Sulfate Reduction In Marine Sediments Research Articles

Sulfate distribution and sulfate reduction in global marine sediments

The quantification of anaerobic oxidation of organic matter in the global seabed is to a large extent based on transport-reaction modeling of pore water ions involved in the mineralization processes. As the predominant of these processes, sulfate reduction can be modeled from the depth distribution of sulfate and other relevant solutes. An active organoclastic sulfate reduction is typically characterized by sulfate profiles with a concave-down shape. We use here a comprehensive database to show that half of all competent sulfate profiles have an opposite, concave-up shape, which may appear inconsistent with this concept. We also show that there is a strong discrepancy between sulfate profiles obtained by gravity coring and by IODP coring, with generally much higher sulfate flux and shallower sulfate penetration depth in the gravity cores (the term IODP here covers the international ocean drilling programs, DSDP, ODP and IODP). The discrepancy is correlated with a selectivity for coring sites in the published studies, by which gravity coring is focused towards the ocean margins and continental shelf and towards high surface water productivity and high sedimentation rate. In contrast, IODP coring is more frequently done in low sedimentation regions and is largely absent from the continental shelf. As a result, estimates of sulfate reduction in the global seabed may be biased by a selection of only IODP core data for the modeling. Furthermore, the modeling of pore water profiles shows net rates of sulfate reduction, while 35S-radiotracer experiments show the higher gross rates of the process. Estimates from 35S-radiotracer measurements suggest that dissimilatory sulfate reduction oxidizes 77 Tmol yr−1 of organic carbon globally, which is 3- to 4-fold higher than recent data based on modeling of IODP data. The potential biases by global estimates of sulfate reduction in marine sediments are discussed.

Sediment mineralogy influences the rate of microbial sulfate reduction in marine sediments

Sedimentary microbial communities play a critical role in the global carbon cycle, oxidizing deposited organic carbon and thus influencing the type of carbon buried from Earth's surface. The rate of microbial metabolism within sedimentary microbial communities is often linked to the lability and amount of organic carbon deposited. Here we show that, in pure culture, for sulfate-reducing bacteria (Desulfovibrio bizertensis) the rate of microbial sulfate reduction is a function of the proportion of clay minerals present in the incubation vials. We argue that the presence of clay minerals stimulates the growth of the sulfate-reducing bacteria and the rate at which sulfate is consumed; we conclude that this is not linked to nutrients and carbon on the clay minerals but rather is a function of the high specific surface area of clay minerals. We further use a global compilation of sedimentary pore fluid data to demonstrate that these observations can be seen in marine sediments, that is pore fluid sulfate concentration gradients in marine sediments correlate with the percentage of clay minerals in the sediment. Our findings suggest that sediment mineralogy influences the rate of microbial activity in marine sediments, which has heretofore not been considered.

Sulfate reduction rates in the sediments of the Mediterranean continental shelf inferred from combined dissolved inorganic carbon and total alkalinity profiles

Microbial sulfate reduction in marine sediments is coupled either to anaerobic oxidation of methane (S-AOM) or organic material (organoclastic sulfate reduction, OSR). These two pathways of sulfate reduction are important components of the geochemical cycles of both sulfur and carbon in marine systems because they change the redox state of both elements and increase the dissolved inorganic carbon (DIC) and total alkalinity (TA) concentrations in sediment pore water. Here, we determine reaction rates of OSR and S-AOM in the sulfate methane transition zone (SMTZ) of the sediment based upon the different effect of each process on DIC and TA. Although TA has been used as a diagnostic proxy for the identification and determination of the governing chemical processes in numerous previous studies, we show that it can also be applied to disentangle and quantify net sulfate reduction rates in the SMTZ, provided that OSR and S-AOM are the principal processes that affect DIC and TA, and that the effects of other processes, such as carbonate mineral precipitation, are quantified by additional data. By integrating the obtained rates, we determine the contribution of each pathway to the total sulfate reduction. Calculated results from pore water profiles from the Southeastern Mediterranean continental shelf indicate that within the SMTZ, located at a depth of approximately 1 m, each pathway accounted for about half of the total sulfate reduction. The calculated OSR and S-AOM rates were similar to estimations of sulfate reduction rates based upon pore water sulfate profiles from studies in other sedimentary systems, in spite of the differences between these environments and the Southeastern Mediterranean continental shelf, showing that TA and DIC are a robust method for calculating net sulfate reduction. Furthermore, we show here that TA considerations can be used to quantitatively constrain the fractions of reduced sulfate that eventually precipitates as pyrite and FeS, versus the fraction that is oxidized and precipitates as elemental sulfur.

