[Show abstract][Hide abstract] ABSTRACT: Remineralization of organic carbon in deep-sea sediments is thought to proceed primarily via aerobic respiration and sulfate reduction because the supply of nitrate and metal oxides is not usually significant in deep-sea sediments. Dissimilatory metal reduction, on the other hand, may represent a dominant pathway in coastal and continental shelf sediments where delivery of terrigenous Fe(III) and Mn(IV/III) oxides is sufficiently high or where physical mixing processes near the sediment–water interface can result in the reoxidation of Fe2+ or Mn2+. Passive continental margin sediments receiving outflow from large rivers are well-known deposition centers for organic carbon and may also be hotspots for metal-reducing microbial activity, considering the simultaneous rapid deposition of unconsolidated metal oxides of terrigenous origin. Despite its potential, only a few studies have examined the role of microbial metal reduction in Corg remineralization in these environments. To investigate carbon remineralization processes in continental margin sediments, shallow cores across channels and levees in the Congo River fan (~5000 m) and Louisiana slope (b1800 m) were profiled for the main redox species involved in early diagenesis using a combination of voltammetric gold mercury (Au/Hg) microelectrodes and conventional analyses. Interestingly, metal reduction dominated carbon remineralization processes in the top ~20 cm of sediment subject to high deposition, while evidence for sulfate reduction was lacking. These findings suggest that dissimilatory Fe(III) reduction may be more significant than previously thought in continental slope sediments, which may have important implications on carbon cycling in marine environments. In addition, these findings may have geological implications in controlling atmospheric oxygen levels over geological time and the evolution of microbial life on Earth.
[Show abstract][Hide abstract] ABSTRACT: The ability to predict the success of the microbial reduction of soluble U(VI) to highly insoluble U(IV) as an in situ bioremediation strategy is complicated by the wide range of geochemical conditions at contaminated sites and the strong influence of aqueous uranyl speciation on the bioavailability and toxicity of U(VI) to metal-reducing bacteria. To determine the effects of aqueous uranyl speciation on uranium bioreduction kinetics, incubations and viability assays with Shewanella putrefaciens strain 200 were conducted over a range of pH and dissolved inorganic carbon (DIC), Ca2+, and Mg2+ concentrations. A speciation-dependent kinetic model was developed to reproduce the observed time series of total dissolved uranium concentration over the range of geochemical conditions tested. The kinetic model yielded the highest rate constant for the reduction of uranyl non- carbonate species (i.e., the 'free' hydrated uranyl ion, uranyl hydroxides, and other minor uranyl complexes), indicating that they represent the most readily reducible fraction of U(VI) despite being the least abundant uranyl species in solution. The presence of DIC, Ca2+, and Mg2+ suppressed the formation of more bioavailable uranyl non-carbonate species and resulted in slower bioreduction rates. At high concentrations of bioavailable U(VI), however, uranium toxicity to S. putrefaciens inhibited bioreduction, and viability assays confirmed that the concentration of non-carbonate uranyl species best predicts the degree of toxicity. The effect of uranium toxicity was accounted for by incorporating the free ion activity model of metal toxicity into the bioreduction rate law. Overall, these results demonstrate that, in the absence of competing terminal electron acceptors, uranium bioreduction kinetics can be predicted over a wide range of geochemical conditions based on the bioavailability and toxicity imparted on U(VI) by solution composition. These findings also imply that the concentration of uranyl non-carbonate species, despite being extremely low, is a determining factor controlling uranium bioreduction at contaminated sites.
