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Aerobic and anaerobic decomposition of organic matter in marine sediment: Which is fastest?
Limnology and Oceanography
Abstract
The enigma of aerobic vs. anaerobic decomposition in marine sediments was addressed by means of a thin-layer incubation technique. Two different 14C-labeled plant materials, aged diatoms (Skeletonema cos- tatum) and fresh barley hay, were each mixed into intertidal sediment and spread in a 1.5-mm layer on the bottom of oxic and anoxic chambers. After a 27-d incubation, conditions in all chambers were switched from aerobic to anaerobic and vice versa for 11 d. Rates of 14C0, evolution in diatom chambers showed that aerobic carbon mineralization was - 10 times faster than anaerobic both before and after the switch. Low rates of (14C)DOC release suggested that the limiting step of an,aerobic decay was the initial hydrolytic and fermentative enzymatic attack on the predecomposed diatoms. Initial carbon mineralization of barley hay was not affected by the presence or absence of oxygen. Leaching of DOC from the fresh barley hay supplied anaerobic respirers with labile substrates. When leaching ceased and after the aerobic-anaerobic switch, the rate of anaerobic mineralization was reduced. Mineralizat:.on of leachable and easily hydrolyzable compounds from fresh plant detritus is equally fast under aerobic ant! anaerobic conditions. When structural components dominate the particulate remains, anaerobic processes are hampered by inefficient and slow bacterial hydrolysis of structurally complex macromolecules.
... Sulfate reduction has been reported to be the predominant electron-accepting process in marine-dominated coastal sediments such as the Ashtamudi estuary (Nealson, 1997;Vincent et al., 2017). In the presence of significant levels of organic matter and sulfate, anoxic circumstances favour the anaerobic process of sulfate reduction, in which sulfate is reduced to sulfide by utilising electrons from organic compounds (Kristensen et al., 1995). Sulfate reduction rates with depth are inversely correlated with SRB abundance. ...
... SRB are crucial in the transformation of sulfur by dissimilatory sulfate reduction (He et al., 2015;Muyzer and Stams, 2008). According to Kristensen et al. (1995), this finally mineralizes the organic material and raises the concentration of sulfide in the sediment, exerting a significant effect on the pore water chemistry (Nickerson and Thibodeau, 1985). Although numerous species of anaerobic microbes have been identified, the structural and functional diversity of SRB in tropical coastal environments is mostly unexplored. ...
... Other kelp species have been tested for example, aerobic in-situ decomposition of M. integrifolia had slightly faster breakdown rates than the kelp tested in the present study (k = 0.032) (Albright et al., 1980). Because the present study was conducted under anaerobic conditions, it might be expected to produce a significantly slower degradation rate than those of aerobic incubations since aerobic carbon mineralisation can be up to ten times faster than anaerobic (Kristensen et al., 1995). Anoxia in benthic areas is common throughout coastal and deeper parts of the oceans and anoxic areas are believed to increase the incorporation of organic carbon into sediments even when compared to areas where input of organic matter is greater (Pedersen & Calvert, 1990;Stein, 1990). ...
... The early loss of labile materials in the present study shows that macroalgal decomposition follows previous descriptions of decomposition and provides valuable details about the compounds released (Canfield, 1994;Kristensen et al., 1995). The initial stage is the leaching of soluble organic compounds, followed by rapid remineralisation of labile compounds (Findlay, 2021). ...
The ecological and biological importance as well as economic, and cultural value of macroalgae is becoming more prominent. Introduction of the term ‘blue carbon’ (BC) has drawn attention to natural coastal ecosystems, the habitats they provide, and their capacity to fix CO2. The overall aim of this thesis was to place the importance of carbon sequestration within the already essential services that macroalgae provide to the biosphere. This thesis focused on: 1) Quantifying the amount of detritus produced by species in Scottish macroalgae habitats and providing annual figures of total carbon leaving kelp forests in fluxes, 2) understanding the processes of degradation of detritus from three dominant kelp species and estimating the pathways of carbon loss of the detritus, and 3) identifying the sources of sediment carbon using biomarkers and environmental DNA primers specific to the class Phaeophyceae.
Macroalgae in the North-East Atlantic: 1) fix significant amounts of CO2 through photosynthesis thus removing it from the atmosphere, 2) release the carbon fixed through photosynthesis as detritus which accumulates and is buried, broken down by bacteria, and contributes to food webs, and 3) contributes carbon to sediment stores in Scotland and the wider North-East Atlantic shelf. These three criteria are fundamental blue carbon habitat characteristics. It is thereby recommended that macroalgae are henceforth included in blue carbon frameworks and directives, particularly in Scotland, where the contribution to long-term carbon stores in fjord and shelf systems is potentially greater than any other BC habitat in the region. It is estimated that 0.2 Mt C yr-1 is transferred to sediments from macroalgae in Scotland, the equivalent of 0.04 g C m-2 of kelp forest.
... As an alternative phenomenological approach, if the relationship between the total 137 Cs desorption rate and influential environmental factors (e.g., temperature, oxygen concentration) can be determined, amount of 137 Cs desorbed from the lake bottom can be sufficiently obtained to predict the meteorological input. Microbial activity in sediment is generally promoted by increased temperature and oxygen content (Hackett et al., 1977;Hansen and Blackburn, 1991;Kristensen et al., 1995). In terms of the chemical balance, the increased temperature shifts the chemical equilibrium of 137 Cs between the mineral particles and pore water in the desorption direction because of the negative standard enthalpy of reaction for the exchange of Cs hydrate with the cations in mineral particles (Martin and Laudelout, 1963;Schramm and Kwak, 1982;Liu et al., 2003). ...
... The decomposition rate in the aerobic incubation experiment could not be evaluated to assess its influence on organic decomposition by DO. If the aerobic decomposition rate is faster than anaerobic decomposition, as shown in previous studies (Hackett et al., 1977;Hansen and Blackburn, 1991;Kristensen et al., 1995), the amount of 137 Cs desorbed in the organic matter decomposition process should have been higher in Aero10 than in Anaero10. However, the 137 Cs concentrations in Aero10 were lower than those in Anaero10, as well as in the 10 cm surface layer of the control column (Fig. 4b). ...
The temperature and oxygen environment play important roles in the desorption of ¹³⁷Cs from freshwater lake sediment to lake water. In this study, 12 quarterly surveys were performed to measure the dissolved ¹³⁷Cs concentration in surface and bottom lake water, the vertical distribution of water temperature, and the dissolved oxygen (DO) concentration at the upstream, midstream, and downstream sites of the Yokokawa Dam Lake in Fukushima Prefecture, Japan. Higher concentrations of dissolved ¹³⁷Cs were detected in the bottom water than in the surface water, especially in the summer and midstream lake regions at depths of 8–21 m owing to higher temperatures, which activated the bacterial decomposition of organic matter, and anaerobization, which enhanced the NH4⁺ in the pore water and ¹³⁷Cs desorption from mineral particles. To compare the effects of anaerobization and increasing temperature on ¹³⁷Cs desorption from sediment particles, intact sediment core samples were collected from the lake midstream and incubated for 1–14 days in a chamber under three controlled temperature and oxygen environment conditions: aerobic +10 °C; anaerobic +10 °C; and anaerobic +20 °C. The vertical distribution of ¹³⁷Cs in the sediment pore water showed a similar profile as NH4⁺ and K⁺, and both the increased temperature and reduced DO concentration enhanced the ¹³⁷Cs desorption. A comparison of the standard partial regression coefficients of temperature and DO concentration in the multiple regression equation for the ¹³⁷Cs concentration in pore water shows that the reduction of DO from saturation to zero at the water–sediment interface accelerated the desorption of ¹³⁷Cs more strongly than did the temperature rise from 10 to 20 °C. The experimental results show a nearly inverse proportional relationship between NH4⁺ and the distribution coefficients of ¹³⁷Cs after thermodynamic correction, except in the surface layer. These findings indicate that the ¹³⁷Cs concentration in pore water can be explained by the temperature-dependent chemical distribution between the solid–aqueous phase and its equilibrium with NH4⁺.
... O 2 seemed to facilitate a more complete degradation process. The result was in agreement with the previous observation in marine sediments, where aerobic mineralization of complex organic material was 2− 3 times faster than that under anoxic conditions (Kristensen et al., 1995). The relatively incomplete decomposition in the anoxic zones of most sediments can be explained by the limited ability of anaerobic microorganisms to hydrolyze certain classes of structurally complex and aromatic organic compounds (Kristensen et al., 1995). ...
... The result was in agreement with the previous observation in marine sediments, where aerobic mineralization of complex organic material was 2− 3 times faster than that under anoxic conditions (Kristensen et al., 1995). The relatively incomplete decomposition in the anoxic zones of most sediments can be explained by the limited ability of anaerobic microorganisms to hydrolyze certain classes of structurally complex and aromatic organic compounds (Kristensen et al., 1995). ...
The discharge of produced water from offshore oil platforms is an emerging concern due to its potential adverse effects on marine ecosystems. In this study, we investigated the feasibility and capability of using marine sediments for the bioremediation of produced water. We utilized a combination of porewater and solid phase analysis in a series of sediment batch incubations amended with produced water and synthetic produced water to determine the biodegradation of hydrocarbons under different redox conditions. Significant removal of benzene, toluene, ethylbenzene and xylene (BTEX) compounds was observed under different redox conditions, with biodegradation efficiencies of 93−97% in oxic incubations and 45−93% in anoxic incubations with nitrate, iron oxide or sulfate as the electron acceptor. Higher biodegradation rates of BTEX were obtained by incubations dominated by nitrate reduction (104−149 nmolC/cm3/d) and oxygen respiration (52−57 nmolC/cm3/d), followed by sulfate reduction (14−76 nmolC/cm3/d) and iron reduction (29−39 nmolC/cm3/d). Chemical fingerprint analysis showed that hydrocarbons were biodegraded to smaller alcohols/acids under oxic conditions compared to anoxic conditions with nitrate, indicating that the presence of oxygen facilitated a more complete biodegradation process. Toxicity of treated produced water to the marine copepod Acartia tonsa was reduced by half after sediment incubations with oxygen and nitrate. Our study emphasizes the possibility to use marine sediment as a biofilter for treating produced water at sea without extending the oil and gas platform or implementing a large-scale construction.
... According to the present study, there was a possibility that humic-like substances, via polymerization between hydrophobic aromatic compounds and active Fe/Al, might occur because there were significant and positive correlations between the stable OC in the WSA and oxalate extractable Fe (r = 0.87**) and dithionite extractable Al (r = 0.81**). The work [29] suggested that under prolonged anaerobic conditions, organic substances with a high molecular weight and complexed molecular structures, such as phenolic compounds, would decompose slowly by microbial activity and thus enhance the accumulation of these stable OC components. Overall, the paddy soils from Inceptisols, Mollisols, and Vertisols containing higher organic and inorganic stabilizing agents/flocculating agents generally enhanced a higher accumulation of stable OC, such as hydrophobic aromatic compounds, in the WSA, and thus could enhance the formation and stabilization of soil aggregates through their higher hydrophobicity and possibly higher formation of humic-like substances that might help to increase the hydrophobicity of the soil aggregates to a greater extent than for paddy soils from Alfisols and Ultisols. ...
Evidence has suggested that either labile organic carbon (OC) or stable OC play a role in improving aggregate stability. Therefore, this study determined the OC fractions in water stable aggregates (WSA) and their contribution to the formation of the WSA in paddy soils, on the Central Plain of Thailand. Analysis of the OC fractions in the WSA was determined using wet oxidation with hydrogen peroxide (H2O2). The chemical composition of the organic
compounds in the WSA was investigated using Fourier transform midinfrared (FT-IR) spectroscopy. The results showed that the WSA content of the studied soils significantly increased with increasing organic and inorganic cementing/flocculating agents, such as soil organic carbon, clay, polyvalent cations, and sesquioxides. The labile OC and stable OC contents in the WSA also significantly increased with increased WSA content, suggesting the physical protection of OC fractions against microbial decomposition. The FT-IR analysis revealed that labile OC in the WSA, both before and after wet oxidation with H2O2, was dominated by polysaccharides, supporting the physical protection of labile OC by the WSA. Paddy soils containing higher organic and inorganic cementing/flocculating agents had higher stable OC, such as hydrophobic aromatic compounds, in the WSA, compared to paddy soils containing lower organic and inorganic cementing/flocculating agents. In turn, the WSA content of the soils in this study significantly increased with increases in the hydrophobic aromatic compounds in the WSA, suggesting the important role of stable hydrophobic organic compounds in enhancing the formation and stabilization of soil aggregates in these paddy soils.