The sulfur cycle below the sulfate-methane transition of marine sediments

The study of sulfate reduction below the sulfate-methane transition (SMT) in marine sediments requires strict precautions to avoid sulfate contamination from seawater sulfate or from sulfide oxidation during handling. We experimented with different methods of sampling porewater sulfate and found that modifications to our sampling procedure reduced the measured sulfate concentrations from hundreds of micromolar to ten micromolar or less. We here recommend some key modifications to porewater sampling to avoid contamination or oxidation artifacts, for example when measuring very low sulfate concentrations below the SMT of marine sediments. At three sites in Aarhus Bay, the sulfate concentrations below the SMT remained around ten micromolar. The calculated free energy change, ΔGr, available for sulfate reduction by such low concentrations is between −17.9 and −11.9 kJ mol−1 sulfate. This is near or below the energy yields that have previously been calculated for microbial sulfate reduction in marine sediments. The three sites are characterized by measurable and very different sulfate reduction rates depending on the depth and sediment age of the SMT. Our data show that sulfate is being consumed below the SMT in spite of the low sulfate concentrations. As sulfate is not drawn down to even lower concentrations, it must be continually regenerated below the SMT, most likely by Fe(III)-driven sulfide oxidation concurrent with the sulfate reduction. We conclude that the low sub-SMT sulfate concentrations are in steady state between reduction and production and are thermodynamically controlled by the minimum ΔGr requirements by sulfate reducing bacteria while sulfate reduction rates are controlled by the rate of sulfide oxidation. This study deals with the general sampling of uncontaminated pore water and hints to a systemic problem with low levels of contamination in porewater induced by coring and pore water extraction. Below the SMT, this contamination could be detected but above the SMT it goes unnoticed. Low levels of contamination as observed in this study may affect other low concentration or redox-sensitive elements in pore water.

Methane-metabolizing microbial communities in sediments of the Haima cold seep area, northwest slope of the South China Sea.

Cold seeps are widespread chemosynthetic ecosystems in the deep-sea environment, and cold seep microbial communities of the South China Sea are poorly constrained. Here we report on the archaeal communities, particularly those involved in methane metabolization, in sediments of a newly discovered cold seep (named 'Haima') on the northwest slope of the South China Sea. Archaeal diversity, abundance and distribution were investigated in two piston cores collected from a seep area (QDN-14B) and a non-seep control site (QDN-31B). Geochemical investigation of the QDN-14B core identified an estimated sulfate-methane transition zone (Estimated SMTZ) at 300-400 cm below sea floor (cmbsf), where a high abundance of anaerobic methane-oxidizing archaea (ANME) occurred, as revealed by analysis of the 16S rRNA gene and the gene (mcrA) encoding the α-subunit of the key enzyme methyl-coenzyme M reductase. ANME-2a/b was predominant in the upper and middle layers of the estimated SMTZ, whereas ANME-1b outcompeted ANME-2 in the sulfate-depleted bottom layers of the estimated SMTZ and the methanogenic zone. Fine-scale phylogenetic analysis further divided the ANME-1b group into three subgroups with different distribution patterns: ANME-1bI, ANME-1bII and ANME-1bIII. Multivariate analyses indicated that dissolved inorganic carbon and sulfate may be important factors controlling the composition of the methane-metabolizing community. Our study on ANME niche separation and interactions with other archaeal groups improves our understanding of the metabolic diversity and flexibility of ANME, and the findings further suggest that ANME subgroups may have evolved diversified/specified metabolic capabilities other than syntrophic anaerobic oxidation of methane coupled with sulfate reduction in marine sediments.

