003;Roy and Wells, 2011;Santana-Casiano et al., 2000Theis and Singer, 1974). Roy et al. (2008) proposed the complexation of Fe(II) by organic ligands as an underlying mechanism to explain the decrease in Fe(II) oxidation rates in the Western Subarctic Pacific. ...
The Fe(II) oxidation rate was studied in different water masses present in the subarctic North Atlantic ocean along the 59.5° N transatlantic section. Temperature, pH, salinity and total organic carbon (TOC) in natural conditions, fixed temperature conditions and both fixed temperature and pH conditions, were considered in order to understand the combined effects of the variables that control the Fe(II) oxidation kinetics in the ocean. The study shows that in natural conditions, temperature was the master variable which controlled 75% of the pseudo-first order kinetics rate (k′). This value rose to 90% when pHF (free scale) and salinity were also considered. At a fixed temperature, 72% of k′ was controlled by pH and at both fixed temperature and pH, salinity controled 62% of the Fe(II) oxidation rate. Sources and characteristics of TOC are important factors influencing the oxidation of Fe(II). The organic matter had both positive and negative effects on Fe(II) oxidation. In surface and coastal waters, TOC accelerated k′, decreasing the Fe(II) half-life time (t1/2). In Subpolar Mode Water, Labrador Sea Water (for the Irminger Basin) and Denmark Straight Overflow Water, TOC slowed down k′, increasing Fe(II) t1/2. This shifting behaviour where TOC affects Fe(II) oxidation depending on its marine or terrestrial origin, depth and remineralization stage proves that TOC cannot be used as a variable in an equation describing k′. The temperature dependence study indicated that the energy requirement for Fe(II) oxidation in surface waters was 32% lower than the required for bottom waters at both pH 7.7 and 8.0. This variability confirmed the importance of the organic matter composition of the selected samples. The Fe(II) oxidation rate constants in the region can be obtained from an empirical equation considering the natural conditions of temperature, pHF and salinity for the area, producing an error of estimation of 0.0072 min⁻¹. This equation should be incorporated in global Fe models.
... Thus, numerous studies have been devoted to the quantification and thermodynamic modeling of reversible metal cation adsorption on the cell wall of aquatic microorganisms [21][22][23][24]. Extensive research over past decades has provided a comprehensive picture of metal binding to cell walls of most model aquatic microorganisms, including autotrophic and heterotrophic bacteria and diatoms [25][26][27][28][29][30][31][32][33]. However, there is no study, to our knowledge, of AuNPs interaction with freshwater periphytic diatoms. ...
The rising concern about the potential toxicity of synthetic gold nanoparticles (AuNPs) in aquatic environments requires a rigorous estimation of physico-chemical parameters of reactions between AuNPs and major freshwater microorganisms. This study addresses the interaction of 10-nm size, positively charged AuNPs with periphytic freshwater diatoms (Eolimna minima). The adsorption experiments on viable cells were performed in 10 mM NaCl and 5 mM NaCl + 5 mM NaHCO3 solution at a variable pH (3–10), at an AuNPs concentration from 1 µg/L to 10,000 µg/L, and an exposure time from a few minutes to 55 days. Three types of experiments, adsorption as a function of time (kinetics), pH-dependent adsorption edge, and constant-pH “Langmuirian” type isotherms, were conducted. In addition, long-term interactions (days to weeks) of live diatoms (under light and in the darkness) were performed. The adsorption was maximal at a pH from 3 to 6 and sizably decreased at a pH of 6 to 10. Results of adsorption experiments were modeled using a second order kinetic model, a Linear Programming Model, Freundlich isotherm, and a ligand binding equation for one site competition. The adsorption of AuNPs(+) most likely occurred on negatively-charged surface sites of diatom cell walls such as carboxylates or phosphorylates, similar to previously studied metal cations. Under light exposure, the AuNPs were stabilized in aqueous solution in the presence of live cells, probably due to the production of exometabolites by diatoms. The adsorbed amount of AuNPs decreased after several days of reaction, suggesting some AuNPs desorption. In the darkness, the adsorption and assimilation were stronger than under light. Overall, the behavior of positively charged AuNPs at the diatom–aqueous solution interface is similar to that of metal cations, but the affinity of aqueous AuNPs to cell exometabolites is higher, which leads to the stabilization of nanoparticles in solution in the presence of diatoms and their exudates. During photosynthetic activity and the pH rising above 9 in the vicinity of diatom cells, the adsorption of AuNPs strongly decreases, which indicates a decreasing potential toxicity of AuNPs for photosynthesizing cells. The present study demonstrates the efficiency of a thermodynamic and kinetic approach for understanding gold nanoparticles interaction with aquatic freshwater peryphytic microorganisms.
... Previous experiments with Synechococcus PCC 7002 cells demonstrated that sorption to cells is a major fate for aqueous iron, although the oxidation state of sorbed iron was not determined in those experiments, so we cannot rule out that some Fe(II) was also sorbed to cells. 34 However, sorption onto cells has previously been documented as a fate for aqueous iron with diverse cyanobacteria, with Fe(III) more commonly detected at the cell surface than Fe(II), 22 The other surface in our experiments that could have sorbed iron extracted as Fe NaAc was Fe ppt . The three techniques we used to address mineralogy indicate that our Fe ppt was a mixture of 58% ferrihydrite, 22% goethite, and 20% lepidocrocite (Figure 2), and ferrihydrite was likely the predominant mineral present during the experiments (see Supporting Information). ...
In this study, we couple iron isotope analysis to microscopic and mineralogical investigation of iron speciation during circumneutral Fe(II) oxidation and Fe(III) precipitation with photosynthetically produced oxygen. In the presence of the cyanobacterium Synechococcus PCC 7002, aqueous Fe(II) (Fe(II)aq) is oxidized and precipitated as amorphous Fe(III) oxyhydroxide minerals (iron precipitates, Feppt), with distinct isotopic fractionation (ε56Fe) values determined from fitting the δ56Fe(II)aq (1.79 and 2.15 ‰) and the δ56Feppt (2.44 and 2.98 ‰) data trends from two replicate experiments. Additional Fe(II) and Fe(III) phases were detected using microscopy and chemical extractions, and likely represent Fe(II) and Fe(III) sorbed to minerals and cells. The iron desorbed with sodium acetate (FeNaAc) yielded heavier δ56Fe compositions than Fe(II)aq. Modeling of the fractionation during Fe(III) sorption to cells and Fe(II) sorption to Feppt, combined with equilibration of sorbed iron and with Fe(II)aq using published fractionation factors are consistent with our resulting δ56FeNaAc. The δ56Feppt data trend is inconsistent with complete equilibrium exchange with Fe(II)aq. Because of this and our detection of microbially-excreted organics (e.g. exopolysaccharides) coating Feppt in our microscopic analysis, we suggest that electron and atom exchange is partially suppressed in this system by biologically-produced organics. These results indicate that cyanobacteria influence the fate and composition of iron in sunlit environments via their role in Fe(II) oxidation through O2 production, the capacity of their cell surfaces to sorb iron, and via the interaction of secreted organics with Fe(III) minerals.
... Journal of Geophysical Research: Biogeosciences 10.1002/2016JG003394 González et al., 2014]. At the beginning of our Fe coating experiments, the concentration of these binding sites was 6.0 × 10 À10 mol L À1 when calculated from the initial bacterial biomass of 0.08 μmol C L À1 and 3.1 × 10 À4 moles of proton-donating surface sites per gram of cells reported by Ha et al. [2010]. ...
Iron (Fe) may alter the biodegradation of dissolved organic matter (DOM), by interacting with DOM, phosphorus (P), and microbes. We isolated DOM and a bacterial community from boreal lake water and examined bacterial growth on DOM in laboratory experiments. Fe was introduced either together with DOM (DOM-Fe) or into bacterial suspension, which led to the formation of insoluble Fe precipitates on bacterial surfaces (Fe coating). In the latter case, the density of planktonic bacteria was an order of magnitude lower than that in the corresponding treatment without introduced Fe. The association of Fe with DOM decreased bacterial growth, respiration, and growth efficiency compared with DOM alone at the ambient concentration of dissolved P (0.16 μmol L�-1), indicating that DOM-associated Fe limited the bioavailability of P. Under a high concentration (21 μmol L�-1) of P, bacterial biomass and respiration were similar or several times higher in the treatment where DOM was associated with Fe than in a corresponding treatment without Fe. Based on the next generation sequencing of 16S rRNA genes, Caulobacter dominated bacterial communities grown on DOM-Fe. This study demonstrated that association of Fe with a bacterial surface or P reduces bacterial growth and the consumption of DOM. In contrast, DOM-Fe is bioavailable and bound Fe can even stimulate bacterial growth on DOM when P is not limiting.
... The diffusion of toxic Ag + through the mineral frustule should be slow and thus, despite the low biomass and high accumulation of Ag, the metal becomes less toxic to diatoms compared to other species. With time, the cyanobacteria and green algae develop sufficient biomass with thick EPS capsules and sheaths (González et al. 2014). This helps external layers, virtually absent in diatoms, to efficiently protect green algae and cyanobacteria from metal stress. ...
Although the industrial use of nanoparticles has increased over the past decade, the knowledge about their interaction with benthic phototrophic microorganisms in the environment is still limited. This study aims to characterize the toxic effect of ionic Ag+ and Ag nanoparticles (citrate-coated silver nanoparticles, AgNPs) in a wide concentration range (from 1 to 1000 μg L−1) and duration of exposure (2, 5 and 14 days) on three biofilm-forming benthic microorganisms: diatom Nitzschia palea, green algae Uronema confervicolum and cyanobacteria Leptolyngbya sp. Ag+ has a significant effect on the growth of all three species at low concentrations (1–10 μg L−1), whereas the inhibitory effect of AgNPs was only observed at 1000 μg L−1 and solely after 2 days of exposure. The inhibitory effect of both Ag+ and AgNPs decreased in the course of the experiments from 2 to 14 days, which can be explained by the progressive excretion of the exopolysaccharides and dissolved organic carbon by the microorganisms, thus allowing them to alleviate the toxic effects of aqueous silver. The lower impact of AgNPs on cells compared to Ag+ can be explained in terms of availability, internalization, reactive oxygen species production, dissolved silver concentration and agglomeration of AgNPs. The duration of exposure to Ag+ and AgNPs stress is a fundamental parameter controlling the bioaccumulation and detoxification in benthic phototrophic microorganisms.
... Human activities such as atmospheric deposition, phosphate fertilizers, and sewage sludge disposal are the main sources of soil Cd and Ni contamination (Smolders and Mertens 2013). It has been reported that iron (Fe) is one of the important metals in soils and aquatic environments (Sunda and Huntsman 1997;Maldonado and Price 2001;González et al. 2014). ...
The sorption, desorption, and speciation of cadmium (Cd), nickel (Ni), and iron (Fe) in four calcareous soils were investigated at the pH range of 2–9. The results indicated that sorption of Fe by four soils was higher than 80 % at pH 2, while in the case of Cd and Ni was less than 30 %. The most common sequence of metal sorption at pH 2–9 for four soils was in the order of Fe ≫ Ni > Cd. Cadmium and Ni sorption as a function of pH showed the predictable trend of increasing metal sorption with increase in equilibrium pH, while the Fe sorption trend was different and characterized by three phases. With regard to the order of Cd, Ni, and Fe sorption on soils, Cd and Ni showed high affinity for organic matter (OM), whereas Fe had high tendency for calcium carbonate (CaCO3). Results of metal desorption using 0.01 M NaCl demonstrated that metal sorption on soils containing high amounts of CaCO3 was less reversible in comparison to soils containing high OM. In general, Cd and Ni desorption curves were characterized by three phases; (1) the greatest desorption at pH 2, (2) the low desorption at pH 3–7, and (3) the least desorption at pH > 7. The MINTEQ speciation solubility program showed that the percentage of free metals declined markedly with increase of pH, while the percentage of carbonate and hydroxyl species increased. Furthermore, MINTEQ predicted that saturation index (SI) of metals increased with increasing pH.
... The LCA method is widely used in a number of research fields such as environmental sciences [11,15,21,25,27], cultural heritage [16,41,42], biology [43][44][45], or material science [9,[12][13][14][15][16]. As a recent example, XANES spectroscopy has been used to follow the biomineralization process of magnetotactic bacteria Magnetospirillum gryphiswaldense strain MSR-1 [18]. ...