Full-text · Article · Feb 2015 · Geochimica et Cosmochimica Acta
[Show abstract][Hide abstract] ABSTRACT: The redox chemistry and speciation of Fe in both solid and dissolved phases were characterized in the organic- and Fe-rich sediments of the Satilla River estuary in South-East Georgia (USA) on a series of four cruises between July 2007 and January 2008. Results indicate that dissolved Fe is present in relatively high concentration in the overlying waters at the freshwater end of the estuary and flocculates along the river as the salinity increases downstream. Soluble organic-Fe(III) complexes comprise the majority of dissolved Fe (< 0.2 μm) in the suboxic pore waters of the upriver stations that are characterized by high concentrations of poorly crystalline Fe(III) (oxy)hydroxides. In contrast, SO42--reducing conditions downstream prevent the accumulation of organic-Fe(III) in the pore waters by titrating Fe from the sediment. Separation of dissolved Fe by size exclusion chromatography revealed that Fe(II) is complexed by organic ligands in the pore waters while the organic-Fe(III) complexes are either small or highly reactive with the column matrix. Finally, dissimilatory Fe(III) reduction, stimulated by inoculating anaerobic sediments with a Fe(III)-reducing bacterium (FeRB), Shewanella putrefaciens strain 200, increased production of soluble organic-Fe(III) complexes, and addition of reactive Fe(III) hydroxides accelerated the non-reductive dissolution of Fe(III) (oxy)hydroxides irrespective of the presence of exogenous FeRB. These findings suggest soluble organic-Fe(III) complexes in suboxic pore waters may be produced both as intermediates during the dissimilatory reduction of Fe(III) (oxy)hydroxides by Fe(III)-reducing microorganisms and during the oxidation of organic-Fe(II) complexes by Fe(III) (oxy)hydroxides. These soluble organic-Fe(III) complexes are stable in pore waters and may flux from the sediments to the continental shelf.
No preview · Article · Jan 2015 · Geochimica et Cosmochimica Acta
[Show abstract][Hide abstract] ABSTRACT: Background
Radionuclide- and heavy metal-contaminated subsurface sediments remain a legacy of Cold War nuclear weapons research and recent nuclear power plant failures. Within such contaminated sediments, remediation activities are necessary to mitigate groundwater contamination. A promising approach makes use of extant microbial communities capable of hydrolyzing organophosphate substrates to promote mineralization of soluble contaminants within deep subsurface environments.
Uranium-contaminated sediments from the U.S. Department of Energy Oak Ridge Field Research Center (ORFRC) Area 2 site were used in slurry experiments to identify microbial communities involved in hydrolysis of 10 mM organophosphate amendments [i.e., glycerol-2-phosphate (G2P) or glycerol-3-phosphate (G3P)] in synthetic groundwater at pH 5.5 and pH 6.8. Following 36 day (G2P) and 20 day (G3P) amended treatments, maximum phosphate (PO43−) concentrations of 4.8 mM and 8.9 mM were measured, respectively. Use of the PhyloChip 16S rRNA microarray identified 2,120 archaeal and bacterial taxa representing 46 phyla, 66 classes, 110 orders, and 186 families among all treatments. Measures of archaeal and bacterial richness were lowest under G2P (pH 5.5) treatments and greatest with G3P (pH 6.8) treatments. Members of the phyla Crenarchaeota, Euryarchaeota, Bacteroidetes, and Proteobacteria demonstrated the greatest enrichment in response to organophosphate amendments and the OTUs that increased in relative abundance by 2-fold or greater accounted for 9%–50% and 3%–17% of total detected Archaea and Bacteria, respectively.
This work provided a characterization of the distinct ORFRC subsurface microbial communities that contributed to increased concentrations of extracellular phosphate via hydrolysis of organophosphate substrate amendments. Within subsurface environments that are not ideal for reductive precipitation of uranium, strategies that harness microbial phosphate metabolism to promote uranium phosphate precipitation could offer an alternative approach for in situ sequestration.
[Show abstract][Hide abstract] ABSTRACT: The reduction of Mn(IV) oxides coupled to the anaerobic oxidation of NH4+ has been proposed for more than a decade to contribute to the fixed nitrogen pool in marine sediments, yet the existence of this process is still under debate. In this study, surface sediments from an intertidal salt marsh were incubated with MnO2 in the presence of elevated concentrations of NH4+ to test the hypothesis that the reduction of Mn(IV) oxides catalyzes anaerobic NH4+ oxidation to NO2- or NO3-. Geochemical factors such as the ratio of Mn(IV) to NH4+, the type of Mn(IV) oxides (amorphous or colloidal MnO2), and the redox potential of the sediment significantly affect the activity of anaerobic nitrification. Incubations show that the net production of NO3- is stimulated under anaerobic conditions with external addition of colloidal but not amorphous MnO2 and is facilitated by the presence of high concentrations of NH4+. Mass balance calculations demonstrate that anaerobic NH4+ oxidation contributes to the net consumption of NH4+, providing another piece of evidence for the occurrence of Mn(IV)-catalyzed anaerobic nitrification in coastal marine sediments. Finally, anaerobic nitrification is stimulated by the amendment of small concentrations of NO3- or the absence of sulfate reduction, suggesting that moderately reducing conditions favor anaerobic NH4+ oxidation. Overall, these findings suggest that Mn(IV)-catalyzed anaerobic nitrification in suboxic sediments with high N/Mn concentration ratios and highly reactive manganese oxides may be an important source of NO2- and NO3- for subsequent marine nitrogen loss via denitrification or anammox.