Evaluation and optimization of PCR primers for selective and quantitative detection of marine ANME subclusters involved in sulfate-dependent anaerobic methane oxidation

Since the discovery that anaerobic methanotrophic archaea (ANME) are involved in the anaerobic oxidation of methane coupled to sulfate reduction in marine sediments, different primers and probes specifically targeting the 16S rRNA gene of these archaea have been developed. Microbial investigation of the different ANME subtypes (ANME-1; ANME-2a, b, and c; and ANME-3) was mainly done in sediments where specific subtypes of ANME were highly enriched and methanogenic cell numbers were low. In different sediments with higher archaeal diversity and abundance, it is important that primers and probes targeting different ANME subtypes are very specific and do not detect other ANME subtypes or methanogens that are also present. In this study, primers and probes that were regularly used in AOM studies were tested in silico on coverage and specificity. Most of the previously developed primers and probes were not specific for the ANME subtypes, thereby not reflecting the actual ANME population in complex samples. Selected primers that showed good coverage and high specificity for the subclades ANME-1, ANME-2a/b, and ANME-2c were thoroughly validated using quantitative polymerase chain reaction (qPCR). From these qPCR tests, only certain combinations seemed suitable for selective amplification. After optimization of these primer sets, we obtained valid primer combinations for the selective detection and quantification of ANME-1, ANME-2a/b, and ANME-2c in samples where different ANME subtypes and possibly methanogens could be present. As a result of this work, we propose a standard workflow to facilitate selection of suitable primers for qPCR experiments on novel environmental samples.

Microbial Sulfate Reduction Potential in Coal-Bearing Sediments Down to ~2.5 km below the Seafloor off Shimokita Peninsula, Japan.

Sulfate reduction is the predominant anaerobic microbial process of organic matter mineralization in marine sediments, with recent studies revealing that sulfate reduction not only occurs in sulfate-rich sediments, but even extends to deeper, methanogenic sediments at very low background concentrations of sulfate. Using samples retrieved off the Shimokita Peninsula, Japan, during the Integrated Ocean Drilling Program (IODP) Expedition 337, we measured potential sulfate reduction rates by slurry incubations with 35S-labeled sulfate in deep methanogenic sediments between 1276.75 and 2456.75 meters below the seafloor. Potential sulfate reduction rates were generally extremely low (mostly below 0.1 pmol cm−3 d−1) but showed elevated values (up to 1.8 pmol cm−3 d−1) in a coal-bearing interval (Unit III). A measured increase in hydrogenase activity in the coal-bearing horizons coincided with this local increase in potential sulfate reduction rates. This paired enzymatic response suggests that hydrogen is a potentially important electron donor for sulfate reduction in the deep coalbed biosphere. By contrast, no stimulation of sulfate reduction rates was observed in treatments where methane was added as an electron donor. In the deep coalbeds, small amounts of sulfate might be provided by a cryptic sulfur cycle. The isotopically very heavy pyrites (δ34S = +43‰) found in this horizon is consistent with its formation via microbial sulfate reduction that has been continuously utilizing a small, increasingly 34S-enriched sulfate reservoir over geologic time scales. Although our results do not represent in-situ activity, and the sulfate reducers might only have persisted in a dormant, spore-like state, our findings show that organisms capable of sulfate reduction have survived in deep methanogenic sediments over more than 20 Ma. This highlights the ability of sulfate-reducers to persist over geological timespans even in sulfate-depleted environments. Our study moreover represents the deepest evidence of a potential for sulfate reduction in marine sediments to date.

Concurrent low- and high-affinity sulfate reduction kinetics in marine sediment

Bacterial sulfate reduction in marine sediments generally occurs in the presence of high millimolar concentrations of sulfate. Published data indicate that low sulfate concentrations may limit sulfate reduction rates below 0.2–2 mM. Yet, high sulfate reduction rates occur in the 1–100 μM range in freshwater sediments and at the sulfate–methane transition in marine sediments. Through a combination of 35S-tracer experiments, including initial velocity experiments and time course experiments, we searched for different sulfate affinities in the mixed community of sulfate reducers in a marine sediment. We supported the radiotracer experiments with a highly sensitive ion chromatographic technique for sulfate with a detection limit of 0.15 μM SO 4 2− in marine pore water. Our results showed that high and low affinities for sulfate co-occur and that the applied experimental approach may determine the observed apparent half saturation constant, K m. Our experimental and model data both show that sulfate reduction in the studied marine sediment could be explained by two dominating affinities for sulfate: a low affinity with a mean half saturation constant, K m, of 430 μM SO 4 2− and a high affinity with a mean K m of 2.6 μM SO 4 2−. The high-affinity sulfate reduction was thermodynamically un-constrained down to <1 μM SO 4 2−, both in our experiments and under in situ conditions. The reduction of radio-labeled sulfate was partly reversible due to concurrent re-oxidation of sulfide by Fe(III) and possibly due to a reversibility of the enzymatic pathway of sulfate reduction. A literature survey of apparent K m values for sediments and pure cultures is presented and discussed.