X-ray absorption fine structure spectroscopy (XAFS) is a local probe, element-specific technique that provides the electronic nature and the local structure around the absorber atom. In this chapter, we present a brief introduction to XAFS technique, both in the near edge, X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) region. Both regions contain complementary information. The XANES gives information on the electronic properties and the local geometry around the absorber atom, and the EXAFS provides the local structure parameters such as coordination numbers, distances, and disorder around the absorber atom. In particular, we focus on the potential of XAFS technique to the study of Fe oxides and oxyhydroxides, this technique being especially useful in nanoparticulate systems, even in very diluted conditions.
... The composite and abiotic systems displayed similar patterns in over time, but there was significantly more Fe(II) aq present in the composite system until $7 h, just after FeCl 2 solution addition ceased. The inhibition of Fe(II) aq oxidation in the composite system was attributed to Fe(II) sorption onto the bacterial cells, which has been previously inferred to stabilize Fe in the reduced state (Daughney et al., 2011;Fakih et al., 2008;Franzblau et al., 2014Franzblau et al., , 2015González et al., 2014). This process facilitated the subsequent surface reduction of sorbed Cu(II), as was found via XANES analysis of the composites over time. ...
There is currently poor understanding of metal removal by composites of bacteria and iron oxide minerals, even though they commonly co-occur and are among the most important sorbents in near-surface fluid–rock environments. This study evaluated Cu removal by composites of Anoxybacillus flavithermus and iron oxide over time during the addition, oxidation, and hydrolysis of Fe(II) aq and precipitation of the mineral, in comparison to Cu removal in the two single-sorbent end-member systems. In the absence of iron oxide, Cu removal by A. flavithermus was well described by a previously published surface complexation model, after inclusion of additional reactions describing aqueous complexation by exudate ligands released by the bacteria. In the absence of bacterial cells, Cu removal by iron oxide synthesized in the presence of the bacterial exudate ligands demonstrated the formation of ternary surface complexes. Removal of Cu by the A. flavithermus–iron oxide composites was ca. 20% greater than the prediction based on assumption of additivity in the two end-member systems. This non-additive behavior was attributed to (1) progressive physical blockage of bacterial surface sites by the iron oxide particles, (2) physical blockage of adsorption sites as a result of self-aggregation of the iron oxide particles, and (3) the reduction of Cu(II) to Cu(I) at the bacterial cell surface, as demonstrated by X-ray absorption spectroscopy. The extent of reduction of Cu(II) to Cu(I) was proportional to the concentration of solid phase Fe(II), suggesting that iron oxidation and copper reduction are linked. This study has shown that Cu removal by bacteria–iron oxide composites is greatly affected by redox processes such as Cu(II) reduction on the cell surface both by other bacterial surface ligands and the oxidation of sorbed Fe(II), as well as Fe(II) redox interactions, and aging effects of the mineral (i.e. surface site masking).
... The electrical properties of bacterial cells are determined by the type and concentration of specific functional groups on bacterial cell walls, pH value, composition and ionic strength of the electrolyte, and separation distance from nearby surfaces [1][2][3][4][5][6][7][8][9][10]. These properties induce important environment characteristics of the bacteria such as mobility [1][2][3][4][5], zeta potential [1][2][3][4]6], metal binding [3][4][5][11][12][13][14][15][16][17][18] and adhesion to surfaces [19][20][21][22][23]. The bacterial cell surface charge is originated from the acidic and basic functional groups, including carboxyl, phosphoryl, hydroxyl, and amine groups [11][12][13]. ...
The Donnan potential can be observed in many biological systems due to the presence of polyelectrolytes as proteins and nucleic acids. The aim of this work was to present a useful tool to describe the fixed and charge-regulated volume charge density profile through the use of a smoothing function and to obtain the electrostatic potential profile as well as the Donnan potential of this system by solving Poisson-Boltzmann (PB) equation. When we use the smoothing function, the Donnan potential arises automatically from the solution of only one Poisson-Boltzmann equation and it is not necessary to impose this potential for treating charged system in the presence of a membrane. The electrostatic behavior across the Bacillus brevis wall considering the dependence on the ionization of the cell wall functional groups as a function of the solution pH was analyzed. An important issue was to show that potentiometric titration data could be used together with the Poisson-Boltzmann equation to predict the electrostatic behavior (e.g., zeta potential) of the bacterial cell surface.
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... Moreover, the carboxylic and hydroxo-groups have been described as the most active groups, binding cationic metals, such as copper and iron, in aqueous solutions in numerous aquatic microorganisms. 61,62 These groups are two strong bases that complex Fe(II) decreasing the Fe(II) oxidation rate. ...
The role played by the natural organic ligands excreted by the green algae Dunaliella tertiolecta on the Fe(II) oxidation rate constants was studied at different stage of growth. The concentration of dissolved organic carbon increased from 2.1 to 7.1 mgL-1 over time of culture. The oxidation kinetics of Fe(II) was studied at nanomolar levels and under different physico-chemical conditions of pH (7.2-8.2), temperature (5-35ºC), salinity (10-37) and dissolved organic carbon produced by cells (2.1-7.1 mgL-1). The experimental rate always decreased in the presence of organic exudates respect to that in the control seawater. The Fe(II) oxidation rate constant was also studied in the context of Marcus theory, where ∆Gº was 39.31-51.48 kJmol-1. A kinetic modelling approach was applied for computing the equilibrium and rate constants for Fe(II) and exudates present in solution, the Fe(II) speciation and the contribution of each Fe(II) species to the overall oxidation rate constant. The best fit model took into account two acidity equilibrium constants for the Fe(II) complexing ligands with pKa,1=9.45 and pKa,2=4.9. The Fe(II) complexing constants were KFe(II)-LH=3∙1010 and KFe(II)-L=107 and the corresponding computed oxidation rates were 68±2 and 36±8 M-1min-1, respectively.
... Enhanced effect of organic ligands on olivine dissolution with pH increase in acidic and circumneutral solutions has been also reported in previous studies ( Hänchen et al., 2006;Olsen and Rimstidt, 2008). Note that the possibility of «passivation» of the olivine surface by Fe(OH) 3 in basic solutions is very unlikely at the conditions of this study: oxidation of Fe(II) into Fe(III) and Fe oxy(hydr)oxide precipitation should be significantly retarded by high concentration of dissolved organic matter (5-25 mg DOC/L, see González et al., 2011). Besides, organic ligand-enhanced dissolution of Fe oxy(hydr)oxides is much more efficient than that of Mg(OH) 2 and basic silicates, as a results, the newly-formed iron oxy(hydr)oxides will be rapidly dissolving in the presence of organic ligands. ...
This work is aimed at quantification of forsteritic olivine (Fo92) dissolution kinetics in batch and mixed-flow reactors in the presence of aerobic gram-negative bacteria (Pseudomonas reactans HK 31.3) isolated from an instrumented well located within a basaltic aquifer in Iceland. The release rate of mineral constituents was measured as a function of time in the presence of live and dead cells in constant-pH (4–9), bicarbonate-buffered (0.001–0.05 M), nutrient-rich and nutrient-free media in batch reactors at 0–30 atm of CO2 partial pressure (pCO2). In batch reactors at 30 atm pCO2, 0.1 M NaCl and 0.05 M NaHCO3 the rates were weakly affected by the presence of bacteria. In nutrient media, the SEM observation of reacted grains revealed the presence of biofilm-like surface coverage that does not modify Mg and Si release rate at the earlier stages of reaction but significantly decreased the dissolution after prolonged exposure.Olivine dissolution rates measured in flow-through reactors are not affected by the presence of dead and live bacteria at pH ⩾9 in 0.01 M NaHCO3 solutions. In circumneutral, CO2-free solutions at pH close to 6, both live and dead bacteria increase the dissolution rate, probably due to surface complexation of exudates and lysis products. In most studied conditions, the dissolution was stoichiometric with respect to Mg and Si release and no formation of secondary phases was evidenced by microscopic examination of post-reacted grains. Obtained results are consistent with known molecular mechanism of olivine dissolution and its surface chemistry. Overall, this work demonstrates negligible effect of P. reactans on olivine reactivity under conditions of CO2 storage in the wide range of aqueous fluid composition.
Despite the importance of structural control on metal stable isotope fractionation in inorganic and abiotic systems, the link between metal structural changes and related isotopic fractionation during reactions with organic...
While it has been recognized that sulfidation can effectively improve the reactivity of microscale zero valent iron (mZVI), there is limited understanding of nitrobenzene (ArNO2) removal by sulfidated mZVI. To understand the reduction capacity and pathway of ArNO2 by sulfidated mZVI, ball-milling sulfidated mZVI (S-mZVIbm) with different S/Fe molar ratios (0-0.2) was used to conduct this experiment. The results showed that sulfidation could efficiently enhance ArNO2 removal under iron-limited and iron excess conditions, which was attributed to the presence of FeSx sites that could provide higher Fe(0) utilization efficiency and stronger passivation resisting for S-mZVIbm. The optimum ArNO2 reduction could be obtained by S-mZVIbm with S/Fe molar ratio at 0.1, which could completely transform ArNO2 to aniline (ArNH2) with a rate constant of 4.36 × 10-2 min-1 during 120-min reaction. FeSx phase could act as electron transfer sites for ArNO2 reduction and it could still be reserved in S-mZVIbm after reduction reaction. The product distribution indicated that sulfidation did not change the types of reduction products, while the removal of ArNO2 by S-mZVIbm was a step-by-step reduction progress along with the adsorption of ArNH2. In addition, a faster reduction of ArNO2 in groundwater/soil system further demonstrated the feasibility of S-mZVIbm in the real field remediation.
Iron species have essential influence on the environmental/geochemical behaviors of arsenic species in water and soil. Colloidal ferric hydroxide (CFH) induces photooxidation of arsenite (As(III)) to arsenate (As(V)) in water at neutral pH through surface complexation and ligand-to-metal charge transfer (LMCT). However, the effect of the co-existing natural organic matter (NOM) on the complexation-photolysis in this process has remained unclear. In the present work, the photooxidation of As(III) induced by CFH was investigated in the presence of various carboxylic acids and polyphenols as simple model compounds of NOM. Two different light sources of ultraviolet A (UVA) (λmax = 365 nm) and ultraviolet B (UVB) (λmax = 313 nm) were used for photooxidation treatment of the experimental ternary system and the control binary system respectively. The obtained results demonstrated that all investigated NOM inhibited the photooxidation of As(III) in the As(III)/CFH system at pH 7. Moreover, the correlation analysis between the pseudo-first order rate constant kobs and various property parameters of NOM showed that the stable constant for the complexation between Fe(III) and NOM (logKFe-NOM) as well as the molecular weight of NOM and the percentages of total acidity of NOM exhibited significant correlations. A simple quantitative structure-activity relationship (QSAR) model was established between kobs and these three parameters utilizing a multiple linear regression method, which can be employed to estimate the photooxidation efficiency of As(III) in the presence of ferric iron and NOM. Thus, the present work contributes to the understanding of the environmental interactions between NOM and iron.
Bio-degradation and photolysis of dissolved organic matter (DOM) in boreal high-latitude waters are the two main factors controlling aquatic fluxes and residence time of carbon but also metal nutrients associated with DOM such as Fe. The DOM is usually present in the form of organic and organo-mineral colloids that also account for the majority of dissolved Fe. Here we use the stable Fe isotope approach to unravel the processes controlling Fe behavior during bio- and photo-degradation of colloids in boreal Fe- and DOM-rich humic waters (a stream and a fen). The adsorption of Fe colloids onto heterotrophic bacteria P. aureofaciens produced enrichment in +0.4‰ (57Fe) in the heavier isotopes of the cell surface relative to the remaining solution. In contrast, long-term assimilation of Fe by live cells yielded preferential incorporation of lighter isotopes into the cells (-0.7‰ relative to aqueous solution). The sunlight-induced oxidation of Fe(II) in fen water and coagulation of organo-ferric colloids led to removal of heavier Fe isotopes (+1.5 to +2.5‰) from solution, consistent with Fe(III) hydroxide precipitation from Fe(II)-bearing solution. Altogether, bio- and photodegradation of organo-ferric colloids, occurring within a few days of exposure time, can produce several per mil isotopic excursions in shallow lentic and lothic inland waters of high latitude boreal regions. Considerable daily scale variations of Fe isotopic composition should therefore be taken into account during interpretation of riverine flux of Fe isotope to the ocean or tracing weathering processes using Fe isotopes in surface waters at high latitudes.