No preview · Article · May 2014 · Geochimica et Cosmochimica Acta
[Show abstract][Hide abstract] ABSTRACT: Bioreduction, the microbial reduction of soluble U(VI) to highly insoluble U(IV), is considered one of the most promising in situ bioremediation strategies to immobilize uranium from groundwater. Although it has been demonstrated that Ca2+ and dissolved inorganic carbon (DIC) decrease bioreduction rates via the formation of stable U(VI)-carbonato and U(VI)-Ca-carbonato complexes, the inhibition mechanism remains unclear. In this study, incubations with Shewanella putrefaciens strain 200 were conducted over a range of Ca2+ and DIC concentrations in unbuffered growth media to delineate the effect of pH on bioreduction rates. In the absence of Ca2+, pH decreased over time which prompted a shift in U(VI) speciation from U(VI)-carbonato complexes toward more readily-reducible U(VI)-hydroxide and U(VI)-organic complexes and increased bioreduction rates. Ca2+ further suppressed the formation of labile U(VI) complexes and required a larger decrease in pH to achieve comparable rates. These results indicate that the main reducible fraction of U(VI) consists of hydroxide and organic complexes despite being the least abundant species in solution. These findings suggest that the pH decrease associated with U(IV) mineral precipitation may promote U(VI) bioreduction in the presence of Ca2+ and carbonates. A new U(VI) bioreduction rate law that explicitly accounts for the speciation of U(VI) species is able to reproduce bioreduction rates under all pH, carbonate, and calcium conditions.
[Show abstract][Hide abstract] ABSTRACT: Although bioreduction of uranyl ions (U(VI)) and biomineralization of U(VI)–phosphate minerals are both able to immobilize uranium in contaminated sediments, the competition between these processes and the role of anaerobic respiration in the biomineralization of U(VI)–phosphate minerals has yet to be investigated. In this study, contaminated sediments incubated anaerobically in static microcosms at pH 5.5 and 7.0 were amended with the organophosphate glycerol-2-phosphate (G2P) as sole phosphorus and external carbon source and iron oxides, sulfate, or nitrate as terminal electron acceptors to determine the most favorable geochemical conditions to these two processes. While sulfate reduction was not observed even in the presence of G2P at both pHs, iron reduction was more significant at circumneutral pH irrespective of the addition of G2P. In turn, nitrate reduction was stimulated by G2P at both pH 5.5 and 7.0, suggesting nitrate-reducing bacteria provided the main source of inorganic phosphate in these sediments. U(VI) was rapidly removed from solution in all treatments but was not reduced as determined by X-ray absorption near edge structure (XANES) spectroscopy. Simultaneously, wet chemical extractions and extended X-ray absorption fine structure (EXAFS) spectroscopy of these sediments indicated the presence of U–P species in reactors amended with G2P at both pHs. The rapid removal of dissolved U(VI), the simultaneous production of inorganic phosphate, and the existence of U–P species in the solid phase indicate that uranium was precipitated as U(VI)–phosphate minerals in sediments amended with G2P. Thus, under reducing conditions and in the presence of G2P, bioreduction of U(VI) was outcompeted by the biomineralization of U(VI)–phosphate minerals and U(VI) sorption at both pHs.
No preview · Article · Apr 2013 · Geochimica et Cosmochimica Acta
[Show abstract][Hide abstract] ABSTRACT: Mn(IV) and Mn(II) are the most stable and prevalent forms of manganese in natural environments. The occurrence of Mn(III) in minerals and the detection of soluble Mn(III) in natural waters, however, suggest that Mn(III) is an intermediate in both the oxidation of Mn(II) and the reduction of Mn(IV). Mn(III) has recently been proposed as an intermediate during the oxidation of Mn(II) by Mn-oxidizing bacteria but has never been considered as an intermediate during the bio-reduction of Mn(IV). Here we show for the first time that microbial Mn(IV) reduction proceeds step-wise via two successive one-electron transfer reactions with production of soluble Mn(III) as transient intermediate. Incubations with mutant strains demonstrate that the reduction of both solid Mn(IV) and soluble Mn(III) occurs at the outer membrane of the cell. In addition, pseudo-first order rate constants obtained from these incubations indicate that Mn(IV) respiration involves only one of the two potential terminal reductases (c-type cytochrome MtrC and OmcA) involved in Fe(III) respiration. More importantly, only the second electron transfer step is coupled to production of dissolved inorganic carbon, suggesting that the first electron transfer reaction is a reductive solubilization step that increases Mn bioavailability. These findings oppose the long-standing paradigm that microbial Mn(IV) reduction proceeds via a single two-electron transfer reaction coupled to organic carbon oxidation, and suggest that diagenetic models should be revised to correctly account for the impact of manganese reduction in the global carbon cycle.