... To incorporate dissolved oxygen consumption and aqueous-phase redox reactions into the reaction network (Table 1; Figure 1, lower portion), decomposition of litter and SOM pools was modified so that the decomposed fraction previously converted directly to CO 2 was converted instead to DOM with a fixed C:N ratio of 20. Rate constants for decomposition of litter and SOM pools to DOM under anoxic conditions were assumed to be 10% of the default values under oxic conditions, representing the decreased activity of hydrolytic enzymes under anoxic conditions (Kristensen et al., 1995). Multiple aqueous-phase chemical reactions were added representing alternative pathways of DOM decomposition, with liberated N, Fe, and sulfate content of organic matter included based on fixed stoichiometry of DOM (C:N:S:Fe = 2000:100:20:1), based on measurements of C, S, and Fe content Spartina alterniflora litter from Massachusetts sites (Breteler et al., 1981) and a global synthesis showing a median plant litter Fe concentration of 0.2 g kg 1 (Peng et al., 2023). ...
Redox processes, aqueous and solid‐phase chemistry, and pH dynamics are key drivers of subsurface biogeochemical cycling and methanogenesis in terrestrial and wetland ecosystems but are typically not included in terrestrial carbon cycle models. These omissions may introduce errors when simulating systems where redox interactions and pH fluctuations are important, such as wetlands where saturation of soils can produce anoxic conditions and coastal systems where sulfate inputs from seawater can influence biogeochemistry. Integrating cycling of redox‐sensitive elements could therefore allow models to better represent key elements of carbon cycling and greenhouse gas production. We describe a model framework that couples the Energy Exascale Earth System Model (E3SM) Land Model (ELM) with PFLOTRAN biogeochemistry, allowing geochemical processes and redox interactions to be integrated with land surface model simulations. We implemented a reaction network including aerobic decomposition, fermentation, sulfate reduction, sulfide oxidation, methanogenesis, and methanotrophy as well as pH dynamics along with iron oxide and iron sulfide mineral precipitation and dissolution. We simulated biogeochemical cycling in tidal wetlands subject to either saltwater or freshwater inputs driven by tidal hydrological dynamics. In simulations with saltwater tidal inputs, sulfate reduction led to accumulation of sulfide, higher dissolved inorganic carbon concentrations, lower dissolved organic carbon concentrations, and lower methane emissions than simulations with freshwater tidal inputs. Model simulations compared well with measured porewater concentrations and surface gas emissions from coastal wetlands in the Northeastern United States. These results demonstrate how simulating geochemical reaction networks can improve land surface model simulations of subsurface biogeochemistry and carbon cycling.
... Redox state has been suggested as another key driver of OC burial. For instance, under oxic environments fresh OC is typically degraded more quickly than stable OC; however, in anoxic conditions fresh OC is degraded slowly and stable OC only minimally Burdige, 2007;Cowie et al., 1995;Kristensen et al., 1995). Consequently, low oxygen and anoxic environments are expected to have greater OC preservation than oxic environments (Arndt et al., 2013;Middelburg & Levin, 2009). ...
Fjords are net carbon sinks with high organic carbon (OC) burial rates; however, the key drivers of OC burial in these systems are not well constrained. To study the role of water column redox condition and OC composition on OC preservation in fjord sediments, we determined OC accumulation rates (OCAR), OC source, and OC degradation in three Swedish fjords with variable redox conditions (long‐term oxic, seasonally hypoxic, and long‐term anoxic). Average OCARs were variable between and within the fjords studied (2–122 g OC m⁻² yr⁻¹), but highest rates were at the mouth for each fjord. Based on a δ¹³C mixing model, Swedish fjords bury predominantly marine‐derived OC (∼83% of the total OC burial) likely because of relatively gentle slopes, low riverine discharge, and high marine inflow. Using a multi‐biomarker approach (lignin, photosynthetic pigments, and total hydrolyzable amino acids) we found, terrestrially‐ and marine‐derived OC were moderately degraded under the various redox conditions sampled, suggesting water column redox and OC source are not primary drivers of OC burial in these fjords. Rather, high sediment accumulation rates, common to fjords globally, lead to low oxygen exposure times, thus promoting efficient burial of OC regardless of its chemical composition.
... Future work could also assess whether there are species-specific differences in excretion rates, and how redox potential and soil nitrogen content are affected by changes to bioturbating infaunal species in crab treatments, since early establishing salt marsh vegetation may rely on nutrients from infaunal invertebrate wastes as well (Holdredge et al. 2010). Organic matter decomposition rates are also increased in more oxygenated soils (Kristensen et al. 1995). Foraging activities by C. maenas could promote aeration of the sediment promoting uptake and resulting in lower organic matter. ...
... With increasing water depth and distance from the coast, sedimentation rate decreases, and the organic matter delivered to the seafloor gets exposed to O 2 for longer periods of time. As a consequence, irrespective of the O 2 concentration of the bottom-water, faster breakdown of complex organic matter into simpler compounds takes place in the sediment surface (Canfield 1994;Kristensen et al. 1995;Burdige 2007;Middelburg 2019;Bhattacharya et al. 2021). Across the sediment depths, this leads to an overall intensification of the simple-fatty-acids-requiring anaerobic metabolisms such as sulfate reduction and methanogenesis (Fernandes et al. 2018;Bhattacharya et al. 2021). ...
Sediments underlying marine hypoxic zones are huge sinks of unreacted complex organic matter, where despite acute O2 limitation, obligately aerobic bacteria thrive, and steady depletion of organic carbon takes place within a few meters below the seafloor. However, little knowledge exists about the sustenance and complex carbon degradation potentials of aerobic chemoorganotrophs in these sulfidic ecosystems. We isolated and characterized a number of aerobic bacterial chemoorganoheterotrophs from across a ~ 3 m sediment horizon underlying the perennial hypoxic zone of the eastern Arabian Sea. High levels of sequence correspondence between the isolates’ genomes and the habitat’s metagenomes and metatranscriptomes illustrated that the strains were widespread and active across the sediment cores explored. The isolates catabolized several complex organic compounds of marine and terrestrial origins in the presence of high or low, but not zero, O2. Some of them could also grow anaerobically on yeast extract or acetate by reducing nitrate and/or nitrite. Fermentation did not support growth, but enabled all the strains to maintain a fraction of their cell populations over prolonged anoxia. Under extreme oligotrophy, limited growth followed by protracted stationary phase was observed for all the isolates at low cell density, amid high or low, but not zero, O2 concentration. While population control and maintenance could be particularly useful for the strains’ survival in the critically carbon-depleted layers below the explored sediment depths (core-bottom organic carbon: 0.5–1.0% w/w), metagenomic data suggested that in situ anoxia could be surmounted via potential supplies of cryptic O2 from previously reported sources such as Nitrosopumilus species.
... However, this was not the case, which indicates that OM was clearly removed or degraded in the 0 -4 cm sediments. The oxic state of the anodic sediments can chemically degrade OM more rapidly compared to the reducing conditions in the cathodic sediments (Kristensen et al., 1995;Sahrawat, 2003). The alkaline conditions in the cathodic sediments also promote OM dissolution, thus explaining lower OM contents compared to the initial sediment. ...
The use and reuse of natural and waste materials after treatment has become increasingly crucial as a means of achieving sustainable and environmentally-friendly solutions, and is part of a broader trend towards embracing circular economy principles. This study aims to understand the behavior of different elements (metal(loid)s and non-metals) and minerals during and after electrokinetic remediation (EKR) and to develop an effective approach to monitor its progress and overcome unwanted occurrences. In this regard, estuarine sediments, collected from Tancarville (Seine River estuary, France), were electrokinetically treated using a 64 L laboratory reactor; treatment was done 8 h per day for 21 days. The physico-chemical properties (pH, electric conductivity, and oxido-reduction potential) and current were monitored during treatment. The spatial evolution of the physico-chemical, physical (grain size distribution), mineral (mainly carbonates), organic, and elemental (As, Ca, Cl, Mg, Na, Pb, Sr, Zn, and Zr) characteristics was studied to assess the treatment efficiency. The results showed that the acidic conditions in the anodic sediments caused the dissolution of carbonates (calcite, dolomite, and aragonite), resulting in a considerable reduction in As, Zn, and Pb. Additionally, Cl as well as electric conductivity were significantly reduced from most sediments, which is essential in agricultural practices. Furthermore, materials had precipitated and settled in the anolyte and catholyte chambers, which acted as sorbents for elements that were released from the sediments (mainly Zn and As). Finally, three distinct phases occurred during treatment and were mainly linked to the current intensity and electric conductivity on the one hand, and the dissolution of carbonates and metal(loid) release on the other. This approach can be used to treat sediments and other media to improve the overall efficiency of remediation processes and create an end product with desired characteristics.
... A much more plausible cause is the production and uptake by L. orbiculatis of 'old carbon' produced by microbial breakdown of autochtonic organic matter (Roth et al., 2023) in the sediment column of coastal systems, like the process encountered in peat bogs (Raghoebarsing et al., 2005;Steinmann et al., 2008), which has been shown to lead to erroneously old 14 C dates for submerged Sphagnum plants. As published by Kristensen et al. (1995) and confirmed by many subsequent studies (e.g., Migliore et al., 2012), even under truly anaerobic conditions a significant amount of CO 2 (aq) is produced by the microbial breakdown of organic matter in the sediment column (described as biologically controlled diagenesis) with methanogenesis as a well-established process (e.g., Konhauser et al., 2011). Methanogenesis is particularly known from anaerobic sediments in coastal marine Fig. 6. a) Uncalibrated versus calibrated ages plots for the terrestrial (green) and marine (navy) samples from Puntone. ...
... Sea ice presence and breakup in Arctic tidal flat areas can strongly impact sediment redistribution and deposition in the intertidal zone, and decreases in ice presence and sediment freezing with climate change could lead to strong changes in the colonization patterns (McCann and Dale 1986 , W ęsławski et al. 2011 ). A better understanding of the environmental factors that drive surface sediment communities, where most degradation of organic molecules occurs (Kristensen et al. 1995, Holmer 1999, is k e y for improving our knowledge of processes in estuarine tidal flats. ...
Climate change is altering patterns of precipitation, cryosphere thaw, and land-ocean influxes, affecting understudied Arctic estuarine tidal flats. These transitional zones between terrestrial and marine systems are hotspots for biogeochemical cycling, often driven by microbial processes. We investigated surface sediment bacterial community composition and function from May to September along a river-intertidal-subtidal-fjord gradient. We paired metabarcoding of in-situ communities with in-vitro carbon-source utilization assays. Bacterial communities differed in space and time, alongside varying environmental conditions driven by local seasonal processes and riverine inputs, with salinity emerging as the dominant structuring factor. Terrestrial and riverine taxa were found throughout the system, likely transported with runoff. In-vitro assays revealed sediment bacteria utilized a broader range of organic matter substrates when incubated in fresh and brackish water compared to marine water. These results highlight the importance of salinity for ecosystem processes in these dynamic tidal flats, with the highest potential for utilization of terrestrially derived organic matter likely limited to tidal flat areas (and times) where sediments are permeated by freshwater. Our results demonstrate that intertidal flats must be included in future studies on impacts of increased riverine discharge and transport of terrestrial organic matter on coastal carbon cycling in a warming Arctic.