Subterranean estuaries (STEs) are land-ocean interfaces where meteoric fresh groundwater mixes with intruding seawater in a coastal aquifer, before discharging into the adjacent water column. In contrast to surface estuaries, STEs have the potential to amplify concentrations of constituents such as copper (Cu) and iron (Fe) due to long residence times and reductive dissolution of mineral phases along the groundwater flowpaths. However, oxidative precipitation of Fe and Mn at the sediment-water interface may scavenge many constituents again before they reach the coastal water column. Hence, the geochemical impact of the suboxic to anoxic submarine groundwater discharge (SGD) on the oxygenated coastal ocean relies on the capability of constituents such as Cu and Fe to stay in solution across redox boundaries. Here, we propose that dissolved organic matter (DOM) in the STE plays a pivotal role in the speciation of Cu and Fe through (i) fueling reductive dissolution and (ii) providing ligands to form stable metal-DOM complexes, increasing their transfer from the STE into the coastal ocean. We investigated the concentrations and speciation of Cu and Fe, and DOM chemical characteristics, in two beach STEs of a barrier island. By combining well-established techniques with novel quantification and speciation approaches from both the inorganic and organic geochemical realm (size-fractionation filtration, ferrozine detection, voltammetry, sequential DOM extraction, and ultra-high resolution mass spectrometry) we characterized metal-DOM associations down to the molecular level. Overall, pore water from both STEs was enriched with Cu and Fe compared to seawater, which indicated transfer potential for both trace metals across the sediment-water interface. However, Fe gradients from pore water to surface were steeper than those for Cu, indicating a larger net transfer of the latter compared to the former. Our voltammetry data showed that Cu was exclusively organically bound in both STEs and the water column, mostly in soluble form (<20 nm). The majority of >60 newly identified Cu-containing complexes had primarily aliphatic character and N and S in their molecular formulae resembling labile marine DOM, while two Cu-DOM complexes had polyphenol (“humic-like”) molecular formulae indicative of terrestrial vascular plant-derived material. In contrast to Cu, the Fe pool consisted of either reduced, soluble (<20 nm), likely free Fe(II) in the anoxic STE, or of larger colloids (<200 nm and >20 nm) in the fresh groundwater and seawater endmembers, likely as Fe(III)(hydr)oxides stabilized by DOM. Furthermore, while Fe and humic-like DOM seemed to share common sources, all directly identified mobile Fe-DOM complexes appeared to have marine origins. Therefore, organic forms of Fe in the STE may primarily consist of immobile humic-Fe coagulates, partially mobile Fe-nanocolloids, and mobile, N-containing, marine aliphatic Fe-complexes. Our study indicates that aliphatic, N-containing ligands may play an important role in the organic complexation and stabilization of Fe and particularly Cu in the STE, and enable them to cross redox boundaries at the sediment-water interface.
The mechanism of metal adsorption onto bacteria can change as a function of metal loading of the binding sites, and therefore the mechanisms of adsorption and the thermodynamic stability constants for the important bacterial surface complexes must be determined in order to accurately model metal behavior in near-surface geologic systems. In this study, batch metal adsorption experiments were conducted as a function of pH from 4 to 8, and Hg loading from 10 to 200 μmol Hg/g (wet weight bacteria). The adsorption data are in poor agreement with predictions of the extent of Hg adsorption that used previously published Hg-bacteria stability constants generated from high metal loading experiments. The extent that the thermodynamic model underpredicts the measured Hg adsorption increases with decreasing Hg loading and pH, suggesting that the mechanism of adsorption changes as a function of Hg loading and that an additional Hg-bacterial surface complex becomes important under low, <200 μmol/g, Hg loading and low, <6.5, pH conditions. The new Hg-bacterial surface complex involves a high affinity, low abundance site, which is likely a sulfhydryl moiety based on x-ray absorption spectroscopy, and is important only under low Hg loading conditions. We model the enhanced adsorption onto this high affinity site using an additional Hg-bacterial surface complex to those considered in the model derived from the high Hg loading data, and we use our data to constrain values of the stability constants for this new complex. Our modeling results provide reasonable fits to the data, and the proposed set of stability constants should improve the accuracy of quantitative models of Hg complexation with bacterial surfaces as a function of Hg loading and pH.
Bacterial cell membranes display a strong affinity for a wide variety of aqueous metal cations and have the potential to affect the mass transport of these cations through adsorption reactions in water–rock systems. Prior studies have focused on determining the thermodynamic stability constants of heavy metals and radionuclides; however, the constants for Mn-bacterial surface complexes formed on manganese oxidizing bacteria remain unmeasured. We measured the rate, extent, and reversibility of Mn(II) adsorption onto a bacterial species capable of Mn-oxidization (Pseudomonas putida), and onto a bacterial species that does not promote Mn-oxidization (Bacillus subtilis). The extent of adsorption was measured as a function of both pH and metal loading in order to determine the thermodynamic stability constants of the Mn-bacterial surface complexes that form on the bacteria and to test whether Mn oxidizers exhibit unusual Mn adsorption behavior relative to species that do not oxidize Mn. Furthermore, we determined the effect of bacterial extracellular polymeric substances (EPS) on Mn(II) adsorption by conducting experiments with and without EPS removal from the biomass. The experimental results indicated that Mn(II) adsorption onto B. subtilis and P. putida was rapid and reversible. The extent of Mn(II) adsorption onto both bacterial species increased with increasing pH, but P. putida adsorbed significantly more Mn(II) than did B. subtilis across most of the pH range studied. Both the adsorption measurements and the calculated stability constants indicate that P. putida, a Mn oxidizing bacterial species, exhibited significantly enhanced Mn adsorption relative to that observed for B. subtilis. The enhanced Mn(II) adsorption exhibited by P. putida suggests that bacteria may adapt the metal binding environments within their cell envelopes in order to optimize bioavailability of metals that are beneficial to their metabolism. Experiments conducted at low metal-loading conditions yielded stability constants for the Mn(II)-bacterial surface complexes that were less than or similar to those calculated for the high metal-loading conditions, suggesting that Mn(II)-sulfhydryl binding does not dominate under low metal loading conditions as it does for other metals. Removal of EPS molecules from P. putida led to significantly reduced extents of Mn(II) adsorption, suggesting that Mn(II)-EPS binding plays at least some role in the overall adsorption of Mn(II) onto P. putida biomass.
The role of sulfhydryl sites within bacterial cell envelopes is still unknown, but the sites may control the fate and bioavailability of metals. Organic sulfhydryl compounds are important complexing ligands in aqueous systems and they can influence metal speciation in natural waters. Though representing only approximately 5-10% of the total available binding sites on bacterial surfaces, sulfhydryl sites exhibit high binding affinities for some metals. Due to the potential importance of bacterial sulfhydryl sites in natural systems, metal-bacterial sulfhydryl site binding constants must be determined in order to construct accurate models of the fate and distribution of metals in these systems. To date, only Cd-sulfhydryl binding has been quantified.
Bacteriogenic iron oxide (BIOS) particles have a high affinity to adsorb inorganic nutrients such as Ca, Mg, Zn, and Ni, raising the question of how Fe(II) oxidizing bacteria (FeOB) compete for these same cations while the BIOS are in such close proximity to the cells. Answering this question requires a detailed understanding of metal binding properties of FeOB.
In this study, the adsorption behaviors of aqueous Ni, Cu, Zn, Sr, Cd, and Pb onto the sheathless FeOB Leptothrix cholodnii SP-6SL were measured separately, and the adsorption behaviors were compared to that of Bacillus subtilis in order to determine if the iron oxidizing bacterial species exhibits similar binding properties to those of most previously studied bacterial species. The experiments for both species were performed aerobically; ionic strength was held constant with 0.1 M NaClO4; the experiments were conducted as a function of pH over the range of 2 to 9; and biomass and metal concentrations were 10 g (wet weight)/L and 2 ppm, respectively.
Our results show that the two studied bacterial species exhibit similar adsorption of the tested metals, and that although some iron oxides formed during L. cholodnii SP-6SL liquid culture growth, they are present in low enough concentrations not to significantly affect the extent of metal adsorption. We apply a linear free-energy approach to define relationships between the stability constants for site-specific metal-bacterial surface complexes and corresponding metal-acetate binding constants. We use these relationships to estimate binding constants for L. cholodnii for some metals that have not been studied, and we apply these results to calculate the ability of L. cholodnii to compete for nutrients with BIOS particles that form during Fe(II) oxidation by the bacteria.
The respiration of dissolved organic matter (DOM) by aerobic heterotrophic bacterioplankton in boreal surface waters is one of the major factors that regulate CO2 exchange of lakes and rivers with the atmosphere in arctic and subarctic zones. The DOM that originates from topsoil leaching and vegetation degradation is brought to the lakes by surface flow and is subjected to coagulation and degradation by heterotrophic bacteria, which are well-established processes in the majority of boreal aquatic settings. The behavior of colloids and organic complexes of trace metals during this process is virtually unknown. In this work, we studied the interaction of two model heterotrophic bacteria, soil Pseudomonas aureofaciens and aquatic Pseudomonas reactans, with peat and Sphagnum moss leachates from the permafrost region under controlled laboratory conditions in nutrient-free media. The moss leachate was the better substrate for bacterial survival, with P. reactans exhibiting an order of magnitude higher live cell number compared with P. aureofaciens. In eight-day experiments, we analyzed organic carbon and ∼40 major and trace elements during heterotrophic bacteria growth. The total net decrease of the concentration of Dissolved Organic Carbon (DOC) was similar for both bacteria and ranged from 30 mg gwet−1 to ≤ 10 mg gwet−1 during 8 days for the moss and peat leachate, respectively. Despite significant evolutions of pH, DOC, Dissolved Inorganic Carbon (DIC) and cell number, most major (Mg, K, Ca) and trace elements remained nearly constant (within ± 30% of the control).Only Fe, Al, P, Zn, Mn, Co, Ba and, to a much lesser extent, Cd, Pb, REEs, U, Ti and Zr, were affected (p < 0.05) by the presence of bacteria relative to the control and exhibited slight to moderate decreases during the experiment. Adsorption onto bacterial surfaces produced fast initial removal of Al, Mn, Ba and, to a lesser degree, Cd, Pb, REE and U. Intracellular metabolic assimilation mostly affected P, Zn, and Co and progressively decreased their concentrations. Finally, coagulation as individual Fe/Al hydroxides due to DOM removal or pH change could also affect elements that were precipitated with organo-mineral colloids (Ti, Zr). The degrees of major and trace element susceptibility to bacterial activity based on concentration changes during the experiment in both substrates ranged over three orders of magnitude from mg L−1 to μg L−1 and followed the order DOC >> P >> Ba > Zn ≥ Fe ≥ Al > Mn > Cu ≥ Sr > Zr ≥ Ti > Ni ≥ Co > REEs ≥ U > Hf∼Th, which reflected the abundance of the elements in the two substrates. Generally, the soil exopolysaccharide (EPS)-producing bacterium P. aureofaciens in the peat leachate had the greatest impact of the four combinations investigated in this study (two bacteria with two substrates). Under on-going environmental changes in the boreal zone, the autochthonous processes of bacterioplankton activity are able to decrease the concentrations of a very limited number of trace elements, including mainly Fe and several macro- (P) and micro- (Zn, Mn, Ba) nutrients.
The effects of nonmetabolic bacterial cell wall adsorption and the presence of bacterial exudates on the precipitation of mineral phases from solution is not well constrained experimentally. In this study, we measured the extent of Hg(II) removal from solution, in the presence and absence of nonmetabolizing cells of Bacillus subtilis in both Cl-free and Cl-bearing systems with Hg concentrations ranging from undersaturation to supersaturation with respect to montroydite [HgO(s)]. Total Hg molalities ranged from 10−5.00 to 10−2.00 M at pH 4.50 and 7.00; the ionic strength of the experiments was kept constant using 0.01 M NaClO4, and the wet mass of bacteria was held constant at 5 g/L for each biotic experiment.