The significance of isotope specific diffusion coefficients for reaction-transport models of sulfate reduction in marine sediments

Modeling isotopic signatures in systems affected by diffusion, advection, and a reaction which modifies the isotopic abundance of a given species, is a discipline in its infancy. Traditionally, much emphasis has been placed on kinetic isotope effects during biochemical reactions, while isotope effects caused by isotope specific diffusion coefficients have been neglected. A recent study by Donahue et al. (2008) suggested that transport related isotope effects may be of similar magnitude as microbially mediated isotope effects. Although it was later shown that the assumed differences in the isotope specific diffusion coefficients were probably overstated by one or two orders of magnitude ( Bourg, 2008), this study raises several important issues: (1) Is it possible to directly calculate isotopic enrichment factors from measured concentration data without modeling the respective system? (2) Do changes in porosity and advection velocity modulate the influence of isotope specific diffusion coefficients on the fractionation factor α? (3) If one has no a priori knowledge whether diffusion coefficients are isotope specific or not, what is the nature and magnitude of the error introduced by either assumption? Here we argue (A) That the direct substitution of measured data into a differential equation is problematic and cannot be used as a replacement for a reaction-transport model; (B) That the transport related fractionation scales linearly with the difference between the respective diffusion coefficients of a given isotope system, but depends in a complex non-linear way on the interplay between advection velocity, and downcore changes of temperature and porosity. Last but not least, we argue that the influence of isotope specific diffusion coefficients on microbially mediated sulfate reduction in typical marine sediments is considerably smaller than the error associated with the determination of the fractionation factor.

Methanogenesis and sulfate reduction in marine sediments: A new model

A number of studies have shown that methanogens are active in the presence of sulfate under some conditions. This phenomenon is especially exemplified in carbonate sediments of the southern Australian continental margin. Three sites cored during Ocean Drilling Program (ODP) Leg 182 in the Great Australian Bight have high concentrations of microbially-generated methane and hydrogen sulfide throughout almost 500 m of sediments. In these cores, the sulfate-reducing and methanogenic zones overlap completely; that is, the usual sulfate-methane transition zone is absent. Amino acid racemization data show that the gassy sediments consist of younger carbonates than the low-gas sites. High concentrations of the reduced gases also occur in two ODP sites on the margin of the Bahamas platform, both of which have similar sedimentary conditions to those of the high-gas sites of Leg 182. Co-generation of these reduced gases results from an unusual combination of conditions, including: (1) a thick Quaternary sequence of iron-poor carbonate sediments, (2) a sub-seafloor brine, and (3) moderate amounts of organic carbon. The probable explanation for the co-generation of hydrogen sulfide and methane in all these sites, as well as in other reported environments, is that methanogens are utilizing non-competitive substrates to produce methane within the sulfate-reducing zone. Taken together, these results form the basis of a new model for sulfate reduction and methanogenesis in marine sediments. The biogeochemical end-members of the model are: (1) minimal sulfate reduction, (2) complete sulfate reduction followed by methanogenesis, and (3) overlapping sulfate reduction and methanogenesis with no transition zone.