No preview · Article · Dec 2012 · Geochimica et Cosmochimica Acta
[Show abstract][Hide abstract] ABSTRACT: The upper basin of Effingham Inlet possesses permanently anoxic bottom
waters, with a water column redox transition zone typically occurring at
least 40 m above the sediment-water interface. During our sampling
campaign in April and July 2007, this redox transition zone was
associated with sharp peaks in a variety of parameters, including
soluble reactive phosphorus (SRP) and total particulate phosphorus
(TPP). Based on sequential extraction results, TPP maxima exhibited
preferential accumulation of an operationally defined class of loosely
adsorbed organic phosphorus (P), which may contain a substantial
fraction of polyphosphate (poly-P). This poly-P may furthermore be
involved in the redox-dependent remobilization of SRP. For example,
direct fluorometric analysis of poly-P content revealed that particulate
inorganic poly-P was present at concentrations ranging from 1 to 9 nM P
within and several meters above the TPP maximum. Below the depth of 1%
oxygen saturation, however, particulate inorganic poly-P was
undetectable (<0.8 nM in situ). Assuming this concentration profile
reflects the remineralization of inorganic poly-P to SRP across the
redox transition, inorganic poly-P degradation accounted for as much as
4 ± 3% (average ± standard deviation) to 9 ± 8% of
the vertical turbulent diffusive SRP flux. This finding is a
conservative estimate due in part to sample storage effects associated
with our analysis of poly-P content. By comparison, iron-linked P
cycling accounted for at most 65 ± 33% of the diffusive SRP flux,
leaving ˜25% unaccounted for. Thus, while redox-sensitive poly-P
remineralization in Effingham Inlet appears modest based on our direct
conservative estimate, it may be higher from a mass balance viewpoint.
Poly-P cycling may therefore be an overlooked mechanism for the
redox-sensitive cycling of P in some hypoxic/anoxic boundaries,
especially iron-poor marine oxygen minimum zones.
Full-text · Article · Jun 2012 · Global Biogeochemical Cycles
[Show abstract][Hide abstract] ABSTRACT: Soils and groundwater contaminated with heavy metals and radionuclides
remain a legacy of Cold War nuclear weapons development. Due to the
scale of environmental contamination, in situ sequestration of heavy
metals and radionuclides remain the most cost-effective strategy for
remediation. We are currently investigating a remediation approach that
utilizes periplasmic and extracellular microbial phosphatase activity of
soil bacteria capable promoting in situ uranium phosphate sequestration.
Our studies focus on the contaminated soils from the DOE Field Research
Center (ORFRC) in Oak Ridge, TN. We have previously demonstrated that
ORFRC strains with phosphatase-positive phenotypes were capable of
promoting the precpitation of >95% U(VI) as a low solubility
phosphate mineral during growth on glycerol phosphate as a sole carbon
and phosphorus source. Here we present culture-independent soil slurry
studies aimed at understanding microbial community dynamics resulting
from exogenous organophosphate additions. Soil slurries containing
glycerol-2-phosphate (G2P) or glycerol-3-phosphate (G3P) and nitrate as
the sole C, P and N sources were incubated under oxic growth conditions
at pH 5.5 or pH 6.8. Following treatments, total DNA was extracted and
prokaryotic diversity was assessed using high-density 16S
oligonucleotide microarray (PhyloChip) analysis. Treatments at pH 5.5
and pH 6.8 amended with G2P required 36 days to accumulate 4.8mM and 2.2
mM phosphate, respectively. In contrast, treatments at pH 5.5 and pH 6.8
amended with G3P accumulated 8.9 mM and 8.7 mM phosphate, respectively,
after 20 days. A total of 2120 unique taxa representing 46 phyla, 66
classes, 110 orders, and 186 families were detected among all treatment
conditions. The phyla that significantly (P<0.05) increased in
abundance relative to incubations lacking organophosphate amendments
included: Crenarchaeota, Euryarchaeota, Bacteroidetes, and
Proteobacteria. Members from the classes Bacteroidetes, Sphingobacteria,
α-proteobacteria, and γ-proteobacteria increased in relative
abundance by 10 to 406-fold. These are the first PhyloChip studies that
identify unique subsurface community responses to organophosphate
substrates as well as demonstrate the diversity of the extant ORFRC
microbial community capable of promoting in situ uranium phosphate
sequestration. These studies also indicate that concentrations of
phosphate released into extracellular space can be controlled by the
type of substrate supplied to soil microbial communities. Additionally,
we will present data summarizing the two Rahnella genome sequencing
projects (Rahnella sp. Y9602 and the Rahnella aquatilis ATCC 33071)
completed by the Joint Genome Institute.