... The deposition and degradation dynamics, i.e. the concentration, chemical nature, and fate of the organic matter that is delivered to the seafloor, determine the microbiome structure and function in the sediment system across the continental margin (Bhattacharya et al. 2021). With increasing water-depth (distance from the 80 coast), lower sedimentation rate, so greater O2-exposure time (irrespective of the O2 concentration of the bottom-water) for the breakdown of the buried organic matter into simpler compounds, is encountered in the sediment surface (Canfield 1994;Kristensen et al. 1995;Burdige 2007;Middelburg 2019;Bhattacharya et al. 2021); consequently, down the sediment depths there is a core-wide intensification of the simple-fatty-acids-requiring anaerobic 85 metabolisms of the carbon-sulfur cycle (Fernandes et al. 2018). ...
Sediments underlying marine hypoxias are huge sinks of unreacted complex organic matter, where despite acute O2-limitation aerobic bacterial communities thrive, and near-complete depletion of organic carbon takes place within a few meters below the seafloor. However, little knowledge exists about how aerobic chemoorganotrophs survive in these sulfidic ecosystems, and what may be their potentials for degrading complex carbon compounds. To elucidate these questions, we isolated and characterized a number of aerobic bacterial chemoorganoheterotrophs from across a ~3-m sediment horizon underlying the perennial hypoxia of the eastern Arabian Sea. High levels of sequence correspondence between the isolates’ genomes and the habitat’s metagenomes and metatranscriptomes illustrated that the strains were widespread and active across the sediment cores explored. The isolates could catabolize several complex organic compounds of marine and terrestrial origins, via aerobic respiration at high as well as low O2 concentrations. Some of them could also grow anaerobically on yeast extract or acetate by reducing nitrate and/or nitrite. Fermentation did not support growth in any of the strains, but enabled all of them to maintain a fraction of the cell population amidst prolonged anoxia. Under extreme oligotrophy, robust growth followed by protracted stationary phase was observed for all the isolates at low cell density, irrespective of whether O2 was high or low. While metabolic deceleration was apparently central to the strains’ adaptation to dwindling organic carbon, O2-limitation could be potentially surmounted via supplies from known producers of biogenic O2, such as Nitrosopumilus species, which had copious footprints in the habitat’s metagenomes.
... Nonetheless, the introduction of oxygen in the anode chamber due to water oxidation might promote organic matter dissolution through chemical degradation. Indeed, the rate of chemical degradation of OM under oxic conditions (in the anodic zone) is faster than in anoxic conditions (Kristensen et al. 1995). Finally, the 20-40-cm sediments showed slightly lower OM than the initial sediment. ...
The treatment and beneficial use of polluted or contaminated environmental matrices have become major issues, especially as the world strives toward a zero-waste policy. In this regard, dredged sediments need to be treated before they can be used in an environmentally safe and sustainable manner. Therefore, this work aims to treat estuarine sediments and, more importantly, use physicochemical, mineral, organic, and chemical information to understand the reactions that occur upon treatment. Dredged estuarine sediments were collected from Tancarville (Seine River estuary, France) and subjected to electrokinetic (EK) remediation using a 128-L laboratory-scale reactor. The sediments were treated 8 h per day for 21 days. The electric (voltage and current) and physicochemical (pH and electric conductivity) parameters were monitored during treatment. Sediments were collected from various sections in the reactor at the end of the experiment (lengthwise, widthwise, and depthwise). The spatial variation was investigated in terms of organic, mineral, and metal contents. Statistical analyses proved that the variation occurred only in the lengthwise direction. Furthermore, three main phases described the treatment, which were mainly linked to carbonate dissolution and pH variation. The results also showed that the trace elements Ni and Zn were reduced by 21% and 19%, respectively, without a direct link to pH, while Ca and Mg were only redistributed. The buffering capacity of the anodic sediment was reduced due to carbonate dissolution. The treated sediments showed reduced contents in trace metals without affecting major elements that can be useful in agriculture (i.e., Ca and Mg).
... Soil organic matter can decrease availability of Cd in soil (Khan et al. 2017). Microbial decomposition of organic matter under aerobic conditions is around 10 times faster when compared to anaerobic conditions (Kristensen 1995). This means that organic matter turnover is lower in soils with lower Eh and indeed rice paddy soils therefore accumulate more carbon when compared to non-flooded arable cropping systems and orchards (Wu 2010). ...
Previous studies have shown that arsenic and cadmium can accumulate in rice grain to levels that cause health concerns. Furthermore, geographical survey has shown that there is considerable variation (~ 100-fold) in accumulation of these carcinogens in rice grain. This variance must be due to heterogeneity in soil biogeochemistry and contrasting rice management regimens. Here we present the first systematic global study to investigate the impact of soil biogeochemistry on accumulation of these elements in rice grain. Matched grain, shoot, root and soil samples were collected across a latitudinal gradient from East Africa to Europe and soil, shoot, grain chemistry and soil microbial community (prokaryotes and fungi) assessed within the context of arsenic and cadmium biogeochemistry. European and Vietnamese grain sum of arsenic species (inorganic arsenic plus dimethylarsonic acid) concentration medians, ~ 0.1 mg/kg, were found to be around ten-fold higher compared to those in East Africa and Sri Lanka. Arsenic concentrations were linked to higher levels of soil arsenic, and to higher abundance of soil sulphur-oxidising and sulphate reducing bacteria and methanogenic archaea. For cadmium, Sri Lanka showed highest (median 0.0156 mg/kg) and Europe lowest (median of 0.001 mg/kg) levels in grain, with the other regions showing intermediate values. Interestingly, grain cadmium was unrelated to soil cadmium concentrations, with Europe having the highest levels of cadmium in soil. Instead, grain cadmium correlated with higher oxidation/reduction potential, lower -log[hydrogen ion], lower soil calcium, and to a higher abundance of aerobic bacteria and fungi (lowest abundance of these organisms in European soils).
... These microorganisms have mostly unlimited activity. They are able to transform many organic or mineral molecules thanks to their extreme richness in enzymes which catalyze the reactions necessary on the one hand, for either their respiration, and for the synthesis of living matter by biodegradation of the environment [14]. Overall, the organic matter metabolization reaction can be summarized by the equation [15] ( Eq.1) : ...
Moroccan industry, like most industries around the world, today faces a water scarcity. Water is one of the basic utilities for all kinds of production, especially in the agro-food sector. Morocco, a country of bioclimatic and ecological transition with fragile resources, is threatened with rapid degradation in the event of overexploitation of underground water reserves. Indeed, the evolution of the water supply over the last decade already testifies to the irregularity of the outlook and confirms climate failure. “From 1990 to 2000, the resources in m ³ per inhabitant per year fell from 1200 to 950. In 2020 the water resources are only around 600m ³ / inhabitant / year, at a time when the demand for water total has reached the ceiling of 20 to 21km ³ of water that can be mobilized. The country will drop to the shortage threshold for 500m ³ / inhabitant / year around the year 2030".In fact, Morocco has launched programs to save water and review some resource allocations in order to meet growing needs. One of these programs is the launch of 10-95 law. It about the regulation of water discharges by establishing a financing instrument based on “the polluter pays” principle. This law has forced several industries, to install wastewater treatment plants (WWTP) for their water discharges to the extent that they meet the standards defined by Law 10-95. On this work, we will present an analysis of the operation of a wastewater treatment plant in a Moroccan dairy, the issues that have arisen and the solutions that have been provided. The analysis of the influential of the WWTP during the different phases of treatment gave results, which allowed us to optimize the performance of each phase and reduce the cost of running the WWTP.
... A lower OC burial efficiency with increased O 2 exposure is supported by both field observations [6] and theoretical arguments [8]. Experimental studies have demonstrated that some fractions of organic matter are degraded less efficiently under anoxic conditions [9][10][11]. This observation is in line with thermodynamic arguments that certain OC compounds only yield enough energy to support microbial growth if they are coupled to the reduction of O 2 [12] and that some OC bonds only can be broken by oxygenase enzymes that require O 2 as substrate [13]. ...
Organic carbon (OC) burial efficiency, which relates the OC burial rate to respiration in the seafloor, is a critical parameter in the reconstruction of past marine primary productivities. The current accepted theory is that sediments underlying oxygen-deficient (anoxic) bottom waters have low respiration rates and high OC burial efficiencies. By combining novel in situ measurements in anoxic basins with reaction-transport modelling, we demonstrate that sediments underlying anoxic bottom waters have much higher respiration rates than commonly assumed. A major proportion of the carbon respiration is concentrated in the top millimeter—the so-called ‘reactive surface layer’—which is likely a feature in approximately 15% of the coastal seafloor. When re-evaluating previously published data in light of our results, we conclude that the impact of bottom-water anoxia on OC burial efficiencies in marine sediments is small. Consequently, reconstructions of past marine primary productivity in a predominantly anoxic ocean based on OC burial rates might be underestimated by up to an order of magnitude.
... Soil depth and marsh zone had no effects on k (two-way ANOVA, p>0.1), which shows that the initial decomposition rate of labile plant inputs is not influenced by hydrology or the soil redox status. This finding agrees with previous studies demonstrating the effects of oxygen availability on OM decomposition depends on OM quality and that labile materials decompose at similar rates in oxic and anoxic environments (Benner et al., 1984;Kristensen et al., 1995). By contrast, S increased with flooding frequency (i.e., from the high marsh to pioneer zone) and with soil depth, indicating that the stabilization of labile materials does depend on the soil redox status. ...
Salt marshes play an important role in the global carbon cycle due to the large amount of organic carbon stored in their soils. Soil organic carbon formation in these coastal wetland ecosystems is strongly controlled by the plant primary production and initial decomposition rates of plant be-lowground biomass and litter. This study used a field warming experiment to investigate the response of belowground litter breakdown to rising temperature (+1.5 and +3.0 • C) across whole-soil profiles (0-60 cm soil depth) and the entire intertidal flooding gradient ranging from the pioneer zone via the low marsh to high marsh. We used standardized plant materials, following the Tea Bag Index approach, to assess the initial decomposition rate (k) and the stabilization factor (S) of labile organic matter inputs to the soil system. While k describes the initial pace at which labile (= hydrolyzable) organic matter decomposes, S describes the part of the la-bile fraction that does not decompose during deployment in the soil system and stabilizes due to biochemical transformation. We show that warming strongly increased k consistently throughout the entire soil profile and across the entire flooding gradient, suggesting that warming effects on the initial decomposition rate of labile plant materials are independent of the soil aeration (i.e., redox) status. By contrast, negative effects on litter stabilization were less consistent. Specifically, warming effects on S were restricted to the aer-ated topsoil in the frequently flooded pioneer zone, while the soil depth to which stabilization responded increased across the marsh elevation gradient via the low to high marsh. These findings suggest that reducing soil conditions can suppress the response of belowground litter stabilization to rising temperature. In conclusion, our study demonstrates marked differences in the response of initial decomposition rate vs. stabilization of labile plant litter to rising temperature in salt marshes. We argue that these differences are strongly mediated by the soil redox status along flooding and soil-depth gradients.
... Without organic matter (OM) addition during the experiment, we also observed that the total organic carbon (TOC) content 275 was lower in the upper sediment layer than in the deep layer likely due to the positive influence of oxygen availability on the mineralization of OM in sediments. Indeed, the aerobic mineralization of sedimentary OM is known to be faster than anaerobic mineralization, irrespective of the degree of lability of OM (Kristensen et al., 1995). The vertical distribution of dissolved oxygen in sediments was thus determinant on OM dynamics and the structure of microbial communities. ...
Bioturbation processes influence particulate (sediment reworking) and dissolved (bioirrigation) fluxes at the sediment-water interface. Recent works showed that benthic foraminifera largely contribute to sediment reworking in 20 intertidal mudflats; yet their role in bioirrigation processes remains unknown. In a laboratory experiment, we showed that foraminifera motion-behavior increased the oxygen penetration depth and decreased the total organic content. Their activity in the top 5 mm of the sediment also affected prokaryotic community structure. Indeed, in bioturbated sediment, bacterial richness was reduced and sulfate reducing taxa abundance in deeper layers was also reduced, probably inhibited by the larger oxygen penetration depth. Since foraminifera can modify both particulate and dissolved fluxes, their role as bioturbators can 25 no longer be neglected. They are further able to mediate the prokaryotic community, suggesting that they play a major role in the benthic ecosystem functioning and may be the first described single-celled eukaryotic ecosystem engineers.