Evidence for Anaerobic CH 4 Oxidation in Freshwater Peatlands

This study involved in vitro assays of peat soil to investigate the occurrence, importance and potential mechanism(s) of anaerobic methane oxidation (AOM) in several northern peatlands ranging from ombrotrophic bog to minerotrophic fen. Although strong evidence suggests that AOM is linked to sulfate reduction in marine sediments, very little is known about AOM in freshwater systems such as northern peatlands, which have large methane (CH 4 ) production and are a significant source of atmospheric CH 4 . Our results showed a mean net AOM rate of 17 ± 2.6 nmol kg − 1 s − 1 with a maximum rate of 176 nmol kg − 1 s − 1 for a minerotrophic fen in central New York. AOM was demonstrated with three independent methods to verify our results: (a) additions of methanogenic inhibitors, (b) stable isotope enrichment ( 13 C-CH 4 ), and (c) natural abundance stable isotope analysis ( 13 C-CH 4 ). These experiments confirmed that AOM occurs simultaneously with methanogenesis, consumes a significant portion of gross CH 4 production, and significantly fractionates C isotopes (∼ −127‰). Experiments using a variety of potential electron acceptors demonstrated that Fe(III) and SO4 2 − are not quantitatively important, while the role of NO 3 − is uncertain and deserves more attention. The exact mechanism(s) for AOM in peat soils remains unclear; however the AOM rates reported in this study are similar to those reported for CH 4 production and aerobic CH 4 oxidation in northern peatlands, suggesting that AOM may be an important control on CH 4 fluxes in northern peatland ecosystems.

High activity and low temperature optima of extracellular enzymes in Arctic sediments: implications for carbon cycling by heterotrophic microbial communities

The rate of the initial step in microbial remineralization of organic carbon, extracellular enzymatic hydrolysis, was investigated as a function of temperature in permanently cold sediments from 2 fjords on the west coast of Svalbard (Arctic Ocean). We used 4 structurally distinct polysaccha- rides (chondroitin sulfate, fucoidan, xylan and pullulan) to determine the temperature-activity re- sponses of hydrolysis of a related class of compounds. All 4 enzyme activities showed similarly low temperature optima in the range of 15 to 18°C. These temperature optima are considerably lower than most previous reports of temperature optima for enzyme activities in marine sediments. At 0°C, close to the in situ temperature, these enzyme activities achieved 13 to 38% of their rates at optimum temperatures. In one experiment, sulfate reduction rates were measured in parallel with extracellular enzymatic hydrolysis in order to determine the relative temperature responses of the initial and ter- minal steps in microbial remineralization of carbon. The temperature optimum of sulfate reduction, 21°C, was considerably lower than previous reports of sulfate reduction in marine sediments, but is consistent with recent studies of psychrophilic sulfate reducers isolated from Svalbard sediments. A calculation of potential carbon flow into the microbial food chain demonstrated that the activity of just one type of polysaccharide-hydrolyzing enzyme could in theory supply 21 to 100% of the carbon con- sumed via sulfate reduction across the temperature range investigated here. These characteristics suggest that these extracellular enzymes are well adapted to permanently cold temperatures.

Organic matter degradation through oxygen respiration, denitrification, and manganese, iron, and sulfate reduction in marine sediments (the Kattegat and the Skagerrak)

Abstract A comprehensive study is reported on organic matter degradation through O2 respiration, denitrification and manganese- iron- and sulfate reduction in continental shelf sediments at 10 localities in the Kattegat and the Skagerrak (Denmark, Sweden and Norway). Total sediment community mineralization ranged from 4 to 31 mmol C m-2 d-1 with an average DIC:O2 ratio of 1.1. Most sediments exhibited an oxygen penetration depth of 0.33-0.6 cm except for a deeper-water (695 m) locality in the Skagerrak where O2 penetrated 1.9 cm into the sediment. The ratio of total O2 uptake measured by intact flux core incubation to diffusive O2 uptake rates measured with microelectrodes varied from -1 to 3.5 due to differences in benthic animal activity at the stations. Denitrification activity ranged from 0.01 to 1.03 mmol N m-2 d-1 and was primarily regulated by sediment nitrification activity. Manganese reduction was only significant at the deep-water locality in the Skagerrak. Iron reduction rates of 0 to 9.6 mmol C m-2 d-1 were found, and the activity was largely determined by the amount of poorly crystalline Fe(III) in the sediment. Sulfate reduction activity ranged from 0.1 to 9.6 mmol SO4 2- m-2 d-1at the stations investigated. A large heterogeneity was found between stations regarding the relative importance of the different carbon oxidation pathways. Within the Kattegat/Skagerrak region one may find sites where any one of the four carbon oxidation pathways, heterotrophic oxygen respiration, manganese reduction, iron reduction and sulfate reduction, is dominant. Thus, the present study demonstrates that even within short geographical distances a large variation in the relative importance of the different carbon oxidation pathways is to be expected in marine sediments.