[Show abstract][Hide abstract] ABSTRACT: The biomineralization of U(VI) phosphate as a result of microbial phosphatase activity is a promising new bioremediation approach to immobilize uranium in both aerobic and anaerobic conditions. In contrast to reduced uranium minerals such as uraninite, uranium phosphate precipitates are not susceptible to changes in oxidation conditions and may represent a long-term sink for uranium in contaminated environments. So far, the biomineralization of U(VI) phosphate has been demonstrated with pure cultures only. In this study, two uranium contaminated soils from the Department of Energy Oak Ridge Field Research Center (ORFRC) were amended with glycerol phosphate as model organophosphate source in small flow-through columns under aerobic conditions to determine whether natural phosphatase activity of indigenous soil bacteria was able to promote the precipitation of uranium(VI) at pH 5.5 and 7.0. High concentrations of phosphate (1-3 mM) were detected in the effluent of these columns at both pH compared to control columns amended with U(VI) only, suggesting that phosphatase-liberating microorganisms were readily stimulated by the organophosphate substrate. Net phosphate production rates were higher in the low pH soil (0.73 ± 0.17 mM d -1 ) compared to the circumneutral pH soil (0.43 ± 0.31 mM d -1 ), suggesting that non-specific acid phosphatase activity was expressed constitutively in these soils. A sequential solid-phase extraction scheme and X-ray absorption spectroscopy measurements were combined to demonstrate that U(VI) was primarily precipitated as uranyl phosphate minerals at low pH, whereas it was mainly adsorbed to iron oxides and partially precipitated as uranyl phosphate at circumneutral pH. These findings suggest that, in the presence of organophosphates, microbial phosphatase activity can contribute to uranium immobilization in both low and circumneutral pH soils through the formation of stable uranyl phosphate minerals.
Full-text · Article · Oct 2011 · Geochimica et Cosmochimica Acta
[Show abstract][Hide abstract] ABSTRACT: Iron speciation in the Satilla River Estuary was investigated using competitive ligand equilibration-adsorptive cathodic stripping voltammetry (CLE-ACSV) with the ligand 1-nitroso-2-naphthol (1N2N). The blackwater Satilla River contains high concentrations of dissolved organic matter and iron, suggesting that it could provide a source of dissolved iron to the continental shelf. Total dissolved iron in the water column decreases along the estuary, likely due to flocculation and precipitation processes associated with the increase in salinity. Simultaneously, the percentage of dissolved organic-Fe(III) complexes measured by CLE-ACSV in overlying waters of sediment cores increases with salinity. The speciation of dissolved iron in the pore waters indicates that these complexes originate in the underlying sediments. Diffusive fluxes calculated from depth profiles in the sediments indicate that only 8% of the sedimentary flux of Fe(III) is delivered to the continental shelf during low riverine discharge. During normal flow conditions, however, the sediment flux of Fe(III) represents 63% of the total riverine flux of Fe(III). These findings suggest that the flux of dissolved iron to the continental shelf is controlled by the sedimentation of iron in the estuary and the remobilization of soluble organic-Fe(III) complexes produced either during iron reduction deep in the sediment or oxidation of Fe(II) close to the sediment-water interface. The Satilla River Estuary generates five to eight times higher concentrations of dissolved Fe(III) than the average major world rivers, suggesting that it is, along with other small blackwater rivers, currently underrepresented in world average river flux calculations.