... This presumably reflect the higher efficiency of aerobic versus anaerobic respiration (e.g. Kristensen et al. 1995). In most studies, the assessment of benthic carbon mineralization is derived from the benthic O 2 consumption rate, and the RQ used for converting O 2 consumption to equivalents of carbon mineralization varies extensively in the literature. ...
Seaweed farming is a growing industry worldwide, and its sustainable management requires detailed knowledge about the environmental implications of detrital release. This study investigates benthic degradation of kelp detritus in defaunated mesocosms. The degradation dynamics were investigated over several weeks by resolving O 2 and dissolved inorganic carbon (DIC) fluxes as a function of detritus amendments (0.15 g wet weight [WW] m ⁻² to 1 kg WW m ⁻² ), temperature (8 and 15°C), and presence of O 2 for 2 commercially important kelp species: Saccharina latissima and Alaria esculenta . Kelp fragments were deposited in 2 different ways to simulate oxic and anoxic degradation: on the sediment surface (surface amendments) and just below the oxic surface sediment layer (subsurface amendments). All amendments resulted in high initial O 2 consumption followed by an exponential decrease in O 2 uptake over time. The degradation rates increased linearly with the amount of kelp added for both species and for both types of amendments. S. latissima expressed higher decay constants across all experiments and had a higher percentage turnover of carbon. In some instances, microbial priming apparently enabled enhanced degradation of pre-existing resilient sedimentary carbon. The absolute degradation rates of kelp were reduced in the absence of O 2, and sulfate reduction resulted in gradual accumulation of iron sulfide. Lower ambient temperature reduced the benthic mineralization rate of both kelp species, particularly during the initial incubation stages. The current study demonstrates the importance of key variables for microbial kelp degradation in marine sediments and their dynamics—variables that should be carefully considered when assessing environmental implications of seaweed farming.
... Such a spatial variation is in accordance with an earlier work 3 . Further, nutrient-richness favours the microbial growth and activities 14,15 . Hence, Manoli Island that is rich in nutrients exhibited high counts of cyanobacteria, and Kurusadi island supported high diversity perhaps due to low nutrient loading of nitrogen and phosphorus as a result of less anthropogenic impact, which is in accordance with earlier work of 16 . ...
The present work analysed diversity of cyanobacteria in rhizosphere soil samples, collected from four mangrove species from seven islands of Gulf of Mannar Biosphere Reserve, southeast India. A total of 103 cyanobacterial species belonging to 17 families were identified with 35 unicellular, 21 heterocystous and 47 non-heterocystous forms. The cyanobacterial count was positively correlated with temperature, nitrogen, phosphorus, potassium and total organic carbon, and negatively correlated with salinity and trace metals. The cyanobacterial counts varied with islands and mangrove species and also with environmental parameters, nutrients and heavy metals. Soil salinity was the maximum (48±4.5 ppt) in Shingle Island and the minimum (25±3.1ppt) in Manoli Island but the temperature exhibited a reverse trend. Soil temperature was the lowest (25.1±3.1OC) in Shingle Island and the highest (31.5±4.1OC) in Manoli Island. The Manoli Island exhibited the maximum soil pH (8.8±1.7) and Eh (-131.2±34 mV). The soil pH was minimum in Kurusadai Island (6.7±1.1) while Eh was the lowest in Pullivasal Island (-198.2±41 mV).
... Usually in the terrestrial OM, the C/N ratio ranges from 20 to 200 (Hedges et al., 1986;Kim et al., 2006) unlike the low ratio of < 4 to 6 that is autochthonously produced in marine ecosystem (Elser et al., 2000). The OM rich in nitrogenous material such as microalgae with low TOC/TN ratio is known to favor net bacterial mineralization, whereas those poor in nitrogen such as of terrestrial origin with high TOC/TN ratio favors net bacterial immobilization (Kristensen et al., 1995). Lobbes et al. (2000) proposes TOC/TN ratio of 9.5 to fresh OM derived from phytoplankton and bacteria., while any ratio above 9.5 must be implicated due to degraded OM derived from marine detrital material (Lobbes et al., 2000). ...
Different fractions of organic matter in surface sediments from three transects along the eastern margin of the Arabian Sea (AS) were quantified to determine the sources of organic matter, and also to study its impact on microbial community structure. From the extensive analyses of different biochemical parameters, it was evident that the distribution of total carbohydrate (TCHO), total neutral carbohydrate (TNCHO), proteins, lipids, and uronic acids (URA) concentrations and yield (% TCHO-C/TOC) are affected by organic matter (OM) sources and microbial degradation of sedimentary OM. Mono-saccharide compositions from surface sediment was quantified to assess the sources and diagenetic fate of carbohydrates, suggesting that the deoxysugars (rhamnose plus fucose) had significant inverse relationship (r = 0.928, n = 13, p < 0.001) with hexoses (mannose plus galactose plus glucose) and positive relationship (r = 0.828, n = 13, p < 0.001) with pen-toses (ribose plus arabinose plus xylose). This shows that marine microorganisms are the source of carbohydrates and there is no influence of terrestrial OM along the eastern margin of AS. During the degradation of algal material, the hexoses seem to be preferentially used by heterotrophic organisms in this region. Arabinose plus galactose (glucose free wt %) values between 28 and 64 wt% indicate that OM was derived from phytoplankton, zooplankton, and non-woody tissues. In the principal component analysis, rhamnose, fucose, and ribose form one cluster of positive loadings while glucose, galactose, and mannose form another cluster of negative loadings which suggest that during OM sinking process, hexoses were removed resulting in increase in bacterial biomass and microbial sugars. Results indicate sediment OM to be derived from marine microbial source along the eastern margin of AS.
... Fe-P fraction controlled by organic matter plays a major role in the release of P from sediments. Indeed, the anaerobic degradation of organic matter or the solubilization of the Fe-coated OP releases Fe-P in the sediments of the bay [58,59]. Authors such as Malecki et al. [60] and Zhang et al. [59] generally reported that at the onset of anaerobic conditions, the P released was relatively large. ...
Rapid economic development and rampant urbanization are accelerating the eutrophication of coastal environments. This is the case of the bay of M'Badon in Côte d'Ivoire, located in an area not far from an open dump, which is under strong anthropogenic pressure. However, no data is available on nitrogen and phosphorus distribution in the water column and sediments of the M'Badon bay. This study not only targets the distribution of phosphorus and nitrogen, but also the specia-tion of phosphorus in sediments in order to assess its bioavailability. In order to do this, the waters of the bay of M'Badon were analyzed with a spectrophotometer and the speciation of phosphorus in the sediments was made according to the method of Williams. The results showed that PO 4 3− , TP, NO 3 − , NO 2 − and TN concentrations in the water column varied significantly among the seasons. While, that of NH 4 + did not vary between the seasons. In the water column, inorganic phosphorus (0.94 ± 0.12 mg L −1) is high and represents 65.3% of the TP. In addition, inorganic nitrogen concentration (0.33 ± 0.14 mg L −1) was low and organic nitrogen constituted 85% of TN in the water column. The results also indicated that Fe-P concentration (522.8 ± 233.4 μg g −1) represented the highest fraction of phosphorus in sediments with a percentage of 59.5%. Potential bioavailable P accounted for an average of 95.4% of TP in sediments. So, M'badon bay is an important reservoir of bioavailable phosphorus which might accentuate the eutrophication during several decades in the bay.
... This was about 12 times faster than the anaerobic OM remineralization (0.09 ± 0.03 mmol•C (L sediment) −1 • d −1 ) estimated based on 34 days of anoxic incubation. Our results are similar to a previous study of fine sands (from False Bay, US), using a thin-layer incubation technique (Kristensen et al., 1995), in which the authors reported that aerobic OM remineralization was~10 times faster than under anoxic conditions. Higher rate of OM remineralization under oxic conditions compared to anoxic conditions is well known (e.g. ...
Permeable sandy sediments cover 50-60 % of the global continental shelf and are important bioreactors that regulate organic matter (OM) turnover and nutrient cycling in the coastal ocean. In sands, the dynamic porewater advection can cause rapid mass transfer and variable redox conditions, thus affecting OM remineralization pathways, as well as the recycling of iron and phosphorus. In this study, North Sea sands were incubated in flow-through reactors (FTRs) to investigate biogeochemical processes under porewater advection and changing redox conditions. We found that the average rate of anaerobic OM remineralization was 12 times lower than the aerobic pathway, and Fe(III) oxyhydroxides were found to be the major electron acceptors during 34 days of anoxic incubation. Reduced Fe accumulated in the solid phase (expressed as Fe(II)) before significant release of Fe2+ into the porewater, and most of the reduced Fe (~96 %) remained in the solid phase throughout the anoxic incubation. Fe(II) retained in the solid phase, either through the formation of authigenic Fe(II)-bearing minerals or adsorption, was easily re-oxidized upon exposure to O2. Excessive P release (apart from OM remineralization) started at the beginning of the anoxic incubation and accelerated after the release of Fe2+ with a constant P/Fe2+ ratio of 0.26. After 34 days of anoxic incubation, porewater was re‑oxygenated and > 99 % of released P was coprecipitated through Fe2+ oxidation (so-called "Fe curtain"). Our results demonstrate that Fe(III)/Fe(II) in the solid phase can serve as a relatively immobile and rechargeable "redox battery" under dynamic porewater advection. This Fe "redox battery" is characteristic for permeable sediments and environments with variable redox conditions, making Fe an important player in OM turnover. We also suggest that P liberated before Fe2+ release can escape the "Fe curtain" in surface sediments, thus potentially increasing net benthic P efflux from permeable sediments under variable redox conditions.
... Soil depth and marsh zone had no effects on k (two-way ANOVA, p > 0.1), which shows that the initial decomposition rate of labile plant inputs is not influenced by hydrology or the soil redox status. This finding agrees with previous studies demonstrating the effects of oxygen availability on OM decomposition depends on OM quality, and that labile materials decompose at similar rates in oxic and anoxic environments (Benner et al., 1984;Kristensen et al., 1995). By contrast, S increased with flooding frequency (i.e. from high marsh to pioneer zone) and with soil depth, indicating that the stabilization 200 of labile materials does depend on the soil redox status. ...
Salt marshes play an important role in the global carbon (C) cycle due to the large amount of C stored in their soils. Soil C input in these coastal wetland ecosystems is strongly controlled by the production and initial decomposition rates of plant belowground biomass and litter. This study used a field warming experiment to investigate the response of belowground litter breakdown to rising temperature (+1.5 °C and +3.0 °C) across whole-soil profiles (0–60 cm soil depth) and the entire flooding gradient ranging from pioneer zone via low marsh to high marsh. We used standardized plant materials, following the Tea Bag Index approach, to assess the initial decomposition rate of (k) and the stabilization factor (S) of labile organic matter (OM) inputs to the soil system. While k describes the initial pace at which labile (= hydrolyzable) OM decomposes, S describes the part of the labile fraction that does not decompose during deployment in the soil system and stabilizes due to biochemical transformation. We show that warming strongly increased k consistently throughout the entire soil profile and across the entire flooding gradient, suggesting that warming effects on the initial decomposition rate of labile plant materials are independent of the soil aeration (i.e. redox) status. By contrast, negative effects on litter stabilization were less consistent. Specifically, warming effects on S were restricted to the aerated topsoil in the frequently flooded pioneer zone, while the soil depth to which stabilization responded increased across the marsh elevation gradient via low to high marsh. These findings suggest that reducing soil conditions can suppress the response of belowground litter stabilization to rising temperature. In conclusion, our study demonstrates marked differences in the response of initial decomposition rate vs. stabilization of labile plant litter to rising temperature in salt marshes. We argue that these differences are strongly mediated by the soil redox status along flooding and soil-depth gradients.