The biotic systems exhibited enhanced Hg(II) removal from solution relative to the abiotic controls in undersaturated conditions. However, thermodynamic modeling of the experimental systems strongly suggests that all of this Hg removal can be ascribed to Hg adsorption onto cell envelope functional groups. There was no evidence for enhanced Hg removal due to precipitation in bulk solutions that were undersaturated with respect to the solid phase. Under the highest total Hg concentrations studied in both the Cl-free and Cl-bearing systems, bacteria inhibit precipitation, maintaining high concentrations of Hg in solution. Cell-free, exudate-bearing control experiments suggest that aqueous complexation between Hg and the bacterially-produced exudates accounts for at least some of the precipitation inhibition. However, a comparison of total available binding sites on the exudates with the concentration of Hg in solution suggests that aqueous complexation alone can not account for the observed elevated final aqueous Hg concentrations in solution, and that the exudates likely exert a kinetic inhibition on the precipitation reaction as well.
A simple procedure for ultrasound-assisted extraction and colorimetric determination of iron in soil samples was developed. The iron concentration in the analyzed samples was determined by the colorimetric method and the results compared with inductively coupled plasma mass spectrometry (ICP-MS). Fifteen soil samples were analyzed and the iron concentration results compared with those obtained by ICP-MS using microwave-assisted sample digestion. The proposed procedure showed good efficiency for iron extraction and the results obtained by colorimetric determination exhibited good agreement with ICP-MS. Moreover, ultrasound-assisted extraction and colorimetric determination is a simple, fast and low-cost procedure for application in routine analysis.
Silver is one of the most toxic metals for aquatic biota although it is relatively rare in aqueous systems. Silver can be found both as nanoparticles (AgNPs) and ionic form (Ag+) in the natural environment. Benthic phototrophic biofilm is the first biological surface that accepts the impact of pollutants in aquatic systems. Natural biofilms are microbial aggregates on solid substrates composed of heterotrophic and phototrophic microorganisms embedded in an extracellular polymeric matrix basically formed by exopolymeric substances (EPS). The present study addresses the effect of AgNPs and AgNO3, used from 0.1 to 100 µg L-1 of Ag, on two samples of biofilms harvested in a Rotating Annular Bioreactor (RAB), a mesocosm environment simulating the natural conditions found in rivers.
Two independent experiments were carried out, one for silver nanoparticles (AgNPs) and the other for ionic silver (AgNO3), and the biofilm structure has been independently characterized for each experiment. The biofilms were initially dominated by three classes of algae: Chlorophyceae, Cyanophyceae and Diatomophyceae. These three classes were represented by: Chlamydomonas sp., Scenedesmus ecornis, Scenedesmus spinosus, Heteroleibleinia sp., Leptolyngbya margaretheana, Achnanthes exigua and Diatoma moniliformis.
The dry biomass did not decrease in the presence of Ag+ and it means that the surface of the biofilm is replacing the loss of Diatomophyceae and Cyanophyceae by Chlorophyceae. Accordingly, Ag+ was poisoning the superficial species on the biofilm, formed by diatoms and cyanobacteria, while keeping these habitats for Chlorophyceae. The toxicity of AgNPs was due to small particles (20 nm) that can pass through the cell wall and reach the plasma membrane. Besides, after adsorption, AgNPs could produce ionic Ag and reactive oxygen species that also invoke cell damages.
In addition to biofilm culture experiments, we studied single-axenic cultures of three species in traditional batch experiments: Uronema confervicolum (green alga), Nitzchia palea (diatom) and Leptolyngbya sp. (Cyanobacteria) in order to find species capable to accumulate AgNPs that can be potentially used as detoxificant under polluted conditions.
Due to the significant increase in nanoparticle production and especially that of silver nanoparticles over the past decade, the toxicity of silver in both ionic (Ag(+)) and nanoparticulate (AgNPs) form must be studied in detail in order to understand their impact on natural ecosystems. A comparative study of the effect of AgNPs and ionic silver on two independent phototrophic biofilms was conducted in a rotating annular bioreactor (RAB) operating under constant conditions. The concentration of dissolved silver in the inlet solution was progressively increased every 4 days of exposure, from 0.1 to 100 μg L(-1). In the course of the 40-day experiment, biofilm samples were collected to determine the evolution of biomass, chlorophyll-a, as well as photosynthetic and heterotrophic enzymatic activities in response to silver addition. Analysis of both dissolved and particulate silver allowed quantification of the distribution coefficient and uptake rate constants. The presence of both AgNPs and Ag(+) produced significant changes in the biofilm structure, decreasing the relative percentage of Diatomophyceae and Cyanophyceae and increasing the relative percentage of Chlorophyceae. The accumulation capacity of the phototrophic biofilm with respect to ionic silver and the corresponding distribution coefficients were an order of magnitude higher than those of the phototrophic biofilm with respect to AgNPs. Higher levels of AgNPs decreased the biomass from 8.6 ± 0.2 mg cm(-2) for 0-10 μg L(-1) AgNPs to 6.0 ± 0.1 mg cm(-2) for 100 μg L(-1) added AgNPs, whereas ionic silver did not have any toxic effect on the biofilm growth up to 100 μg L(-1) of added Ag(+). At the same time, AgNPs did not significantly affect the photosynthetic activity of the biofilm surface communities compared to Ag(+). It can thus be hypothesized that negatively charged AgNPs may travel through the biofilm water channels, thereby affecting the whole biofilm structure. In contrast, positively charged Ag(+) is bound at the cell surfaces and EPS, thus blocking its further flux within the biofilm layers. On the whole, the phototrophic biofilm demonstrated significant capacities to accumulate silver within the surface layers. The main mechanism to avoid the toxic effects is metal complexation with exopolysaccharides and accumulation within cell walls, especially pronounced under Ag(+) stress. The significant AgNPs and Ag(+) uptake capacities of phototrophic biofilm make it a highly resistant ecosystem in silver-polluted river waters.
The sub-optimal growth of phytoplankton and the resulting persistence of
unutilized plant nutrients (nitrate and phosphate) in the surface waters
of certain ocean regions has been a long-standing puzzle,. Of these
regions, the Southern Ocean seems to play the greatest role in the
global carbon cycle,, but controversy exists as to the dominant controls
on net algal production. Limitation by iron deficiency,, light
availability,, and grazing by zooplankton have been proposed. Here we
present the results from culture experiments showing that the amount of
cellular iron needed to support growth is higher under lower light
intensities, owing to a greater requirement for photosynthetic
iron-based redox proteins by low-light acclimatized algae. Moreover,
algal iron uptake varies with cell surface area, such that the growth of
small cells is favoured under iron limitation, as predicted
theoretically. Phytoplankton growth can therefore be simultaneously
limited by the availability of both iron and light. Such a co-limitation
may be experienced by phytoplankton in iron-poor regions in which the
surface mixed layer extends below the euphotic zone-as often occurs in
the Southern Ocean,-or near the bottom of the euphotic zone in more
stratified waters. By favouring the growth of smaller cells, iron/light
co-limitation should increase grazing by microzooplankton, and thus
minimize the loss of fixed carbon and nitrogen from surface waters in
settling particles,.
The attention of environmental decision makers is increasingly being focused on the movement of pollutants into ground water. Of particular importance is the transport and speciation of metals. The MINTEQA2 model is a versatile, quantitative tool for predicting the equilibrium behavior of metals in a variety of chemical environments. MINTEQA2 is a geochemical speciation model capable of computing equilibria among the dissolved, adsorbed, solid, and gas phases in an environmental setting. MINTEQA2 includes an extensive database of reliable thermodynamic data that is also accessible to PRODEFA2, an interactive program designed to be executed prior to MINTEQA2 for the purpose of creating the required MINTEQA2 input file. The report describes how to use the MINTEQA2 model. The chemical and mathematical structure of MINTEQA2 and the structure of the database files also are described. The use of both PRODEFA2 and MINTEQA2 are illustrated through the presentation of an example PRODEFA2 dialogue reproduced from interactive sessions and the presentation of MINTEQA2 output files and error diagnostics. The content and format of database files also are explained.
The oxidation of Fe II has been studied in the presence of the natural organic compounds alanine, glutamic acid, w .
Microbes are intimately involved in the iron cycle. First, acquisition of iron by microorganisms for biochemical requirements
is a key process in the iron cycle in oxygenated, circumneutral pH environments, where the solubility of Fe(III) (oxyhydr)oxides
is extremely low. Second, a number of aerobic (using O2) and anaerobic (living in the absence of O2) autotrophic bacteria gain energy for growth from the oxidation of dissolved and solid-phase Fe(II) compounds to Fe(III)
(oxyhydr)oxides. Third, heterotrophic Fe(III)-reducing bacteria close the chemical loop by reducing solid-phase Fe(III) minerals
back to dissolved and solid-phase Fe(II). Together these metabolic processes control the partitioning of the Earth's fourth
most abundant crustal element, and they are additionally tied to the cycling of several major nutrients (e.g. carbon, oxygen,
nitrogen, sulfur) and trace elements (e.g. phosphorus, nickel) in modern and ancient environments.
Iron(III) (hydr)oxides formed at extracellular biosurfaces or in the presence of exopolymeric substances of microbes and plants may significantly differ in their structural and physical properties from their inorganic counterparts. We synthesized ferrihydrite (Fh) in solutions containing acid polysaccharides [polygalacturonic acid (PGA), alginate, xanthan] and compared its properties with that of an abiotic reference by means of X-ray diffraction, transmission electron microscopy, gas adsorption (N2, CO2), X-ray absorption spectroscopy, 57Fe Mössbauer spectroscopy, and electrophoretic mobility measurements. The coprecipitates formed contained up to 37 wt% polymer. Two-line Fh was the dominant mineral phase in all precipitates. The efficacy of polymers to precipitate Fh at neutral pH was higher for polymers with more carboxyl C (PGA ∼ alginate > xanthan). Pure Fh had a specific surface area of 300 m2/g; coprecipitation of Fh with polymers reduced the detectable mineral surface area by up to 87%. Likewise, mineral micro- (<2 nm) and mesoporosity (2–10 nm) decreased by up to 85% with respect to pure Fh, indicative of a strong aggregation of Fh particles by polymers in freeze-dried state. C-1s STXM images showed the embedding of Fh particles in polymer matrices on the micrometer scale. Iron EXAFS spectroscopy revealed no significant changes in the local coordination of Fe(III) between pure Fh and Fh contained in PGA coprecipitates. 57Fe Mössbauer spectra of coprecipitates confirmed Fh as dominant mineral phase with a slightly reduced particle size and crystallinity of coprecipitate–Fh compared to pure Fh and/or a limited magnetic super-exchange between Fh particles in the coprecipitates due to magnetic dilution by the polysaccharides. The pHiep of pure Fh in 0.01 M NaClO4 was 7.1. In contrast, coprecipitates of PGA and alginate had a pHiep < 2. Considering the differences in specific surface area, porosity, and net charge between the coprecipitates and pure Fh, composites of exopolysaccharides and Fe(III) (hydr)oxides are expected to differ in their geochemical reactivity from pure Fe(III) (hydr)oxides, even if the minerals have a similar crystallinity.
Thalassiosira oceanica (Hustedt) Hasle et Heimdal (clone 1003) attained rapid rates of growth in low Fe seawater containing the siderophore ferrioxamine B (FeDFB) as the sole Fe source. Short-term rates of Fe uptake were 109 times faster than those predicted from the equilibrium concentration of inorganic Fe, suggesting that FeDFB was the substrate for the Fe transport system. An extracellular reduction step, mediated by a cell surface reductase, preceded Fe transport from FeDFB and was induced under Fe limitation. The half-saturation constant for the reduction was 0.68 μM. Iron reduction rates were two times faster than uptake rates, so that the activities of the reductase and the transporter were tightly coupled. The rates of Fe reduction of a number of Fe chelators, including synthetic organic ligands (nitrilotriacetate, diethylenetriaminepentaacetate, and EDTA) and fungal siderophores (desferrioxamine B and desferrioxamine E), were inversely proportional to the ratio of the stability constants of their Fe(III) and Fe(II) complexes and varied by a factor of two times, like the redox potentials of the Fe complexes. Platinum (II), a known inhibitor of Fe reductase activity, appeared to reduce the rates of Fe uptake from FeDFB but not from inorganic complexes. The results suggested that reoxidation of Fe(II) produced by reduction may be a necessary part of the Fe internalization reaction. Ferric reductase could be relevant to phytoplankton nutrition in the open sea where organic Fe complexes dominate the dissolved speciation and where the concentration of inorganic Fe is limiting.