No preview · Article · Sep 2011 · Limnology and Oceanography
[Show abstract][Hide abstract] ABSTRACT: A recently developed autonomous benthic lander equipped with an underwater potentiostat and a micromanipulator for in situ voltammetric depth profiling of main redox species in pore waters was deployed for 3 yr at multiple stations along the Satilla River estuary (Georgia, U.S.A.). These measurements revealed that biogeochemical processes in estuarine sediments vary seasonally and are influenced by riverine discharge. A prolonged drought decreased river discharge, altered the salinity gradient in the estuary, and profoundly affected anaerobic respiratory processes in the underlying sediments. Under normal hydrologic conditions, iron reduction was the dominant anaerobic respiratory process across the estuary, likely due to the significant supply of iron from the coastal plane and low salinity of the estuary. Under drought conditions, the salinity of the estuary increased to full seawater strength and carbon remineralization was enhanced significantly. Yet, sulfate reduction was only observed near the mouth of the estuary, whereas a substantial increase in iron reduction was distinctive upriver. Evidence indicates that the increase in iron-reducing activity during the drought resulted mainly from the deposition of mineral particles in the upper estuary when the salinity gradient increased. Altogether, this study demonstrates that the biogeochemical response of estuarine sediments to natural perturbations is rapid and that respiration processes are controlled by a combination of temperature, supply of inorganic and organic substrates, and hydrological processes. These findings also suggest that an increase in the frequency of droughts as a result of climate change may enhance the resiliency of iron-reducing bacteria in river-fed estuarine sediments.
Preview · Article · Sep 2011 · Limnology and Oceanography
[Show abstract][Hide abstract] ABSTRACT: Bioremediation provides a unique in situ strategy for combating radioactive contamination from nuclear facilities. The most widely studied uranium bioremediation process revolves around the biotic reduction of aqueous U(VI) complexes to highly insoluble U(IV) minerals. Previous investigations highlighted the importance of uranium speciation on the rate of bioreduction and showed that the type of ligand and degree of complexation dramatically affect the bioavailability of uranium in solution. These studies, however, only monitored the decrease of total dissolved uranium over time. In the present study, a novel voltammetric method was developed to quantify the speciation of uranyl in solution in the presence and absence of carbonates with a hanging mercury drop electrode. Incubations with Shewanella putrefaciens 200R, a model uranium-reducing organism, were conducted at various pH and uranyl speciation was monitored over time to investigate the mechanism of uranium bioreduction in conditions that mimic a wide variety of subsurface environments.
[Show abstract][Hide abstract] ABSTRACT: Recent voltammetric analyses indicate that Shewanella putrefaciens strain 200 produces soluble organic-Fe(III) complexes during anaerobic respiration of sparingly soluble Fe(III) oxides. Results of the present study expand the range of Shewanella species capable of producing soluble organic-Fe(III) complexes to include Shewanella oneidensis MR-1. Soluble organic-Fe(III) was produced by S. oneidensis cultures incubated anaerobically with Fe(III) oxides, or with Fe(III) oxides and the alternate electron acceptor fumarate, but not in the presence of O(2), nitrate or trimethylamine-N-oxide. Chemical mutagenesis procedures were combined with a novel MicroElectrode Screening Array (MESA) to identify four (designated Sol) mutants with impaired ability to produce soluble organic-Fe(III) during anaerobic respiration of Fe(III) oxides. Two of the Sol mutants were deficient in anaerobic growth on both soluble Fe(III)-citrate and Fe(III) oxide, yet retained the ability to grow on a suite of seven alternate electron acceptors. The rates of soluble organic-Fe(III) production were proportional to the rates of iron reduction by the S. oneidensis wild-type and Sol mutant strains, and all four Sol mutants retained wild-type siderophore production capability. Results of this study indicate that the production of soluble organic-Fe(III) may be an important intermediate step in the anaerobic respiration of both soluble and sparingly soluble forms of Fe(III) by S. oneidensis.