... For example, we only observed significant effects of soil water content and pH on A-values instead of k-values (Table 3). It was probably because, compared to labile organic material, the breakdown of aged and recalcitrant organic matter was more dependent on the actions of heterotrophic microorganisms and thus more sensitive to the absence of oxygen and level of pH (Kristensen et al., 1995;Mueller et al., 2018). Such results allude to the complexity and nuances in the decomposition processes, and highlight the importance of linking decomposition of specific litter components to their corresponding predictors. ...
Litter decomposition is a fundamental process underpinning multiple ecosystem services. Despite a long history of research on decomposition, direct and indirect effects of multiple interactive land management on wetland decomposition yet remain less well understood. Here, we used a long-term whole-ecosystem wetland experiment in south-central Florida to investigate interactive effects of land-use intensification, cattle grazing and prescribed fire on in situ wetland plant litter decomposition. We further examined the direct and indirect pathways of land management effects on litter decomposition through changes in associated litter traits, soil properties, and soil microbial attributes using structural equation models. We used the litterbag technique that quantifies decomposition rates (k-values) and recalcitrant fractions (A-values). Our results showed that land-use intensification increased k-values in ungrazed wetlands and decreased k-values in grazed wetlands, but consistently reduced A-values regardless of other treatments. Prescribed fire individually suppressed litter decomposition by reducing k and increasing A. Further, these effects occurred through altering litter, soil, and microbial properties. Our results revealed that litter traits and soil properties were the first two strongest factors in determining wetland decomposition processes. Particularly, litter P and Mg contents and soil P and K contents were the best predictors for k, while litter Ca and lignin contents and soil pH, N and water content best predicted A. Moreover, microbial traits exhibited interactive effects with litter and soil properties to affect wetland litter decomposition. Our research suggests that cattle grazing could buffer against stimulating effect of land-use intensification on decomposition rates and thus avoid nutrient releases pulses. Our study further indicates that land-use intensification and fire suppression in subtropical wetlands could promote organic matter depletion and thus nutrient loss, highlighting the need to reduce anthropogenic disturbances to natural wetlands to maintain their capacity for providing associated regulating and supporting services.
... On average, about 10% more SOM was degradable when oxygen was available as terminal electron acceptor. This could be explained by the more favourable thermodynamics with the larger energy gain of the aerobic degradation pathway and the fact that under anaerobic conditions hydrolysis and fermentation of structurally complex organic matter are rate-limiting (Kristensen, 1995). Complete mineralization as well as degradation of larger molecules (i.e., lignin) is only accomplished under aerobic conditions. ...
Anaerobic sediment organic matter decay generates methane, delays sediment consolidation, reduces sediment density, viscosity and shear strength, all impacting the sediment rheological parameters and the navigable depth. This study quantifies the share of anaerobically and aerobically degradable sediment organic matter (SOM) in a depth profile and along a transect through the tidal river Elbe in the section of the Port of Hamburg. From exponential organic matter decay functions, organic matter decay rates (mg C gTOC⁻¹ d⁻¹) were derived and clustered with a k-Means Cluster Analysis. The reactivity of different (kinetic) organic matter pools along the river transect were characterized based on their biodegradation rates. A fast, medium, slowly and non-degradable pool (pools 1–4) were identified based on the measured organic matter lability. SOM lability decreased from upstream to downstream, evidenced by the decreasing amount of the easily degradable pool 1 material from upstream to downstream. The size of the slowly degradable pool 3, assumed to be associated with SOM bound to the mineral particles, did not show any spatial gradient and is therefore suggested to represent a baseline share of hardly accessible SOM in the investigation area (about 12%−16% of TOC). Total degradability thus appears to be governed by the amount of SOM present in addition to this basis (pool 3), which in turn follows a source gradient and an age gradient from upstream to downstream. The recalcitrant pool 4 was the largest at any part of the harbour, for any depth, and for both, anaerobic and aerobic conditions (about 75%−85% of TOC). This indicates that the sediment in the investigation area, including the uppermost fluidic and freshly settled layers, mostly comprises stabilised organic matter and contributes largely to storage of organic carbon. Differently sized anaerobic SOM pools with depth were observed as well as seasonal changes of the easily degradable SOM pool 1. The degradability was larger in upper sediment layers, it was also larger under aerobic conditions (by about 10% of TOC) but the differences between aerobic and anaerobic decay decreased from upstream to downstream.
... Finally, we found that δ 15 N (‰) explained part of the gene composition in the dataset, with δ 15 N (‰) values closer to 0 being an indicator of higher organic matter content derived from N 2 -fixation such as by cyanobacteria [83]. These findings explain why the Dead Zones stations had lower δ 15 N (‰) values as these areas can accumulate large quantities of algal material, including diazotrophic organisms [9,79,84] that potentially remains longer in the sediment because degradation is slower under anoxic conditions [85,86]. These findings indicate that the expansion of oxygen deficient waters have long-term effects on benthic microbial communities and the composition of functional genes. ...
Background:
Microorganisms in the seafloor use a wide range of metabolic processes, which are coupled to the presence of functional genes within their genomes. Aquatic environments are heterogenous and often characterized by natural physiochemical gradients that structure these microbial communities potentially changing the diversity of functional genes and its associated metabolic processes. In this study, we investigated spatial variability and how environmental variables structure the diversity and composition of benthic functional genes and metabolic pathways across various fundamental environmental gradients. We analyzed metagenomic data from sediment samples, measured related abiotic data (e.g., salinity, oxygen and carbon content), covering 59 stations spanning 1,145 km across the Baltic Sea.
Results:
The composition of genes and microbial communities were mainly structured by salinity plus oxygen, and the carbon to nitrogen (C:N) ratio for specific metabolic pathways related to nutrient transport and carbon metabolism. Multivariate analyses indicated that the compositional change in functional genes was more prominent across environmental gradients compared to changes in microbial taxonomy even at genus level, and indicate functional diversity adaptation to local environments. Oxygen deficient areas (i.e., dead zones) were more different in gene composition when compared to oxic sediments.
Conclusions:
This study highlights how benthic functional genes are structured over spatial distances and by environmental gradients and resource availability, and suggests that changes in, e.g., oxygenation, salinity, and carbon plus nitrogen content will influence functional metabolic pathways in benthic habitats.
... We did not measure how far oxygen penetrates into the sediment but based on our experiences from other oligotrophic lakes in Indonesia and literature data (Corzo et al. 2018), we assume that penetration depth is much less than 5 mm. Still, even with such a thin oxygenated surface layer, a significant part of the sedimentary organic matter will be degraded aerobically because aerobic degradation is much faster (Kristensen et al. 1995), decreasing the overall concentration of organic matter in deeper anoxic layers and increasing the fraction of more recalcitrant material (Westrich and Berner 1984). The diverse lithology of the different catchments is partially reflected in the element composition (Fig. 3) and the clustering results (Fig. 7). ...
Tropical Lake Sentani in the Indonesian Province Papua consists of four separate basins and is surrounded by a catchment with a very diverse geology. We characterized the surface sediment (upper 5 cm) of the lake’s four sub-basins based on multivariate statistical analyses (principal component analysis, hierarchical clustering) of major element compositions obtained by X-ray fluorescence scanning. Three types of sediment are identified based on distinct compositional differences between rivers, shallow/proximal and deep/distal lake sediments. The different sediment types are mainly characterized by the correlation of elements associated with redox processes (S, Mn, Fe), carbonates (Ca), and detrital input (Ti, Al, Si, K) derived by river discharge. The relatively coarse-grained river sediments mainly derive form the mafic catchment geology and contribution of the limestone catchment geology is only limited. Correlation of redox sensitive and detrital elements are used to reveal oxidation conditions, and indicate oxic conditions in river samples and reducing conditions for lake sediments. Organic carbon (TOC) generally correlates with redox sensitive elements, although a correlation between TOC and individual elements change strongly between the three sediment types. Pyrite is the quantitatively dominant reduced sulfur mineral, monosulfides only reach appreciable concentrations in samples from rivers draining mafic and ultramafic catchments. Our study shows large spatial heterogeneity within the lake’s sub-basins that is mainly caused by catchment geology and topography, river runoff as well as the bathymetry and the depth of the oxycline. We show that knowledge about lateral heterogeneity is crucial for understanding the geochemical and sedimentological variations recorded by these sediments. The highly variable conditions make Lake Sentani a natural laboratory, with its different sub-basins representing different depositional environments under identical tropical climate conditions.
... The degradation was initially dominated by a DOC Porewater plume (0.3 ± 0.3 mol C, or 27% of highly reactive OM, as calculated in the decay model Fig. 2) and this labile DOC was quickly remineralised into inorganic C (alkalinity or CO 2 ). The subsequent production of DIC Porewater (0.25 ± 0.07 mol C) in the second phase showed that the initially released DOC was likely converted into DIC Porewater (Kristensen et al. 1995) at depths of 100 cm. Processing of the DOC can also be observed from the rising CH 4Porewater at a depth of 100 cm from day 20 onwards, as the beginning of the second peak. ...
Marine macroalgae are a key primary producer in coastal ecosystems, but are often overlooked in blue carbon inventories. Large quantities of macroalgal detritus deposit on beaches, but the fate of wrack carbon (C) is little understood. If most of the wrack carbon is respired back to CO2, there would be no net carbon sequestration. However, if most of the wrack carbon is converted to bicarbonate (alkalinity) or refractory DOC, wrack deposition would represent net carbon sequestration if at least part of the metabolic products (e.g., reduced Fe and S) are permanently removed (i.e., long-term burial) and the DOC is not remineralised. To investigate the release of macroalgal C via porewater and its potential to contribute to C sequestration (blue carbon), we monitored the degradation of Ecklonia radiata in flow-through mesocosms simulating tidal flushing on sandy beaches. Over 60 days, 81% of added E. radiata organic matter (OM) decomposed. Per 1 mol of detritus C, the degradation produced 0.48 ± 0.34 mol C of dissolved organic carbon (DOC) (59%) and 0.25 ± 0.07 mol C of dissolved inorganic carbon (DIC) (31%) in porewater, and a small amount of CO2 (0.3 ± 0.0 mol C; ca. 3%) which was emitted to the atmosphere. A significant amount of carbonate alkalinity was found in porewater, equating to 33% (0.27 ± 0.05 mol C) of the total degraded C. The degradation occurred in two phases. In the first phase (days 0–3), 27% of the OM degraded, releasing highly reactive DOC. In the second phase (days 4–60), the labile DOC was converted to DIC. The mechanisms underlying E. radiata degradation were sulphate reduction and ammonification. It is likely that the carbonate alkalinity was primarily produced through sulphate reduction. The formation of carbonate alkalinity and semi-labile or refractory DOC from beach wrack has the potential to play an overlooked role in coastal carbon cycling and contribute to marine carbon sequestration.
Graphical abstract
... Acetate is another important organic carbon source in the ocean and can be produced during the anaerobic degradation of complex organic matter and especially during fermentation (Sansone, 1986;Canfield, 1993;Kristensen et al., 1995). This organic carbon source is especially important in sediments (Boschker, 2001), but also has been reported in the water column (Wu et al., 1997;Zhuang et al., 2019). ...
Anoxic marine zones (AMZs) constitute pelagic systems distinguished from the oxygen minimum zones (OMZs) by the complete absence of detectable oxygen and the accumulation of nitrite in mid-waters. At the top of the oxygen-depleted layer and below the oxycline, nutrients are abundant; light intensity is very much reduced (<1% of incident light) and a secondary chlorophyll maximum (SCM) is developed. The shoaling of the oxygen-depleted layer, product of the AMZ expansion, could enhance this SCM, which has little-known biogeochemical effects. Here, we show that the SCM is contributing a measurable signal in the particulate organic carbon (POC), enough to alter the δ13CPOC in the top of the oxygen-depleted layer. This data showed significant differences among stations with and without the development of a SCM, being 3.0‰ heavier when a SCM is developed, and indicating photosynthetic activity and/or remineralization in the top of the AMZ. More depleted δ13CPOC values were also found when no SCM was present indicating stronger chemoautotrophic activity, potentially driven by anammox and sulfur-oxidizing bacteria activity. Assimilation rate data show that when sufficient light and Prochlorococcus are present, photosynthesis exceeds chemoautotrophic carbon fixation, and can exceed heterotrophic assimilation of glucose or acetate. However, in the majority of the stations, assimilation rates of both glucose and acetate exceeded carbon fixation rates under light stimulation, suggesting that often the SCM is still a net heterotrophic system.