Several recent studies have applied surface complexation models to quantify metal adsorption by bacterial surfaces. Although these models can account for the effects of many abiotic variables (such as pH and ionic strength), to date, the effects of biotic variables (such as growth phase) have not been investigated. In this study, we quantify the effect of growth phase on surface site concentrations, deprotonation constants, and metal-binding constants by performing acid-base titrations and Cd and Fe(III) batch adsorption experiments using suspensions containing Bacillus subtilis cultured to exponential, stationary, and sporulated phase. For each type of surface site, concentrations and pKa values describing deprotonation decrease as the cells move from exponential to stationary phase, but remain constant from stationary to sporulated phase. Due to the variations in site concentrations and deprotonation constants, Cd and Fe(III) binding constants are largest for stationary-phase cells and smallest for sporulated cells, even though cells in stationary phase adsorb roughly 5% to 10% less metal (per unit weight) than exponential-phase cells, and roughly 10% to 20% more metal than sporulated cells. These variations in surface complexation model parameters indicate that any attempt to predict proton or metal adsorption by bacteria must consider the growth phase of the population.
This text presents a new technique for detecting microorganisms, specifically bacteria found in all levels of the biosphere. It also discusses methods for enzymatic release of these microbes as well as their interactions in all ecosystems. Drawings and micrographs help to illustrate this concept. Part one is devoted to the mechanisms of adherence. The second part discusses microbial ecology and the bacterial population of tissues as well as both land and aquatic microbiocenoses in general. Part three specifically covers technique. What is known about the mechanisms of adherence justifies the choice of techniques suggested. Applications in areas such as nature, medicine, environmental hygiene and the food industry are discussed. The explanation of useful techniques, the author’s research results, and practical application methods make this volume an essential reference tool for researchers, technicians and practitioners.
We have attempted to introduce the rich diversity of the microbial world and the activities of the organisms which comprise it. Subsequent chapters will discuss the geological importance of microbial processes in greater depth. It is hoped, however, that we have been able to convey some of the extraordinary variety and abundance of microorganisms in the environment and thereby intimate the profound impact they have on geochemical processes in the biosphere. In addition, we hope that the "case studies" of recent research have introduced the important concept that we still know very little about the microbial world. What we understand currently about the diversity, abundance and activities of organisms in the environment is based on the study of a minority of types. Probably greater than 90% of microbial species are yet undiscovered, and the majority of environments await exploration. The coming years should see a rapid expansion of our understanding of the variety of microbial life and its interactions with the Earth.
The particle-laden surface layer (approx 150-370 mu m) and subsurface waters of a South San Francisco Bay salt marsh were sampled over 2 tidal cycles and analyzed for particle numbers and particulate-associated and total concentrations of Pb and bacteria. Laboratory studies examined the ability of a bacterial isolate from the surface layer and a bacterial 'film-former' to sorb Pb at environmentally significant concentrations in seawater. Degrees by which Pb concentrated in the surface layer relative to the subsurface strongly correlated with enrichments of surface layer bacteria (bacterioneuston). A significant fraction of the bacterioneuston and surface layer Pb were associated with particles. Particle-bound bacterioneuston may interact with Pb at particulate surfaces in this microenvironment.-Authors
The interactions between the polysaccharide alginate and iron(III) were investigated. The solution properties were studied through pH-metry, viscometry, zeta potential and particle size measurements. In the presence of alginate, iron(III) was stabilized and no precipitation was observed. Studies indicate that iron(III)–alginate system was more stable than iron(III) or alginate alone. The binding constant is of the order of 10 4 M −1 . A case for ‘site binding model’ for the interaction between alginate and Fe(III) has been made based on the studies using circular dichroism and zeta potential experiments. The number of binding sites per molecule of alginate has been estimated to be 66. This indicates that the alginate can bind more number of Fe(III) ions and thus provide a stable complex which can find wide industrial applications.
Iron (Fe) is a key element, strongly influencing the biogeochemistry of soils, sediments and waters, but the knowledge about the variety of Fe species present in these systems is still limited. In this work we have used X-ray absorption spectroscopy (XAS) to study the speciation of Fe in soils and waters from a boreal catchment in northern Sweden. The aim was to better understand the controls of Fe speciation across different, but adjacent landscape elements including soil, soil solution, groundwater and stream water draining catchments with contrasting land characteristics. Our results showed that all samples contained mixtures of Fe(II) and Fe(III). The soils consisted of Fe phyllosilicates, Fe (hydr)oxides and Fe complexed by natural organic matter (NOM). All aqueous samples contained Fe(II)– and Fe(III)–NOM complexes, often in combination with Fe(III) (hydr)oxides that were associated with NOM. The variation in contribution from Fe–NOM and Fe (hydr)oxides was controlled by pH and total concentrations of NOM. The XAS spectra suggested formation of mononuclear Fe–NOM complexes consisting of chelate ring structures, but it could not be determined whether they originated solely from Fe(III)– or from a mixture of Fe(II)/Fe(III)–NOM complexes. Our collective results showed that the Fe speciation was highly variable across the different landscape elements and streams. This variation was manifested both in the distribution between mononuclear Fe–NOM complexes and Fe (hydr)oxides associated with NOM and between Fe(II) and Fe(III). These results highlight the complexity of Fe speciation in natural environmental systems and thus the challenges in interpreting Fe reactivity.
Towards a better understanding of acid-base properties and metal adsorption capacities of the first primary producers on Earth, the surface chemistry of non-sulfur anoxygenic phototrophic bacteria (APB) Rhodobacter blasticus f-7 was characterized using a combination of potentiometric acid-base titration methods and electrophoretic mobility measurements as a function of pH (3 to 11) and ionic strength (0.001 to 1.0 M). Surface titrations were performed using limited residence time reactors taking into account the cell-wall bound Ca and Mg from the culture media for net proton balance calculations. Electrophoretic mobilities of live APB cells were investigated in 0.001-0.5 M NaCl at pH of 1 to 11 and different Zn, Cd and Pb concentrations. Adsorption of Zn, Cd, Pb, Cu, Co, Ni, Sr, Al, Ga, Ge, Mo, and W was studied at 25 degrees C in 0.01 M NaNO3 as a function of pH and metal concentration in batch reactors. A competitive Langmuir sorption isotherm in conjunction with a linear programming optimization method (LPM) was used to fit experimental data and assess the number and nature (carboxylate, phosphoryl/phosphodiester and amine) of surface sites and adsorption reaction constants involved in the binding of trace metals to the Rhodobacter blasticus f-7 surface. We found that the total H/OH binding site number (60-120 mu mol/g(wet)) for APB is comparable to that of cyanobacteria studied previously by the same technique (50-200 mu mol/g(wet)). Similarly, LPM adsorption parameters for Zn, Cu, Pb and Cd for APB are in close agreement with those observed for the cyanobacteria. As such, results of the present study indicate similar affinity of both bacteria cells surfaces to divalent metal micronutrients, most likely due to the dominance of carboxylate and phosphorylate binding at rather high metal loading.
Secondary iron-oxides forming by the oxidation of ferrous ions have been observed to accumulate on the surface of bacterial cells in various environments (e.g. lake sediments). It is not always clear to which extent the presence of the bacteria can affect the precipitation process. The cells may play an active (metabolic) role in the oxidation of the ferrous ions. Whether this is the case or not, it does not change the fact that bacterial surfaces can also sorb ferrous/ferric ions and crystal nuclei and therefore influence the properties of the Fe-rich crystals. In order to determine the relative roles of active and passive processes, an important step is to quantify the interactions between ferrous ions and bacterial surfaces. We have measured the kinetics of adsorption and the pH-adsorption/desorption isotherms of ferrous ions on the surface of Bacillus subtilis cells under anaerobic conditions, using various sorbent/sorbate ratios. In conjunction with pH-titrations, this allowed us to estimate the adsorption constants of the ferrous ions onto the various chemical sites that are present on the surface of the cells (e.g. -log(K) ~3.45 for the carboxylic groups). We find that the adsorption of the ferrous ions is a quick but reversible process, which can be modeled by statistical thermodynamics. We will describe our results and discuss their consequences in the example of a simple experience of biomineralization realized in the laboratory with Bacillus subtilis cells.
Aqueous metal cation adsorption onto bacterial surfaces can be successfully modeled by means of a surface complexation approach. However, relatively few stability constants for metal-bacterial surface complexes have been measured. In order to determine the bacterial adsorption behavior of cations that have not been studied in the laboratory, predictive techniques are required that enable estimation of the stability constants of bacterial surface complexes. In this study, we use a linear free-energy approach to compare previously measured stability constants for Bacillus subtilis metal-carboxyl surface complexes with aqueous metal-organic acid anion stability constants. The organic acids that we consider are acetic, oxalic, citric, and tiron. We add to this limited data set by conducting metal adsorption experiments onto Bacillus subtilis, determining bacterial surface stability constants for Co, Nd, Ni, Sr, and Zn.
Bacteria and dissolved humic substances are capable of binding significant concentrations of metals in natural environments. Recent advances in understanding bacteria-metal and humic-metal complexation have provided a framework for directly comparing the binding capacities of these components. In this study, we use chemical equilibrium modeling to construct an internally consistent set of thermodynamic equilibrium constants for proton and Cd binding onto dissolved humic substances, using a variety of published data sets. Our modeling approach allows for the direct comparison of humic substance binding constants and site densities to those previously published for proton and Cd binding onto natural consortia of bacteria. We then combine these constants into a unified model that accounts for the competition between bacterial surfaces and humic and fulvic acids in order to determine the relative importance of each component on the total Cd budget. The combined model is used to examine the relative contributions of bacteria and dissolved humic substances to Cd complexation in natural settings. Calculations are performed for three representative systems: (1) one with a maximum realistic concentration of bacteria and a minimum realistic concentration of humic substance, (2) one with a maximum realistic concentration of humic substance and a minimum concentration of bacteria, and (3) one with an intermediate concentration of both components.
This work reports on a concerted study of diatom-water interfaces for two marine planktonic (Thalassiosira weissflogii= TW, Skeletonema costatum= SC) and two freshwater periphytic species (Achnanthidium minutissimum= AMIN, Navicula minima= NMIN). Proton surface adsorption was measured at 25°C, pH of 3 to 11 and ionic strength of 0.001 to 1.0 M via potentiometric titration using a limited residence time reactor. Electrophoretic mobility of living cells and their frustules was measured as a function of pH and ionic strength. Information on the chemical composition and molecular structure of diatoms surfaces was obtained using FT-IR (in situ attenuated total reflectance) and X-ray Photoelectron Spectroscopy (XPS). The surface area of living cells and their frustules in aqueous solutions was quantified using Small Angle X-ray Scattering Spectroscopy (SAXS).
The importance of extracellular polysaccharide (EPS) on proton and Cd binding was examined by comparing the adsorption behaviors of 4 bacterial species (Pseudomonas putida, Shewenella oneidensis, Rhizobium tropici, and Agrobacterium sp. [ATCC# 21680]) with intact capsular EPS to corresponding adsorption behaviors with the EPS enzymatically removed from the biomass. Potentiometric titrations were conducted to detect any differences in proton binding of the biomass with and without the presence of EPS. Enzymatic removal of the EPS from each of the bacterial species in this study resulted in no significant differences in biomass proton binding behavior. Batch Cd adsorption experiments also showed no significant differences in the adsorption capacities between the EPS and EPS-free systems for all 4 species of bacteria. Our results suggest that EPS contains proton-active functional groups that are similar to those on the cell wall, and that, on a mass-normalized basis, EPS and bacterial cell walls exhibit similar site concentrations and affinities for adsorbing protons and Cd from solution. Because EPS exhibits similar Cd and proton binding properties to bacterial cell walls, and because of the similarity in binding properties between species, it may be possible to model metal and proton binding to biofilms in general using a single set of stability constants. This general modeling approach would obviate the impossible task of determining binding constants for protons and each metal of interest with each EPS component and each bacterial cell wall of interest.