[Show abstract][Hide abstract] ABSTRACT: Shewanella oneidensis MR-1 respires a wide range of anaerobic electron acceptors, including sparingly soluble Fe(III) oxides. In the present study, S. oneidensis was found to produce Fe(III)-solubilizing organic ligands during anaerobic Fe(III) oxide respiration, a respiratory strategy postulated to destabilize Fe(III) and produce more readily reducible soluble organic Fe(III). In-frame gene deletion mutagenesis, siderophore detection assays, and voltammetric techniques were combined to determine (i) if the Fe(III)-solubilizing organic ligands produced by S. oneidensis during anaerobic Fe(III) oxide respiration were synthesized via siderophore biosynthesis systems and (ii) if the Fe(III)-siderophore reductase was required for respiration of soluble organic Fe(III) as an anaerobic electron acceptor. Genes predicted to encode the siderophore (hydroxamate) biosynthesis system (SO3030 to SO3032), the Fe(III)-hydroxamate receptor (SO3033), and the Fe(III)-hydroxamate reductase (SO3034) were identified in the S. oneidensis genome, and corresponding in-frame gene deletion mutants were constructed. DeltaSO3031 was unable to synthesize siderophores or produce soluble organic Fe(III) during aerobic respiration yet retained the ability to solubilize and respire Fe(III) at wild-type rates during anaerobic Fe(III) oxide respiration. DeltaSO3034 retained the ability to synthesize siderophores during aerobic respiration and to solubilize and respire Fe(III) at wild-type rates during anaerobic Fe(III) oxide respiration. These findings indicate that the Fe(III)-solubilizing organic ligands produced by S. oneidensis during anaerobic Fe(III) oxide respiration are not synthesized via the hydroxamate biosynthesis system and that the Fe(III)-hydroxamate reductase is not essential for respiration of Fe(III)-citrate or Fe(III)-nitrilotriacetic acid (NTA) as an anaerobic electron acceptor.
Full-text · Article · Feb 2010 · Applied and Environmental Microbiology
[Show abstract][Hide abstract] ABSTRACT: Depth profiles in the sediment porewaters of the Chattahoochee River (Georgia, USA) show that iron oxides scavenge arsenate in the water column and settle to the sediment–water interface (SWI) where they are reduced by iron-reducing bacteria. During their reduction, these particles seem to release arsenic to the porewaters in the form of arsenate only. Sediment slurry incubations were conducted to determine the effect of low concentrations of arsenic (⩽10 μM) on biogeochemical processes in these sediments. Experiments confirm that any arsenate (As(V)) added to these sediments is immediately adsorbed in oxic conditions and released in anoxic conditions during the microbial reduction of authigenic iron oxides. Incubations in the presence of ⩽1 μM As(V) reveal that arsenate is released but not concomitantly reduced during this process. Simultaneously, microbial iron reduction is enhanced significantly, spurring the simultaneous release of arsenate into porewaters and secondary formation of crystalline iron oxides. Above 1 μM As(V), however, the microbial reductive dissolution of iron oxides appears inhibited by arsenate, and arsenite is produced in excess in the porewaters. These incubations show that even low inputs of arsenic to riverine sediments may affect microbial processes, the stability of iron oxides and, indirectly, the cycling of arsenic. Possible mechanisms for such effects on iron reduction are proposed.
No preview · Article · Oct 2009 · Geochimica et Cosmochimica Acta
[Show abstract][Hide abstract] ABSTRACT: The remediation of uranium from soils and groundwater at Department of Energy (DOE) sites across the United States represents a major environmental issue, and bioremediation has exhibited great potential as a strategy to immobilize U in the subsurface. The bioreduction of U(VI) to insoluble U(IV) uraninite has been proposed to be an effective bioremediation process in anaerobic conditions. However, high concentrations of nitrate and low pH found in some contaminated areas have been shown to limit the efficiency of microbial reduction of uranium. In the present study, nonreductive uranium biomineralization promoted by microbial phosphatase activity was investigated in anaerobic conditions in the presence of high nitrate and low pH as an alternative approach to the bioreduction of U(VI). A facultative anaerobe, Rahnella sp. Y9602, isolated from soils at DOE's Oak Ridge Field Research Center (ORFRC), was able to respire anaerobically on nitrate as a terminal electron acceptor in the presence of glycerol-3-phosphate (G3P) as the sole carbon and phosphorus source and hydrolyzed sufficient phosphate to precipitate 95% total uranium after 120 hours in synthetic groundwater at pH 5.5. Synchrotron X-ray diffraction and X-ray absorption spectroscopy identified the mineral formed as chernikovite, a U(VI) autunite-type mineral. The results of this study suggest that in contaminated subsurfaces, such as at the ORFRC, where high concentrations of nitrate and low pH may limit uranium bioreduction, the biomineralization of U(VI) phosphate minerals may be a more attractive approach for in situ remediation providing that a source of organophosphate is supplied for bioremediation.