... and 72-97% [60]. The average degradation rate constants of the three fractions at the global scale are estimated to be 70, 0.5, and 0.001 yr − 1 , respectively [61], and oxygen deficiency is known to reduce the degradation rate by a factor of 3-10 [62,63]. Thus, to maximize carbon residuals, it is necessary to examine structures that maintain anaerobic conditions. ...
As climate change has attracted increasing global attention, carbon neutral ports (CNPs) have been set as government policies in Japan. In this research, with the aim of providing a more reliable basis for policy making concerning ports, we proposed the carbon storage through stable containment of organic carbon present in dredged soil as a new climate change countermeasure in ports. Furthermore, it was revealed through scenario analysis that the containment effect through beneficial utilization of dredged soil as foundation materials for blue carbon ecosystems (BCEs) and carbon storage by BCEs have significant meanings for the realization of CNPs and sinks for the residual emissions, considering the necessity to promote dredging projects for the development of the next generation of import bases toward the realization of CNPs, which may result in the generation of a large amount of dredged soil. In a country like Japan, whose port policies and port governance make it difficult to actively promote climate change countermeasures by each port, the feasibility of these new countermeasures would depend on the government’s initiative. It would be a world-leading effort to focus on port development in addition to port operation, and utilizing the dredged soil and BCEs as carbon sinks.
... Slow bacterial hydrolysis of structurally complex organic matter and prevailing reducing conditions influences the anaerobic decomposition rates and primarily determines the stability of particulate organic matter in the mangrove water columns (Kristensen et al. 1995). In particular, high DOC concentrations and DOC/POC ratios in the mangrove waters of COR indicated rapid transformation of organic carbon from particulate to more labile dissolved forms. ...
The distribution and possible sources of particulate organic carbon (POC) and particulate nitrogen (PN) in seven mangroves ecosystems along the east and west coast of India were examined, to understand their contribution to coastal biogeochemistry. Suspended particulate matter (SPM) concentration in mangrove waters were about ~ 1.6-fold higher in west coast (Gulf of Kachchh (GOK), Mandovi-Zuari (MA-ZU) and Karwar-Kumta (KR-KU)], whereas the mean POC content in SPM along east coast [Sundarbans (SUN), Bhitarkanika (BHK), Coringa (COR) and Pichavaram-Muthupet (PI-MU)] was nearly two times higher than the west coast (1.97 ± 0.91% and 1.06 ± 0.29%), respectively. The results indicated that the influence of the land-based contaminants on the water quality parameters (dissolved oxygen, pH, salinity, nutrients and chlorophyll-a, etc.), which primarily regulated the distribution and transformation of organic carbon in these mangrove waters. Among the studied systems, an extremely high DOC/POC ratio (5.72 ± 1.64) with low pH and DO in COR waters clearly indicated the labile nature of the organic matter influenced by anthropogenic stress. Strong correlation between POC and PN indicated a similar origin in particulate organic matter. The ratios of POC/PN and POC/Chl-a showed significant spatial variation ranging from 5.5 to 18.7 and 126 to 1057, respectively. The results indicated that significant fraction of in-situ primary production contributed to particulate organic matter (POM) pool in all Indian mangrove waters except the GOK and the SUN waters, where sediment resuspension and mangrove derived organic matter were the dominant POM sources.
The present systematic literature review (SLR) synthesized the literature on mangrove litterfall production and decomposition from studies published between 1985 and 2023 following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Key questions about biophysical, chemical, and anthropogenic/societal factors influencing nutrient cycling via litterfall production and decomposition in mangrove forests were addressed. The SLR included 332 peer-reviewed original and review articles from the ScienceDirect, PubMed, and Google Scholar databases. The United States of America had the highest relative count (RC, 31.32%), followed by Japan (8.79%) and Indonesia (8.24%), and the lowest RCs were found in Bangladesh, Kenya, Philippines, and Thailand. We showed the increasing trend on these topics and discussed the milestones to enhance our understanding of litterfall production and decomposition processes and inform future research endeavors in the context of climate change. A positive trajectory for understanding litterfall production and decomposition for effective decision-making and management strategies towards mangrove conservation and sustainable use is also discussed. Ten-year research prospects were also identified, including studies on impacts of pollution, habitat degradation, climate change, and other destructive human activities. The trend in studies about mangrove litterfall production and decomposition suggests the growing recognition of mangroves’ ecological and societal importance. Future advancements can be made to better understand the biophysical, chemical, and anthropogenic factors influencing litterfall production and decomposition through the identified future research directions. Finally, the findings of the present review are relevant to supporting effective conservation and management strategies for mangroves in a changing climate.
Bioturbation processes influence particulate (sediment reworking) and dissolved (bioirrigation) fluxes at the sediment-water interface. Recent works showed that benthic foraminifera largely contribute to sediment reworking in intertidal mudflats, yet their role in bioirrigation processes remains unknown. In a laboratory experiment, we showed that foraminifera motion behaviour increased the oxygen penetration depth and decreased the total organic content. Their activity in the top 5 mm of the sediment also affected prokaryotic community structure. Indeed, in bioturbated sediment, bacterial richness was reduced, and sulfate-reducing taxa abundance in deeper layers was also reduced, probably inhibited by the larger oxygen penetration depth. Since foraminifera can modify both particulate and dissolved fluxes, their role as bioturbators can no longer be neglected. They are further able to mediate the prokaryotic community, suggesting that they play a major role in the benthic ecosystem functioning and may be the first described single-celled eukaryotic ecosystem engineers.
Sea level rise driven by global warming is threatening low-lying and reclaimed agricultural areas near coasts. The most marginalized of these with low crop yield can be converted into new valuable wetland ecosystems with high CO2 mitigation capacity by removing drainage systems or performing managed realignment. This study assessed CO2 and CH4 dynamics before and after forming two adjacent wetlands by flooding reclaimed agricultural land. The Gyldensteen Coastal Lagoon (214 ha) is an open system flooded with seawater, and the neighboring Lake Engsø (144 ha) is a closed system flooded with freshwater. Before flooding in 2014, the total area was a net source of about 10,350 Mg CO2 yr−1 due to aerobic microbial soil respiration. The temporal and spatial pattern of CO2 and CH4 release changed after flooding in a pattern that depended on soil biogeochemistry, temperature, and other environmental factors. Thus, there were strong exponential temperature dependencies of CO2 and CH4 emissions. Slow anaerobic microbial action in the Coastal Lagoon soil and the presence of sulfate prevented CO2 and CH4 emission, leading to a slight net uptake of CO2 in 2019 (−70 Mg CO2 yr−1). Conversely, methanogenesis near the soil–water interface after freshwater flooding of Lake Engsø drove rapid emission of CH4 (diffusive and ebullitive) that doubled its greenhouse gas emissions. In conclusion, CH4 emissions in Lake Engsø therefore counteracted the CO2 mitigation effect by flooding and the total area remains a net source of greenhouse gases with an emission of 8330 CO2-equivalents yr−1.
Saline lakes across the globe have experienced severe reduction in their surface area as a result of climate change and human-induced perturbations like water diversion and extraction. The changing lake volume is predicted to have large-scale implication on the in-lake biogeochemistry. This study explores the carbon (C) and nitrogen (N) cycling in a desiccating hypersaline lake (Sambhar Lake, India) along with adjacently located brine reservoir and salt pans by measuring concentrations and stable isotopic ratios of different C and N pools during winter and monsoon. Incubation experiments to estimate the net nitrification and mineralization rates in lake sediments were also performed. The Lake witnessed a large decrease in surface area and showed a clear signature of desiccation on lake biogeochemistry. Both particulate and dissolved fractions of C and N in the lake increased as the lake desiccated from monsoon to winter. Low N isotopic composition (δ15N) of particulate organic matter during winter suggested the presence of N2 fixers in this nutrient-rich saline environment. Taken together, significant difference in C and N concentrations and isotopic compositions were observed across the lake, brine reservoir, and salt pans, suggesting considerable modulation of in-lake processes due to human interventions.
Aerobic denitrification is being investigated as a novel biological nitrogen removal process, yet the knowledge on aerobic denitrification is limited to pure culture isolations and its occurrence in bioreactors remains unclear. This study investigated the feasibility and capacity of applying aerobic denitrification in membrane aerated biofilm reactor (MABR) for biological treatment of quinoline-laden wastewater. Stable and efficient removals of quinoline (91.5 ± 5.2%) and nitrate (NO3-) (86.5 ± 9.3%) were obtained under different operational conditions. Enhanced formation and function of extracellular polymeric substances (EPS) were observed at increasing quinoline loadings. MABR biofilm was highly enriched with aerobic quinoline-degrading bacteria, with a predominance of Rhodococcus (26.9 ± 3.7%) and secondary abundance of Pseudomonas (1.7 ± 1.2%) and Comamonas (0.94 ± 0.9%). Metagenomic analysis indicated that Rhodococcus contributed significantly to both aromatic degradation (24.5 ± 21.3%) and NO3- reduction (4.5 ± 3.9%), indicating its key role in aerobic denitrifying quinoline biodegradation. At increasing quinoline loadings, abundances of aerobic quinoline degradation gene oxoO and denitrifying genes of napA, nirS and nirK increased; there was a significant positive correlation of oxoO with nirS and nirK (p < 0.05). Aerobic quinoline degradation was likely initiated by hydroxylation, encoded by oxoO, followed by stepwise oxidations through 5,6-dihydroxy-1H-2-oxoquinoline or 8-hydroxycoumarin pathway. The results advance our understanding of quinoline degradation during biological nitrogen removal, and highlight the potential implementation of aerobic denitrification driven quinoline biodegradation in MABR for simultaneous removal of nitrogen and recalcitrant organic carbon from coking, coal gasification and pharmaceutical wastewaters.
Dredged soil was used as the base material for the construction of tidal flats. We investigated the carbon residual rate when the organic matter contained in dredged soil was incorporated in the tidal flats created in Hannan 2 ku, 17 years after completion. Vertical samples of the dredged soil layer were collected using a handy geoslicer with a length of 3 m. The ORP of the dredged soil layer was from -100 to -150mv. The ignition loss was nearly uniform at about 7%. The carbon residual rate was determind using the ignition loss as alternative indexes for the carbon content. Consequently, the carbon residual rate was found to be 82% based on the difference the ignition loss between at the time of dredging and in this survey. It was shown that using dredged soil as the base material for the tidal flat may store the carbon effectively.
It is crucial to clarifying the interactions among nitrogen, dissolved organic matter, and microorganisms to understand the ecological response mechanisms of degenerating large shallow lakes. This study focused on the spatial patterns of several nitrogen forms in Honghu Lake (Hubei province, China) in combination with the sediment physical and chemical properties. The dissolved organic matter (DOM) composition, the humification degree and the bacterial and eukaryotic communities responses were analyzed. The results showed that the spatial distribution of nitrogen was related to the particle size composition. The maximum mean value of total nitrogen appeared in the southwest region (4813.77 mg·kg⁻¹). DOM was mainly composed of humic-like substances, with a high degree of humification at the entrance of the inflowing lake (projection value peaked at 2.22). Structural equation model analysis also showed that the direct and indirect effects of the microenvironment physical and chemical properties and nitrogen and DOM jointly explained the changes in bacterial community diversity. However, there were no similar spatial distribution characteristics between eukaryotes and environmental parameters, suggesting a single, irregular dominant species and a low level of diversity. This suggests an internal ecological response of nitrogen, DOM, and microorganisms in the degenerating lake environment.