Introduction of exogenous biocolloids such as genetically engineered bacteria in a bioremediation operation can enhance the transport of contaminants in groundwater by reducing the retardation effects. Because of their colloidal size and favorable surface conditions, bacteria are efficient contaminant carriers. In cases where contaminants have a low mobility in porous media because of their high partition with solid matrix, facilitated contaminant transport by mobile bacteria can create high contaminant fluxes. When metabolically active mobile bacteria are present in a subsurface environment, the system can be treated as consisting of three phases: water phase, bacterial phase, and stationary solid matrix phase. In this work a mathematical model based on mass balance equations is developed to describe the facilitated transport and fate of a contaminant and bacteria in a porous medium. Bacterial partition between the bulk solution and the stationary solid matrix and contaminant partition among three phases are represented by expressions in terms of measurable quantities. Solutions were obtained to provide estimates of contaminant and bacterial concentrations. A dimensional analysis of the transport model was utilized to estimate model parameters from the experimental data and to assess the effect of several parameters on model behavior. The model results matched favorably with experimental data of Jenkins and Lion (1993). The presence of mobile bacteria enhances the contaminant transport. However, bacterial consumption of the contaminant, which serves as a bacterial nutrient, can attenuate the contaminant mobility. The work presented in this paper is the first three-phase model to include the effects of substrate metabolism on the fate of groundwater contaminants.
This study assesses the potential use of Mg isotopes to trace Mg carbonate precipitation in natural waters. Salda Lake (SW Turkey) was chosen for this study because it is one of the few modern environments where hydrous Mg carbonates are the dominant precipitating minerals. Stromatolites, consisting mainly of hydromagnesite, are abundant in this lake. The Mg isotope composition of incoming streams, groundwaters, lake waters, stromatolites, and hydromagnesite-rich sediments were measured. Because Salda Lake is located in a closed basin, mass balance requires that the Mg isotopic offset between Lake Salda water and precipitated hydromagnesite be comparable to the corresponding offset between Salda Lake and its water inputs. This is consistent with observations; a δ26Mg offset of 0.8–1.4 ‰ is observed between Salda Lake water and it is the incoming streams and groundwaters, and precipitated hydromagnesite has a δ26Mg 0.9–1.1 ‰ more negative than its corresponding fluid phase. This isotopic offset also matches closely that measured in the laboratory during both biotic and abiotic hydrous Mg carbonate precipitation by cyanobacteria (Mavromatis, V., Pearce, C., Shirokova, L. S., Bundeleva, I. A., Pokrovsky, O. S., Benezeth, P. and Oelkers, E.H.: Magnesium isotope fractionation during inorganic and cyanobacteria-induced hydrous magnesium carbonate precipitation, Geochim. Cosmochim. Acta, 2012a. 76, 161–174). Batch reactor experiments performed in the presence of Salda Lake cyanobacteria and stromatolites resulted in the precipitation of dypingite (Mg5(CO3)4(OH)2·5(H2O)) and hydromagnesite (Mg5(CO3)4(OH)2·4H2O) with morphological features similar to those of natural samples. Concurrent abiotic control experiments did not exhibit carbonate precipitation demonstrating the critical role of cyanobacteria in the precipitation process.
An ion selective electrode was used to monitor binding of Cd2+ on two bacteria, Bacillus subtilis (Gram+) and Escherichia coli (Gram−), as a function of increasing pH. A competitive Langmuir sorption isotherm was used in conjunction with a linear programming method (LPM) or FITEQL to fit experimental data. Results obtained with simulated data showed that LPM is less sensitive than FITEQL to variations in sorption data. Application of the LPM to experimental data found three discrete metal binding sites on B. subtilis and E. coli with −log equilibrium constant (pKS) values of −0.80±0.20, 0.63±0.09, and 2.35±0.10, and −0.60±0.10, 0.25±0.19, and 1.93±0.17, respectively, at a constant ionic strength, I=0.1 M (KNO3). The corresponding site densities were 0.09±0.01, 0.07±0.01, and 0.07±0.01, and 0.01±0.002, 0.02±0.01, and 0.04±0.01 μmol of Cd2+/mg of B. subtilis or E. coli. From FITEQL, pKS values of −1.18±0.15, 0.40±0.11, and 2.31±0.32 for B. subtilis and −1.46±0.34, 0.20±0.12, and 1.87±0.12 for E. coli were recovered with site densities of 0.10±0.07, 0.07±0.06, and 0.06±0.02, and 0.02±0.005, 0.02±0.004, and 0.04±0.04 μmol of Cd2+/mg of B. subtilis or E. coli, respectively. Total site densities of 0.22±0.02 and 0.06±0.01 μmol/mg were obtained by LPM for B. subtilis and E. coli, whereas FITEQL yielded values of 0.23±0.02 and 0.08±0.07 μmol/mg. Both LPM and FITEQL produced feasible results, but LPM was less sensitive to error and did not require an a priori assumption of the number of binding sites.
The exchange mechanism which determines the retention capacity of alginic acid has been investigated. Chromatographic, pH, and viscosity measurements, performed with several metal ions, have permitted us to show that ion exchange is not the only mechanism but that the influence of the two vicinal hydroxyl groups on the retention capacity of alginic acid is also important.
Trace elements in seawater can be limiting factors of biological productivity, tracers of ocean circulation and biogeochemical processes, and proxies for paleoceanography. The global status of trace elements and their isotopes (TEIs) in the ocean is being explored this decade through an international study of the global marine biogeochemical cycles of TEIs (GEOTRACES). Such an international study has become possible due to recent methodological developments in sampling, preconcentration, and measurement of TEIs. Here, we present an overview of recent methodological developments and initial GEOTRACES intercalibration activities for obtaining data about TEIs that are accurate, precise, and intercomparable.
In coastal waters, where conditions are variable and can change rapidly, the kinetics of iron complexation by NOM is a major factor affecting its speciation. In this study we have examined the kinetics of complexation of ferrous and ferric iron by terrigenous NOM in terms of formation rate constants and dissociation rate constants by employing competitive ligand methods with visible spectrophotometry for determination of the complexed iron. Rate constants for organic ferrous complex formation ranged from 500 to 7.5×104 M−1 s−1, and rate constants for complex dissociation ranged from 1×10−6 to 3.6×10−3 s−1. Formation rate constants for organic ferric complexes were comparable to those determined for strong iron binding ligands found in the open oceans, and from 2.1×105 to 9.6×107 M−1 s−1. Ferric complex dissociation data were fitted by a two ligand model, with rate constants for both ligand classes typically higher than those for the strong ligands in the open ocean. Rate constants varied from 2×10−4 to 4×10−3 s−1 for the ‘weak’ ligand class, and from 1.0×10−6 to 1.3×10−4 s−1 for the ‘strong’ class. All rate constants varied by several orders of magnitude between NOM samples of different origin, reflecting the highly variable composition of these substances. Calculated conditional stability constants for organic complexes with ferric species in seawater, Fe′, were generally similar to those measured in the open oceans. Conditional stability constants for the ferrous complexes were less than those for the ferric complexes by two to four orders of magnitude. There was a weak positive correlation between the conditional stability constants for ferrous and ferric complexes, suggesting that similar functional groups are involved in binding each of the two forms of iron. One of the samples of NOM had a conditional stability constant with Fe′ that was comparable with those for siderophores and similar strong iron binding compounds. These results suggest that in the absence of oxidants, complexes between ferrous iron and NOM may be relatively long-lived. Our work also suggests that organic complexes between iron and terrigenous NOM may be quite strong, and will have a major effect on iron solubility in coastal waters.
The hydrous magnesium carbonates, nesquehonite (MgCO3·3H2O) and dypingite (Mg5(CO3)4(OH)2·5(H2O)), were precipitated at 25°C in batch reactors from aqueous solutions containing 0.05M NaHCO3 and 0.025M MgCl2 and in the presence and absence of live photosynthesizing Gloeocapsa sp. cyanobacteria. Experiments were performed under a variety of conditions; the reactive fluid/bacteria/mineral suspensions were continuously stirred, and/or air bubbled in most experiments, and exposed to various durations of light exposure. Bulk precipitation rates are not affected by the presence of bacteria although the solution pH and the degree of fluid supersaturation with respect to magnesium carbonates increase due to photosynthesis. Lighter Mg isotopes are preferentially incorporated into the precipitated solids in all experiments. Mg isotope fractionation between the mineral and fluid in the abiotic experiments is identical, within uncertainty, to that measured in cyanobacteria-bearing experiments; measured δ26Mg ranges from −1.54‰ to −1.16‰ in all experiments. Mg isotope fractionation is also found to be independent of reactive solution pH and Mg, CO32−, and biomass concentrations. Taken together, these observations suggest that Gloeocapsa sp. cyanobacterium does not appreciably affect magnesium isotope fractionation between aqueous fluid and hydrous magnesium carbonates.
Iron oxyhydroxide, especially in its so-called "amorphous" form, plays a key role in the retention and migration of organic and inorganic compounds in soils and aquatic media. The local structure of these "amorphous" species can be directly investigated using synchrotron-based X-ray absorption spectroscopy. In order to study the nucleation mechanisms, FeCl3 solutions were hydrolyzed by NaOH and the precursors obtained at different molar ratios (0 ≤ r = (NaOH)/Fe ≤ 2.7) were studied by EXAFS. For r ≥ 1.5, Fe polymers formed at equilibrium are hexacoordinated and their local structure is the same as β-FeOOH. For r = 1.5, the spectra obtained at different aging times show that the starting nuclei are dimers with edge-sharing octahedra. From t ∼ 50 min, trimers with edge and corner-sharing octahedra are detected in solution. After 1 h, β-FeOOH-like polycations, formed by the coalescence of the trimers, can be observed. These polymers are extremely stable because Cl- ions are still incorporated in the structure and are easily displaced by OH-.
Topsoils are often contaminated by trace metals, and it is important to understand how different processes govern the transport
of such metals to fresh and marine waters. This paper presents measurements of natural nanoparticles and colloidal organic
matter in soil and river samples from Germany and Sweden. In our analytical approach, a nanoparticle separation technique
is combined with multielement detection and applied to soil and river samples to link the macroscale field observations with
detailed molecular studies in the laboratory. It was determined that lead is associated with iron oxide colloids, which are
ubiquitous nanoparticles that can be efficiently transported. Eventually both iron oxides and lead are removed by flocculation
under conditions of estuarine mixing. Iron-rich nanoparticles compete efficiently with natural organic matter (NOM) complexation
for lead binding in both the soil and river systems studied.
Total and labile Fe measurements, and Fe³⁺ titrations were carried out both at sea and in the laboratory with adsorptive cathodic stripping voltammetry (CSV) methods using 1-nitroso-2-naphthol (1N2N) as a complexing ligand to study Fe(III) speciation and the kinetic interaction of Fe³⁺ with naturally occurring organic ligands. On the continental slope and at the shelf/slope front of the Northwest Atlantic ocean, the total dissolved (< 0.4 μm) Fe was predominantly 1N2N nonlabile, with 60% nonlabile at the mouth of Delaware Bay. The exact chemical speciation of this nonlabile Fe is not known; although some of this Fe is likely in strong organic complexes with a KFeL1023.22 as determined by a competitive ligand equilibration/cathodic stripping voltammetry (CLE/CSV) method.
The particle-laden surface layer ( similar to 150-370 mu m) and subsurface waters of a South San Francisco Bay salt marsh were sampled over two tidal cycles and analyzed for particle numbers and particulate-associated and total concentrations of lead and bacteria. Laboratory studies examined the ability of a bacterial isolate from the surface layer and a bacterial "film-former" to sorb Pb at environmentally significant concentrations in seawater. A significant fraction of the bacterioneuston and surface layer Pb were associated with particles. Data suggest that particle-bound bacterioneuston may interact with Pb at particulate surfaces in this microenvironment.
We have used room temperature and cryogenic 57Fe Mössbauer spectroscopy, powder X-ray diffraction (pXRD), mineral magnetometry, and transmission electron microscopy (TEM), to study the synthetic precipitation of hydrous ferric oxides (HFOs) prepared either in the absence (abiotic, a-HFO) or presence (biotic, b-HFO) of nonmetabolizing bacterial cells (Bacillus subtilis or Bacillus licheniformis, ∼108 cells/mL) and under otherwise identical chemical conditions, starting from Fe(II) (10−2, 10−3, or 10−4 mol/L) under open oxic conditions and at different pH (6–9). We have also performed the first Mössbauer spectroscopy measurements of bacterial cell wall (Bacillus subtilis) surface complexed Fe, where Fe(III) (10−3.5–10−4.5 mol/L) was added to a fixed concentration of cells (∼108 cells/mL) under open oxic conditions and at various pH (2.5–4.3). We find that non-metabolic bacterial cell wall surface complexation of Fe is not passive in that it affects Fe speciation in at least two ways: (1) it can reduce Fe(III) to sorbed-Fe2+ by a proposed steric and charge transfer effect and (2) it stabilizes Fe(II) as sorbed-Fe2+ against ambient oxidation. The cell wall sorption of Fe occurs in a manner that is not compatible with incorporation into the HFO structure (different coordination environment and stabilization of the ferrous state) and the cell wall-sorbed Fe is not chemically bonded to the HFO particle when they coexist (the sorbed Fe is not magnetically polarized by the HFO particle in its magnetically ordered state). This invalidates the concept that sorption is the first step in a heterogeneous nucleation of HFO onto bacterial cell walls. Both the a-HFOs and the b-HFOs are predominantly varieties of ferrihydrite (Fh), often containing admixtures of nanophase lepidocrocite (nLp), yet they show significant abiotic/biotic differences: Biotic Fh has less intraparticle (including surface region) atomic order (Mössbauer quadrupole splitting), smaller primary particle size (magnetometry blocking temperature), weaker Fe to particle bond strength (Mössbauer center shift), and no six-line Fh (6L-Fh) admixture (pXRD, magnetometry). Contrary to current belief, we find that 6L-Fh appears to be precipitated directly, under a-HFO conditions, from either Fe(II) or Fe(III), and depending on Fe concentration and pH, whereas the presence of bacteria disables all such 6L-Fh precipitation and produces two-line Fh (2L-Fh)-like biotic coprecipitates. Given the nature of the differences between a-HFO and b-HFO and their synthesis condition dependences, several biotic precipitation mechanisms (template effect, near-cell environment effect, catalyzed nucleation and/or growth effect, and substrate-based coprecipitation) are ruled out. The prevailing present view of a template or heterogeneous nucleation barrier reduction effect, in particular, is shown not to be the cause of the large observed biotic effects on the resulting HFOs. The only proposed mechanism (relevant to Fh) that is consistent with all our observations is coprecipitation with and possible surface poisoning by ancillary bacteriagenic compounds. That bacterial cell wall functional groups are redox active and the characteristics of biotic (i.e., natural) HFOs compared to those of abiotic (i.e., synthetic) HFOs have several possible biogeochemical implications regarding Fe cycling, in the photic zones of water columns in particular.
P K-edge EXAFS spectroscopy has been used to determine the local environment of phosphorus during the hydrolysis of FeCl3 in the presence of phosphate. Measurements were performed on liquid samples and in the fluorescence mode. With the detection geometry adopted during experiments, the self absorption of fluorescence has been quantified and does not appear to be an important phenomenon. Thus no correction was made. In order to clearly identify the neighboring atoms around P, a multiple scattering approach has been used. Multiple scattering seems to be an important phenomenon in PO4/FeCl3 clusters. P K-edge EXAFS data show that even for very acidic solutions, pH < 1, all the phosphate ions are complexed to Fe. For a P/Fe molar ratio of 0.2 one phosphate progressively bonds one, two, and three irons when n (=[OH]/[Fe]) increases from 0 to 2.0. At n = 2, one phosphate bridges three iron dimers and two kinds of PO4−Fe linkages are detected. For P/Fe = 0.5, the number of irons linked to PO4 increases when n increases but does not exceed 2. Thus P K-edge EXAFS spectroscopy combined with a multiple scattering approach allowed us to confirm previous results obtained by EXAFS at the Fe K-edge as well as to describe more precisely the type of linkage between the PO4 tetrahedron and the Fe octahedra.
The structure of fresh sols of partially hydrolyzed ferric nitrate salt were investigated by dynamic light scattering and small-angle X-ray scattering. The size of the colloids formed vs the hydrolysis ratio n = (NaOH)/(Fe) varied with the age of the solutions. Just after the end of the mixing of NaOH with Fe-(NO3)3.9H2O (t < 10 min) the size distribution is bimodal in n = 2, n = 2.2, and n = 2.5 with size particles in the range 40-500 nm. At t > 10 min, the size distribution is monomodal without further evolution with time. The sizes are close to 10-20 nm. Small-angle X-ray scattering curves yield two results: (1) the colloids are linear at the semilocal range order when n = 1.5 or 2.0, but with n = 2.2 or 2.5, they are semilinear or with a structure of fractal aggregates formed by small subunits; (2) the size of the subunits varies from 7 angstrom (n = 1.5) up to 13.5 angstrom (n = 2.5). This size variation with n is contrary to observations when using ferric chloride salts. NO3- ions are rapidly exchanged for OH- or O- ligands in a homogeneous nucleation process.
Bacteria are a widespread, abundant, geochemically reactive component of aquatic environments. However, their role in the formation of secondary reactive surface phases such as iron oxides or in the direct sorption of metal contaminants has yet to be quantitatively described. Here, we compare the formation of iron oxides on bacterial cell surfaces to their formation abiotically (no bacteria present) over a range of both Fe(III) concentration (10-2−10-4.5 M) and pH (2−4.5) in the laboratory. Iron sorption and subsequent precipitation reactions at bacterial surfaces were modeled using current geochemical approaches. Solid-phase partitioning of Fe(III) as hydrous ferric oxide (HFO) was enhanced in the presence of a variety of bacteria over that seen in abiotic controls. The onset of HFO formation occurred at lower pH values and in greater quantities at any given pH in the bacterial treatments. Fe(III) reactions at bacterial surfaces follow a clear continuum between sorption and precipitation that can be quantitatively described using geochemical principles and modeled using surface precipitation theory; to date only demonstrated for inorganic surfaces. These results show that the reactions at biological surfaces are likely to be important in determining the spatial distribution of iron oxides in nature and thus the reactive transport of metals in aqueous environments.
Fungal cell walls possess strong complexing properties, which make them valuable biosorbents to remove heavy metals from wastewaters. The binding mechanisms of Zn and Pb to Penicillium chrysogenum cell walls have been studied by solution chemistry and extended X-ray absorption fine structure (EXAFS) spectroscopy as a function of the complexation rate. It is shown that Zn and Pb bind to the predominant phosphoryl (≈95%) and minor carboxyl groups (≈5%) with a reversed affinity. Zn is predominantly complexed to four PO4 groups in a tetrahedral configuration at low (7.6 × 10-3 mmol/g) to high (0.15 mmol/g) Zn concentration and additionally to COOH groups at total saturation of reactive sites (0.22 mmol/g). In contrast, carboxyl complexes of Pb ((COO)n−Pb) are formed at low Pb concentration (5.6 10-3 mmol/g), and their formation is followed by (PO4)n−Pb complexes at higher complexation rate. The difference in complexation affinity by reactive PO4 and COOH groups observed by EXAFS provides a molecular level explanation for the differences in Pb and Zn isotherms. The Pb isotherm exhibits two plateaus, which correspond to the successive saturation of COOH and PO4 sites, whereas the Zn isotherm has a single-site Langmuir shape because low affinity minor (COO)n−Zn complexes formed at high metal concentration are masked by more abundant (PO4)4−Zn complexes, which readily form.
J.L. GEDDIE AND I.W. SUTHERLAND. 1993. The binding of cations by a range of bacterial polysaccharides was examined. Comparison of native and deacetylated polymers indicated the influence of polysaccharide acetylation on ion uptake and selectivity. The effects of temperature and pH on ion uptake were also examined. Metal ion uptake was carried out by dialysis and samples were analysed using ion chromatography. The native acetylated polymers showed a selectivity for Ca2+ > Mg2+ > monovalent cations, whereas samples lacking acetyl groups showed a selectivity for monovalent cations > Mg2+ > Ca2+. Increased temperatures reduced the capacity for several of the polymers to bind the cations; The Zoogloea ramigera polymer appeared least affected. The pH value also affected uptake.
The flocs formed by coagulation of the NOM contained in two natural waters by iron chloride were studied by X-ray absorption spectroscopy (XANES and EXAFS) and small-angle X-ray scattering (SAXS). The Fe K-edge XANES data clearly showed that the Fe within the flocs is octahedrally coordinated and that its oxidation state is III. The EXAFS results revealed that the NOM is strongly complexed with Fe(III), thus limiting the Fe hydrolysis to the oligomeric stage (typically trimers). The Fe speciation determined from SAXS data confirmed these findings: single-corner-sharing trimers are the predominant Fe species within the flocs. The pH and the nature of the NOM did not influence the Fe speciation or the level of complexation, but played a major role in the structuring of the flocs.
Numerous studies have utilized surface complexation theory to model proton adsorption behaviour onto mesophilic bacteria. However, few experiments, to date, have investigated the effects of pH and ionic strength on proton interactions with thermophilic bacteria. In this study, we characterize proton adsorption by the thermophile Anoxybacillus flavithermus by performing acid–base titrations and electrophoretic mobility measurements in NaNO3 (0.001–0.1 M). Equilibrium thermodynamics (Donnan model) were applied to describe the specific chemical reactions that occur at the water–bacteria interface. Acid–base titrations were used to determine deprotonation constants and site concentrations for the important cell wall functional groups, while electrophoretic mobility data were used to further constrain the model. We observe that with increasing pH and ionic strength, the buffering capacity increases and the electrophoretic mobility decreases. We develop a single surface complexation model to describe proton interactions with the cells, both as a function of pH and ionic strength. Based on the model, the acid–base properties of the cell wall of A. flavithermus can best be characterized by invoking three distinct types of cell wall functional groups, with pKa values of 4.94, 6.85, and 7.85, and site concentrations of 5.33, 1.79, and 1.42 × 10−4 moles per gram of dry bacteria, respectively. A. flavithermus imparts less buffering capacity than pure mesophilic bacteria studied to date because the thermophile possesses a lower total site density (8.54 × 10−4 moles per dry gram bacteria).
The purpose of the present work is to extend our knowledge of metal–cyanobacteria interactions and to contribute to the database on adsorption parameters of aquatic microorganisms with respect to metal pollutants. To this end, the surface properties of the cyanobacteria (Gloeocapsa sp. f-6gl) were studied using potentiometric acid–base titration methods and ATR-FTIR (attenuated total reflection infrared) spectroscopy. The electrophoretic mobility of viable cells was measured as a function of pH and ionic strength (0.01 and 0.1 M). Surface titrations at 0.01–1.0 M NaCl were performed using limited residence time reactors (discontinuous titration) with analysis of Ca, Mg and dissolved organic C for each titration point in order to account for alkali-earth metal–proton exchange and cell degradation, respectively. Results demonstrate that the cell-wall bound Ca and Mg from the culture media contribute to the total proton uptake via surface ion-exchange reactions. This has been explicitly taken into account for net proton balance calculations. Adsorption of Zn, Cd, Pb and Cu was studied at 25 °C in 0.01 M NaNO3 as a function of pH and metal concentration. The proportion of adsorbed metal increases as a function of culture age with cells of 44 days old having the largest adsorption capacities. A competitive Langmuir sorption isotherm in conjunction with a linear programming method (LPM) was used to fit experimental data and assess the number of surface sites and adsorption reaction constants involved in the binding of metals to the cyanobacteria surface. These observations allowed the determination of the identity and concentration of the major surface functional groups (carboxylate, amine, phosphoryl/phosphodiester and hydroxyl) responsible for the amphoteric behavior of cyanobacterial cell surfaces in aqueous solutions and for metal adsorption. Results of this work should allow better optimizing of metal bioremediation/biosequestration processes as they help to define the most efficient range of pH, cell biomass and duration of exposure necessary for controlled metal adsorption on cyanobacteria cultures. It follows from comparison of adsorption model parameters between different bacteria that technological application of cyanobacteria in wastewater bioremediation can be as efficient as other biological sorbents.