Research on marine biodegradation of plastics often has to rely on laboratory‐accelerated test environments. This chapter addresses the question of biodegradation of commodity plastics varieties typically encountered in the ocean environment. The process of biodegradation of plastics in the marine environment is conveniently described in terms of three main steps, though these steps often occur simultaneously in nature. These are: biofilm formation or surface fouling; biotransformation; and uptake and mineralization. Plastics that float in seawater may become negatively buoyant as a result of adhering encrustation of biofilm species. The thermoplastic aromatic polyester used in plastic bottles as well as in textile fiber is a major contaminant in the marine sediment. Respirometry that allows measuring the of rates of mineralization can readily distinguish between the rapidly biodegradable and effecively nonbiodegradable plastics in the laboratory. In respirometry, biomineralization of the substrate occurs in a bioreactor or a flask maintained at constant temperature in the laboratory.
In rivers, scale-dependent feedbacks resulting from physical habitat modifications control the lateral expansion of submerged plant patches, while the mechanisms that limit patch expansion on a longitudinal dimension remain unknown. Our objective was to investigate the effects of patch length on physical habitat modification (i.e., flow velocity, sediment grain size distribution), the consequences for biogeochemical conditions (i.e., accumulation/depletion of nutrients, microbial respiration), and for individual plants (i.e., shoot length). We measured all of these parameters along natural patches of increasing length. These measurements were performed at two sites that differed in mean flow velocity, sediment grain size, and trophic level. The results showed a significant effect of patch length on organic matter content and nutrient concentrations in interstitial water. For the shortest patches sampled, all of these parameters had similar values to those measured at the upstream control position. For longer patches, organic matter content and orthophosphate and ammonium concentrations increased within the patch compared to the upstream bare sediment, whereas nitrate concentrations decreased, suggesting changes in vertical water exchanges and an increase in anaerobic microbial activities. Furthermore, plant height was related to patch length by a quadratic pattern, probably due reduced hydrodynamic stress occurring for increasing patch length, combined with conditions that are less favourable for plants over a threshold length, possibly due to the light limitation or to the high concentration of ammonium that in the concentration range we measured may be toxic for plants. The threshold lengths over which patches influence the nutrient concentrations were reduced for the site with higher nutrient levels. We demonstrated that the plant-induced modifications of the physical habitat exert important effects on biogeochemical conditions, with possible consequences for patch dynamics and ecosystem functioning.
Anoxic and oxic degradation pathways of sedimentary chloropigments were examined by spiking marine sediment with ¹⁴ C‐labeled algal cells and purified chloropigments from the diatom Skeletonema costatum. These experiments suggest that Chl a degrades through multiple pathways. Under oxic conditions, most bulk sedimentary Chl a degraded to various colorless compounds and only a minor fraction degraded to pheophytin a ; added ¹⁴ C‐labeled Chl a also degraded quickly, but 30–40% of this Chl a was converted to pheophytin a. Under anoxic conditions, only a small fraction of bulk Chl a degraded, but added ¹⁴ C‐labeled Chl a continuously degraded and ~30–40% of it was converted to pheophytin a. Pheophytin a is relatively stable under anoxic conditions but degrades under oxic conditions, thus it is a potential end product of chloropigment degradation in anoxic environments.
Degradation pathways are likely dependent on the relative proportion of unassociated Chl a to chlorophyll complexes present in the sediment. Only unassociated Chl a appears to be available for anoxic decomposition. Under oxic conditions, some colorless products were further degraded and solubilized; none of the ¹⁴ C label added as purified pigments was lost under anoxic conditions during the 1‐month incubation. About 80% of the acetone‐extractable ¹⁴ C in labeled cells was lost in 1 month from sediments under oxic conditions and ~30% under anoxic conditions.
The biodegradation of purified radiolabelled membrane lipids from a methanogenic bacterium and a pseudomonad were investigated in mangrove, beach and high marsh marine sediments under aerobic and anaerobic conditions. The effect of organic matter on the amount and rate of degradation was also examined by supplementing beach sediments with humic acids. In aerobic sediments, CO2 was the major product of lipid degradation while under anaerobic conditions both CO2 and CH4 were major end products and the overall rates were reduced (up to 40%) relative to aerobic conditions. Total bacterial numbers increased during all incubations with the largest increases occurring in anaerobic sediments supplemented with humic acids. No lipid degradation occurred in aerobic or anaerobic sediments treated with formaldehyde or autoclaving. In low organic beach sediments, the ester-linked phospholipid of the pseudomonad was degraded much more rapidly than the diphytanyl glycerol diether of the methanogen with 69% of the phospholipid degraded in 96 hours versus only 4% of the methanogen lipid. Lipid degradation in both aerobic and anaerobic sediments was highly correlated to organic matter content with increasing amounts of organic matter inhibiting degradation. Long incubations (75 days) of the diphytanyl glycerol ether resulted in 51% degraded to CO2 in low (0.5%) organic mangrove sediments while only 9% was mineralized in high (10.8%) organic marsh sediments. Physicochemical sorption of membrane lipids to the organic matrix is proposed as a mechanism which protects membrane lipids from microbial attack and degradation.
The relation between productivity, O2 and organic carbon preservation has been the subject of great controversy. One theory has been that anoxic decomposition of organic matter is intrinsically slower than oxic decomposition and thus results in the accumulation of organic carbon in anoxic sediments and waters. However, several past studies suggest that differences in the intrinsic rates of decomposition between the two types of systems are small. Here, I provide further evidence of this by measuring microbial metabolism of individual radiolabeled compounds in the oxic and anoxic waters of stratified water bodies. The lack of a rate difference suggests the need for further explanation of the controls on carbon preservation. The concept is presented that anoxic sediments may sequester organic matter as bacterial biomass, or as bacterially derived products, in the absence of bacterial grazers. Thus, differences in the numbers and diversity of organisms that graze upon bacteria between oxic and anoxic sediments may explain part of the difference in carbon preservation rates that have been observed between the two types of systems.
We used a combination of porewater and solid phase analysis, as well as a series of sediment incubations, to quantify organic carbon oxidation by dissimilatory Fe reduction, Mn reduction, and sulfate reduction, in sediments from the Skagerrak (located off the northeast coast of Jutland, Denmark). In the deep portion of the basin, surface Mn enrichments reached 3.5 wt%, and Mn reduction was the only important anaerobic carbon oxidation process in the upper 10 cm of the sediment. In the less Mn-rich sediments from intermediate depths in the basin, Fe reduction ranged from somewhat less, to far more important than sulfate reduction. Most of the Mn reduction in these sediments may have been coupled to the oxidation of acid volatile sulfides (AVS), rather than to dissimilatory reduction. High rates of metal oxide reduction at all sites were driven by active recycling of both Fe and Mn, encouraged by bioturbation. Recycling was so rapid that the residence time of Fe and Mn oxides, with respect to reduction, ranged from 70-250 days. These results require that, on average, an atom of Fe or Mn is oxidized and reduced between 100-300 times before ultimate burial into the sediment. We observed that dissolved Mn2+ was completely removed onto fully oxidized Mn oxides until the oxidation level of the oxides was reduced to about 3.8, presumably reflecting the saturation by Mn2+ of highly reactive surface adsorption sites. Fully oxidized Mn oxides in sediments, then, may act as a cap preventing Mn2+ escape. We speculate that in shallow sediments of the Skagerrak, surface Mn oxides are present in a somewhat reduced oxidation level (< 3.8) allowing Mn2+ to escape, and perhaps providing the Mn2+ which enriches sediments of the deep basin.
Reductive and oxidative pathways of the sulfur cycle were studied in a marine sediment by parallel radiotracer experiments with SO(4), H(2)S, and S(2)O(3) injected into undisturbed sediment cores. The distributions of viable populations of sulfate- and thiosulfate-reducing bacteria and of thiosulfate-disproportionating bacteria were concurrently determined. Sulfate reduction occurred both in the reducing sediment layers and in oxidized and even oxic surface layers. The population density of sulfate-reducing bacteria was >10 cm in the oxic layer, high enough that it could possibly account for the measured rates of sulfate reduction. The bacterial numbers counted in the reducing sediment layers were 100-fold lower. The dominant sulfate reducers growing on acetate or H(2) were gas-vacuolated motile rods which were previously undescribed. The products of sulfide oxidation, which took place in both oxidized and reduced sediment layers, were 65 to 85% S(2)O(3) and 35 to 15% SO(4). Thiosulfate was concurrently oxidized to sulfate, reduced to sulfide, and disproportionated to sulfate and sulfide. There was a gradual shift from predominance of oxidation toward predominance of reduction with depth in the sediment. Disproportionation was the most important pathway overall. Thiosulfate disproportionation occurred only as cometabolism in the marine acetate-utilizing sulfate-reducing bacteria, which could not conserve energy for growth from this process alone. Oxidative and reductive cycling of sulfur thus occurred in all sediment layers with an intermediate "thiosulfate shunt" as an important mechanism regulating the electron flow.
Among the fundamental goals of microbial ecology is the development of methods that will enable the identification and counting of the important microorganisms in nature, the determination of their physical and chemical microenvironment, and the analysis of their metabolic processes and interactions. Due to the small size of the organisms, much effort has been devoted to the development of high-resolution techniques for the observation and understanding of the world of bacteria on a microscale. Scanning and transmission electron microscopy and fluorescent staining, immunofluorescence and other techniques for light microscopy have been the most successful in terms of reaching a high spatial resolution. With respect to our understanding of the microbial microenvironments and of the nature of the microorganisms that carry out the measured metabolic activities, there is still a long way to go. Most chemical and radiotracer techniques in use today operate on a centimeter or at best on a millimeter scale and in most cases their results cannot be directly related to the relevant microorganisms. One notable exception to this is the combined use of autoradiography and fluorescence microscopy on microbial communities.
The Earth’s atmosphere is affected by the presence of psychotropic chemicals, both licit and illicit substances, not only in major city centres but also in suburban and rural regions. Dedicated analytical procedures, most of them based on gas or liquid chromatography coupled to mass spectrometry, have been optimised for the detection of these substances. Nicotine and caffeine (licit substances), are widespread in the world at concentrations sometimes reaching 100ng/m3. Conversely, drugs of abuse (namely cocaine, cannabinoids, heroin and amphetamines, which are in most countries illicit) rarely exceed 1ng/m3 each. However, their presence in airborne particles is virtually ubiquitous in agreement with what was observed in the past for surface and waste waters. The spatial and temporal variability of psychotropic substances in the atmosphere has been an object of study in different types of urban areas, whereas data are scarcer for rural areas. In the current ambient concentrations, personal exposure to airborne drugs of abuse may be considered negligible, posing no harm to human health. The possibility of drawing abuse prevalence indicators from the drug contents in the air merits, however, to be explored.
The depth variation of total organic carbon (TOC), organic matter composition, and porewater composition in marine sediments suggests that different components of the organic matter undergo decomposition at widely different rates. The decomposition of 14C-labeled organic substances was followed in sediment microcosms in the laboratory. The substances used were chosen to simulate a portion of material settling to the sediment-water interface (a marine diatom) or hypothesized components of refractory sediment organic matter (melanoidins and a bacterial polymer). The microcosms were found to be good models of the sediment-water interface in terms of how well they mimicked sediment decomposition rates and processes. The decomposition of the labeled material and the natural sediment TOC were monitored over 1 month: the water overlying the sediment remained oxic, and net consumption of nitrate was small. There was no detectable sulfate reduction. The algae and the bacterial polymer were decomposed on average 9 x faster than the melanoidins and 90 x faster than the natural sediment TOC. The soluble fraction of the algae was decomposed more rapidly than the particulate material.
This paper addresses three related questions: (1) What factors control the efficiency of carbon burial in sediments? (2) Are rates of anaerobic organic matter degradation intrinsically lower than aerobic rates? (3) How important are anaerobic processes in the global marine sediment carbon economy?Carbon burial efficiency (the ratio of the carbon burial rate and the carbon flux to the sediment surface) was estimated from literature data for a range of environments and was shown to be a function of sedimentation rate. No difference independent of sedimentation rate was found between aerobic and anaerobic sediments.A review of recent microcosm and laboratory studies shows that anaerobic rates are not intrinsically lower than aerobic rates; fresh organic matter degrades at similar rates under oxic and anoxic conditions. Aerobic decomposition rates near the sediment surface are typically greater than anaerobic rates at depth because the most labile carbon is consumed before it can be buried in the anoxic zone.A model approach was taken in estimating the importance of anaerobic processes in the global marine sediment economy, instead of extrapolating measured rates as done previously. The result, 150 Tg C yr, is two to nine times lower than previous estimates. This rate is about 9% of the global aerobic carbon oxidation rate and is about equal to the rate of long‐term carbon burial. The importance of anaerobic processes in marine sediments lies in their role in determining the amount of carbon preserved, not in the amount of carbon remineralized overall.
Laboratory study of the bacterial decomposition of Long Island Sound plankton in oxygenated seawater over a period of 2 years shows that the organic material undergoes decomposition via first‐order kinetics and can be divided into two decomposable fractions, of considerably different reactivity, and a nonmetabolizable fraction. This planktonic material, after undergoing varying degrees of oxic degradation, was added in the laboratory to anoxic sediment taken from a depth of 1 m at the NWC site of Long Island Sound and the rate of bacterial sulfate reduction in the sediment measured by the ³⁵ S radiotracer technique. The stimulated rate of sulfate reduction was in direct proportion to the amount of planktonic carbon added. This provides direct confirmation of the first‐order decomposition, or G model, for marine sediments and proves that the in situ rate of sulfate reduction is organic‐matter limited. Slower sulfate reduction rates resulted when oxically degraded plankton rather than fresh plankton was added, and the results confirm the presence of the same two fractions of organic matter deduced from the oxic degradation studies.
Near‐surface Long Island Sound sediment, which already contains abundant readily decomposable organic matter, was also subjected to anoxic decomposition by bacterial sulfate reduction. The decrease in sulfate reduction rate with time parallels decreases in the amount of organic matter, and these results also indicate the presence of two fractions of organic carbon of distinctly different reactivity. From plots of the log of reduction rate vs. time two first‐order rate constants were obtained that agree well with those derived from the plankton addition experiment. Together, the two experiments confirm the use of a simple multi‐first‐order rate law for organic matter decomposition in marine sediments.
At the 60th day of anaerobic decomposition of dead cells of Scenedesmus sp. at 20C, the 30% of added algal cell carbon was transformed into dissolved organic carbon and 20% mineralized; 50% remained as particulate matter. On the other hand, 8% of the added algal cell nitrogen was transformed into dissolved organic nitrogen, 48% was mineralized, and 44% remained in particulate form.
The dissolved organic compounds consisted mainly of lower fatty acids and yellowish acidic substances. Some proteinaceous material was found. Anaerobic decomposition patterns are compared with those under aerobic conditions and suggest the presence of relatively high concentrations of dissolved organic matter in anaerobic natural environments.
Rates of bacterial sulfate reduction are calculated from the accumulation of reduced sulfur compounds in coastal sediments. The method is found to underestimate the in situ metabolism 10‐fold because it neglects diffusional losses of produced sulfide. In relation to this result, the quantitative connection between the pyrite, HCl‐extractable iron, and organic carbon contents of the sediments and the intensity of sulfate reduction are disussed. A comparison is made between colony counts of sulfate‐reducing bacteria and the rate of sulfate reduction in coastal sediments. The number of bacteria is roughly proportional to their measured rate of metabolism both when different sediment types and different depths are compared. The colony counts, however, seem to underestimate the true numbers of sulfate reducers by 1000‐fold or more.
• The rate and extent of decomposition of both axenic algal cultures and mixed cultures of algae, bacteria, and zooplankton are described. Culture ages ranged from a few days to seven months, and the growth media from a synthetic fresh-water to seawater. Algae and algal-related organic matter were composed of three fractions: a small fraction which respires within a few hours in the dark, a large fraction which decomposes slowly within a year, and a large refractory fraction which decomposes only a few percent per year. The refractory fraction varied from 12 to 87% (mean, 40%) with the higher values obtained for very young or very old cultures. The decomposition of the biodegradable portion of mixed cultures grown in the presence of bacteria and zooplankton followed first-prder kinetics. The decay rate was a function of culture age and was higher for young cultures (K′ = 0.01 to 0.06 day-1) than for old cultures (0.01 to 0.03 day-1). The decomposition of pure cultures followed second-order kinetics during initial stages of decomposition, but subsequent decomposition could be described well with the first-order kinetics.
The major objective was to determine the rate and extent of algal degradation under simulated natural conditions. Decomposition of heterogeneous and unialgal cultures was studied under dark, anaerobic, constant-temperature laboratory conditions. Effects of high sulfate concentration, bacterial seedings, temperature, pH, and cell composition on the rate and extent of degradation were evaluated. After 200 days, decomposition of algal cultures was essentially complete, and the undecomposed particulate organic matter remaining was termed the refractory organic fraction. This fraction ranged from 20 to 60% of the ash-free dry weight for various culture with an average of 40%. The decomposition of the biodegradable organic fraction could be adequately described by first-order decay kinetics with a range for the decay constant k of 0.011-0.032/d with an average 0.022/day. The rate and extent of degradation were similar to those found by other investigators under aerobic decomposition conditions.
Vertical fluxes of bulk particulate material, organic carbon, nitrogen, lignin‐derived phenols, and neutral sugars through the water column and into surface sediments of Dabob Bay, Washington, were determined monthly for 1 yr by sediment trap deployments at 30, 60, and 90 m at a site 110 m deep. Vertical fluxes of sinking bulk particulate material in this marine bay were elevated during winter and increased in consistent proportion to sediment trap deployment depth throughout the year. Although annual average particle fluxes at 30 and 60 m bracketed the mean accumulation rate of the underlying sediment, the flux at 90 m was higher by a factor of 2 due to resuspension, horizontal advection, or both.
The monthly fluxes of lignin‐derived phenols paralleled those of total particulate material, indicating a common riverine origin. The annual average fluxes of vanillyl and cinnamyl phenols through the water column closely matched the corresponding accumulation rates in the underlying sediment, whereas about a third of the total syringyl phenol input was degraded at the water‐ sediment interface. Although p ‐hydroxyacetophenone exhibited a stability typical of lignin‐derived phenols, the distinctly higher reactivities (> 60% degradation) of p ‐hydroxybenzaldehyde and p ‐hydroxybenzoic acid indicate a predominantly nonlignin source.
On average, 60 and 70%, respectively, of the total particulate organic carbon and nitrogen and 65–75% of all neutral sugars settling through the midwater column were degraded at the water‐ sediment interface. The elemental and carbohydrate composition of the degraded material was similar to that of local net plankton except for higher percentages of glucose and total neutral sugars. Land‐derived organic material accounted for about one‐third of the total organic carbon passing through the midwater column and two‐thirds of the organic carbon accumulating in the underlying sediments. The amounts of plankton‐derived organic matter sinking through the midwater column and being preserved in the sediments below corresponded to 14 and 3% of the annual mean primary productivity. Plankton‐derived organic matter exhibited about 5 times the reactivity of local land‐derived organic matter at the water‐sediment interface of Dabob Bay and supported essentially all of the benthic respiration.
The dynamics of benthic primary production and community respiration in a shallow oligotrophic, marine lagoon (Fællestrand, Denmark) was followed for 1·5 years. The shape of the annual primary production cycle was explained primarily by seasonal changes in temperature (r2 = 0·67-0·72) and daylength (r2 = 0·63), whereas temperature almost explained all variation in benthic community respiration (r2 = 0·83-0·87). On a daily basis the benthic system was autotrophic during spring and summer supplied by 'new' and 'regenerated' nitrogen and predominantly heterotrophic during fall and winter caused by light and nutrient limitation. The linear depth-relationship between porewater alkalinity and ammonium indicated that the C:N ratio of mineralized organic matter is low in spring and summer (3-6) and high in fall and winter (9-16). This is inversely related to net primary production and thus the input of labile, nitrogen-rich algal cells. Accordingly, mineralization occurred predominantly in the upper 2-5 cm of the sediment. The pool of reactive material (microalgal cells) was estimated to account for 12% of total organic carbon in the upper 3 cm, and had an average turnover time of less than 1 month in summer. Assimilation of organic carbon by benthic animals was equivalent to about 30% of the annual gross primary production. Grazing reduced chlorophyll a concentration in the sediment during summer and spring to values 30-40% lower than in winter, but maintained a 3-4 times higher specific microalgal productivity. The rapid turnover of organic carbon and nitrogen, and important role of benthic microalgae showed that the benthic community in this oligotrophic lagoon is of a very dynamic nature.
A variety of field and laboratory observations demonstrate that particle reworking and irrigation activities of benthic fauna promote the remineralization of organic matter. Of the many simultaneous factors involved, repetitive oscillation of redox conditions may be one of the most important. In bioturbated Corg-rich sediments with restricted O2 penetration, particles constantly cycle between oxic and anoxic zones but typically spend ∼ 10−100 × longer under anoxic than oxic conditions. Cyclic redox patterns are also common within individual burrow structures and are accompanied by rapid switching in dominant metabolic processes. Geometrically and temporally complex redox mosaics are the rule. Experimental evidence and theoretical considerations indicate that even brief, periodic re-exposure to O2 results in more complete and sometimes rapid decomposition than is possible under constant conditions or unidirectional redox change. Redox oscillation apparently results initially in net remineralization of existing microbial biomass followed by stimulated renewed synthesis (self-priming) in a manner similar to many disturbances or grazing effects. Some properties, such as sedimentary P storage, are comparable under fully oxic and oscillating redox conditions but differ under anoxic. The relative frequency and duration of redox change are presumably critical properties governing response. Redox oscillation common in bioturbated sediments or the terrestrial rhizosphere likely represents a distinct functional environmental state with unique biogeochemical properties. Studies of decomposition and Corg preservation processes should take this possibility into account.
A brief outline is given of the use of the Technicon Autoanalyzer® for the determination of phosphate, silicate and nitrate and the use of this equipment, together with continuous measurements of temperature and chlorophyll, to record the properties of surface water from a ship underway. The continuous recording of surface properties gives promise of a valuable new method for studying nutrient enrichment and biological production in space and time over large areas of the sea surface in eutrophic waters.
Specifically radiolabeled [C-lignin]lignocelluloses and [C-polysaccharide]lignocelluloses were prepared from a variety of marine and freshwater wetland plants including a grass, a sedge, a rush, and a hardwood. These [C]lignocellulose preparations and synthetic [C]lignin were incubated anaerobically with anoxic sediments collected from a salt marsh, a freshwater marsh, and a mangrove swamp. During long-term incubations lasting up to 300 days, the lignin and polysaccharide components of the lignocelluloses were slowly degraded anaerobically to CO(2) and CH(4). Lignocelluloses derived from herbaceous plants were degraded more rapidly than lignocellulose derived from the hardwood. After 294 days, 16.9% of the lignin component and 30.0% of the polysaccharide component of lignocellulose derived from the grass used (Spartina alterniflora) were degraded to gaseous end products. In contrast, after 246 days, only 1.5% of the lignin component and 4.1% of the polysaccharide component of lignocellulose derived from the hardwood used (Rhizophora mangle) were degraded to gaseous end products. Synthetic [C]lignin was degraded anaerobically faster than the lignin component of the hardwood lignocellulose; after 276 days, 3.7% of the synthetic lignin was degraded to gaseous end products. Contrary to previous reports, these results demonstrate that lignin and lignified plant tissues are biodegradable in the absence of oxygen. Although lignocelluloses are recalcitrant to anaerobic biodegradation, rates of degradation measured in aquatic sediments are significant and have important implications for the biospheric cycling of carbon from these abundant biopolymers.
Determination of ammonia in natural waters by the phenolhypochlorite method
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