Iron cycling in the Earth's crust depends on redox reactions, which often trigger the precipitation and dissolution of Fe-rich minerals. Microbial activity is also an integral part of iron cycling, through carbon fixation, respiration and passive sorption reactions. Iron oxides formed in close association with bacteria (either as internal or external precipitates) are referred to as biogenic minerals. They form in several types of environments on Earth, from freshwater to marine systems, aquifers, soils and mining impacted systems. Biogenic iron oxides generally occur as nanocrystals and show a wide range of morphology and mineralogy. These minerals form as a result of the direct metabolic activity of bacteria or as a result of passive sorption and nucleation reactions. The metabolic activity of acidophilic and neutrophilic iron-oxidizing bacteria under oxic conditions promotes the oxidation of Fe(II) to Fe(III) and the precipitation of biogenic iron oxides as extracellular precipitates near or on the bacterial cells. Iron oxidation under anoxic conditions can also occur, as a result of the activity of nitrate-reducers and photoautotrophic bacteria using Fe(II) as an electron donor. Secondary Fe-oxide formation has been reported during the microbial reduction of iron oxides. Passive Fe sorption and nucleation onto bacterial cell walls represents another important mechanism leading to iron oxide formation. The surface reactivity of the bacterial surface under environmental pH conditions confers a net negative charge to the cell wall, which leads to the binding of soluble iron and eventually to the precipitation of iron oxides under saturation conditions. Extracellular polymers produced by bacteria can act as a template for iron sorption and Fe-oxide nucleation. Intracellular iron oxide formation has been observed in natural environments. Magnetotactic bacteria produce intracellular magnetosomes, occurring as chains of magnetite crystals within the cells, and an unidentified iron-rich mineral phase forms inside Shewanella cells during the anaerobic reduction of ferrihydrite. Several studies have clearly shown that biogenic iron oxides form in present-day environments, but they might also be important components of ancient geological formations, such as banded-iron formations (BIF). BIF formation is still being debated, but there is now strong evidence that bacteria, more specifically, phototrophic iron oxidizers and possibly iron reducers might have been involved. Biogenic iron oxides represent a potential tool in the search for past and present life on Earth and other planetary systems. Despite the promising use of Fe-isotopes and magnetosomes, there is still no clear proof that they can form only as a result of biological activity. In fact, Fe isotope fractionation of abiotic iron oxides is often similar to that of biogenic oxides and the specific mineralogical characteristics of magnetite crystals present inside magnetotactic bacteria can be reproduced under abiotic conditions. In summary, the role of bacteria in iron cycling has been the focus of several studies in the last few decades, but clearly, more research is needed in order to fully assess the role of microorganisms in their formation.
The effect of cell wall-associated extracellular polymeric substances (EPS) of the Gram-negative bacterium Shewanella oneidensis strain MR-1 on proton, Zn(II), and Pb(II) adsorption was investigated using a combination of titration/batch uptake studies, surface complexation modeling, attenuated total reflectance – Fourier transform infrared (ATR-FTIR) spectroscopy, and Zn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. Both unmodified (wild-type (WT) strain) and genetically modified cells with inhibited production of EPS (ΔEPS strain) were used. Three major types of functional groups (carboxyl, phosphoryl, and amide groups) were identified in both strains using ATR-FITR spectroscopy. Potentiometric titration data were fit using a constant capacitance model (FITEQL) that included these three functional groups. The fit results indicate less interaction of Zn(II) and Pb(II) with carboxyl and amide groups and a greater interaction with phosphoryl groups in the ΔEPS strain than in the WT strain. Results from Zn(II) and Pb(II) batch adsorption studies and surface complexation modeling, assuming carboxyl and phosphoryl functional groups, also indicate significantly lower Zn(II) and Pb(II) uptake and binding affinities for the ΔEPS strain. Results from Zn K-edge EXAFS spectroscopy show that Zn(II) bonds to phosphoryl and carboxyl ligands in both strains. Based on batch uptake and modeling results and EXAFS spectral analysis, we conclude that the greater amount of EPS in the WT strain enhances Zn(II) and Pb(II) uptake and hinders diffusion of Zn(II) to the cell walls relative to the ΔEPS strain.
The influence of aqueous silica on the hydrolysis of iron(III) nitrate and chloride salts in dilute aqueous solutions (mFe ∼ 0.01 mol/kg) was studied at ambient temperature using X-ray absorption fine structure (XAFS) spectroscopy at the Fe K-edge. Results show that in Si-free iron nitrate and chloride solutions at acid pH (pH < 2.5), Fe is hexa-coordinated with 6 oxygens of H2O- and/or OH-groups in the first coordination sphere of the metal, at an Fe-O distance of 2.00 ± 0.01 Å. With increasing pH (2.7 < pH < 13), these groups are rapidly replaced by bridging hydroxyls (-OH-) or oxygens (-O-), and polymerized Fe hydroxide complexes form via Fe-(O/OH)-Fe bonds. In these polymers, the first atomic shell of iron represents a distorted octahedron with six O/OH groups and Fe-O distances ranging from 1.92 to 2.07 Å. The Fe octahedra are linked together by their edges (Fe-Fe distance 2.92–3.12 Å) and corners (Fe-Fe distance ∼3.47 ± 0.03 Å). The Fe-Fe coordination numbers (Nedge = 1–2; Ncorner = 0.5–0.7) are consistent with the dominant presence of iron dimers, trimers and tetramers at pH 2.5 to 2.9, and of higher-polymerized species at pH > 3.
Adsorption of Fe to bacterial surfaces is known to occur and is a possible precursor to biomineralization of Fe-oxides. However, the extent of adsorption and the resulting effect on saturation conditions in the presence of bacteria have not been adequately explored. We conduct four types of experiments: (1) kinetic precipitation-type experiments with and without bacteria at starting Fe concentrations of 2.5 and 5.0 ppm at a constant of pH of 3; (2) batch adsorption experiments as a function of bacteria concentration and at a total Fe concentration of 2.5 ppm; (3) batch adsorption/desorption experiments at low pH (<4) with 1 ppm Fe and 2 g/L bacteria; (4) abiotic precipitation experiments at low pH (<4) and 1 ppm total Fe.The results of the experiments conducted in oversaturated conditions demonstrate that bacteria significantly enhance the rate and extent of Fe removal from solution relative to the bacteria-free controls. We use the results of the separate batch adsorption experiments to determine stability constants for Fe-bacteria surface complexes assuming zero precipitation at these low Fe, low pH conditions. The observed increase in Fe removed in the presence of bacteria in ‘oversaturated’ conditions can be accounted for by adsorption only, using a surface model based on the reaction:Fe3++R>L1−↔R>L1Fe2+ log K=6.1±0.4where R>L1− represents the deprotonated form of the bacterial surface functional group with a pKa value of 3.3. The stability of the Fe–bacteria sorption reaction is orders of magnitude stronger than that observed for the other metal–bacteria systems, emphasizing the importance of Fe(III) adsorption in bacteria-bearing systems. Some Fe removal by the bacteria was irreversible on the time scale of our experiments, indicating that, although adsorption has the potential to account for the observed Fe removal, the process is likely more complicated than simple metal adsorption onto bacterial cell wall functional groups.
A multi-compartment system, PIGS (Partitioning in Geobiochemical Systems), with five compartments was constructed to study metal distribution between soil constituents. Soil microorganisms (Pseudomonas putida, Trichoderma harzianum) were compared with common soil minerals (kaolin and aluminium oxide) and solid organic matter (peat) with respect to their ability to accumulate Zn, Cd and Hg. Experiments were conducted under conditions that are representative of natural soils concerning pH, metal concentration, ionic strength and microbial activity. Different relative amounts of the solid phases were used to approach natural conditions. Results from the PIGS indicated considerable differences in metal distribution between the various solids, and also indicated that for the different solid phases metal distribution was related to variations in pH and ionic strength of the solutions in different ways. The presence of fulvic acid generally decreased metal accumulation by peat and microorganisms around neutral pH. Accumulation by organic compounds (peat), as well as by microorganisms, was substantial under experimental conditions used, i.e. up to more than 40 and 20% of the added metals was accumulated by these components, respectively. In some cases the considerable accumulation of trace metals by the fungus and the bacterium under acidic conditions is of particular interest, since this process may counteract the metal-mobilizing effects of soil acidification. It is evident from our study that microorganisms should not be overlooked when studying metal interactions with soil constituents.
A method is presented to determine the extent of iron complexation by natural organic ligands in seawater. Catalytic cathodic stripping voltammetry (CSV) is used to take advantage of ligand competition between the added ligand, 1-nitroso-2-napthol (NN), and natural ligands present in seawater. The conditional stability constant for the complexation of iron by NN was calibrated for salinities between 1 and 36 using ligand competition with EDTA. The values of log K′Fe(NN)3 (valid for pH 6.9 seawater) were found to vary linearly with log salinity according to log K′Fe(NN)3 = −1.04 ± 0.08 log(salinity) + 30.12 ± 0.09. The detection window, defined by the iron complexing ability of NN was altered by varying the concentration of NN. Preliminary measurements of iron complexing ligands in samples from coastal and open oceanic origin revealed the presence of natural complexing ligands at concentrations higher than that of total dissolved iron. The stability constants for the complexes were high, log K′FeL falling within the range of 18.8–21.2, indicating that by far the greatest component (99%) of the dissolved iron occurs organically complexed in pH 6.9 seawater. Model calculations showed that it is possible that the organic fraction may be somewhat less at a pH value near 8.
A multi-compartment experimental system (PIGS) was used to study the importance of microorganisms for metal distribution in simulated soil systems. The influence of pH, solute composition (NaCl and CaCl2) and presence of fulvic acid on the distribution of zinc, cadmium and mercury among different phases in the multi-compartment system was studied, i.e., among bacteria, fungi and five other solid soil components and a solution phase. Using the multi-compartment system combined with a factorial design made it possible to study several soil factors as well as solid soil components simultaneously as well as to estimate interaction effects between soil factors. The microorganisms accumulated a considerable part (up to 38%) of the metal, despite the fact that they constituted only a minor fraction (0.4 or 1.7%) of the total solid mass. In contrast, quartz and feldspar, which together constituted 80% of the solid mass, accumulated less than 10%. The fraction associated with peat was generally large (11–57%) and the other solid components accumulated intermediate amounts of metal. Solution pH was the single factor that had the largest effect on the metal distribution. The effect of pH was less pronounced on fungi than on most other solid components, which indicates that the relative importance of fungi as metal sorbents increases as pH decreases. Changing solution composition from NaCl to CaCl2 decreased metal sorption to most sorbents except for the microorganisms where an increased accumulation was observed. The combined effects of pH and fulvic acid were considerable in some cases. This study stresses that the microbial sorbents can respond to changes in the soil solution chemistry in other ways than other solid soil components do. Thus, the microorganism fraction of the solid phase should not be neglected in metal distribution studies of soil.
The equilibrium distribution of Cd, Cu and Pb between the cyanobacterium Anabaena spp. and natural lake water as a function of salinity and degree of algal decomposition, was studied using batch experiments. Adsorption isotherms could be described with the Freundlich equation. Distribution coefficients (Kd) for Anabaena spp. were largest for Pb, followed by Cd and Cu. The variation in Kd values for Cd was explained by a model accounting for metal complexation to the algal surface and complexation by dissolved ligands of variable concentration. The phytoplankton/water distribution of Cd in waters of different salinity appears to be regulated by the free Cd2+ activity in solution. Mineralization of Anabaena spp. for 42 d resulted in a decrease of the solid to water ratio by a factor of 12, and an increase in Kd for Cd (factor four), Cu (factor eight) and Pb (factor 10–20). The increase in Kd is attributed to a higher affinity of the metals for the adsorbent. The use of constant Kd values for mineralizing phytoplankton and detritus in trace metal fate models, may result in underestimations of bound fractions. The extent to which this fraction is underestimated, depends on the time course of the distribution coefficient and the solid to water ratio during the mineralization process.
When Pseudomonas aeruginosa PAO1 biofilms (attached to Sepharose surfaces) were subjected to dissolved Fe3+, most Fe was removed from solution within 25 h by surface complexation with negatively charged functional groups on the bacterial cell wall via a nucleation and mineralization process. Chemical formation of Fe-(hydr)oxides was partially responsible for dissolved Fe removal, which stemmed from a pH increase, facilitated by microbial activity. PAO1 used Fe3+ as an electron acceptor producing Fe2+ under localized anaerobic conditions over the first 50 h. The high ratio of Fe2+ to total Fe in solution produced a high proportion of Fe(II) in Fe precipitates; however, as the formation of Fe-(hydr)oxides started after 50 h, the Fe2+ content in solution began to diminish. Biofilms can so influence the local chemical conditions and metal speciation that the bulk solution phase is also affected, thereby mediating a wide-range (bio)geochemical cycling of iron. Long-term survival of natural biofilms, even under strict oligotrophic conditions, could have a broad lasting effect on the bulk geochemical environment.