Effect of crude oil on denitrification and sulfate reduction in marine sediments

The denitrifying activity was measured in different types of sediment from the Mediterranean coast of France before, and after, a massive contamination (30–100 g kg-1 sediment) of hydrocarbons. A closed system was used in order to maintain anoxic conditions and to control substrates and gaseous products concentrations. We have demonstrated that the respiratory metabolism was inhibited in all cases following an incubation time of 20 to 50 days. At this time, the addition of lactate restore the denitrifying activity. The inhibitory effect of crude oil was not related to an alteration of bacterial cells, but to changes in environmental conditions allowing denitrification. The presence of hydrocarbons in the sediments causes a decrease in the redox potential and a concomitant stimulation of the sulfate reduction.

Sulfate reduction in marine sediments from the Baltic Sea-North Sea Transition

Abstract The spatial distribution and intensity of sulfate reduction was studied in the upper 0-15 cm of coastal sediments by radiotracer technique. The study area extended from the eastern North Sea through Skagerrak and Kattegat into the Belt Sea at the entrance to the Baltic Sea. The 27 sampling stations ranged from pure sand to clayey mud with organic contents from 0.2 to 13% dry weight. Water depths ranged from 7 to 200 m. Depth-integrated sulfate reduction rates were mostly 1-5 mmol m-2 d-1 in the Skagerrak-Kattegat-Belt Sea area. Lowest rates, 0.08-1.0 mmol m-2 d-1, were encountered in low-organic sands from exposed parts of the North Sea. Extremely high rates, 17-30 mmol m-2 d-1, were found in the Helgoland Bight, SE North Sea, near the mouth of the Elbe River. Intensive sulfate reduction was associated with areas offine-grained sediment and with high deposition rates of organic detritus, The degree of pyritization correlated positively to sulfate reduction rates (r = 0.63) and to FeS concentratio...

The effect of temperature on rates of sulfate reduction in marine sediments

Abstract Rates of sulfate reduction in sediments of Long Island Sound have been determined, as a function of temperature, via the 35S radiotracer method. Temperature dependence was measured either by following changes in rates in intact cores with seasonal changes in temperature, or by conducting laboratory experiments on homogenized sediments under controlled temperatures. At constant temperature, a large range of sulfate reduction rates were observed for the study sites. In both the intact cores and the laboratory experiments, sediments with lower rates of sulfate reduction exhibited a more pronounced temperature dependence. Apparent activation energies determined using the Arrhenius equation show a systematic trend with sulfate reduction rate when temperature was normalized. As the rate of sulfate reduction decreased, the apparent activation energy increased from 36 to 132 kJ/mole. Although this observation can be interpreted in a number of ways, it is suggested that susceptibility of organic matter to...

The dependence of bacterial sulfate reduction on sulfate concentration in marine sediments

The effect of dissolved sulfate concentration on the rate of bacterial sulfate reduction in marine sediment from Long Island Sound was examined using a radio-sulfur technique. The experimental results show that the rate is independent of the dissolved sulfate concentration until low levels are reached (<3 mM), and that, when interpreted using a Monod-type rate law, a saturation constant, K s , of 1.62 ± 0.16 M results. This weak dependence implies that the dissolved sulfate exerts only a limited influence on the rate of sulfate reduction in marine sediments. Given such a weak dependence, dissolved sulfate profiles in marine sediments must resemble profiles generated by models with sulfate independent kinetics. Initially, this would suggest that currently used sulfate-independent diagenetic models are appropriate in modelling sulfate profiles. However, comparison of these models with those containing weak sulfate-dependent kinetic terms shows that there exists considerable disagreement between these models when the parameter grouping (D sk) 1 2 /w is larger than ~0.2 and smaller than ~3.0. (Here D s is the SO ; 4 diffusion coefficient, k the organic matter decay constant and w the sediment burial velocity.) When the currently used models are corrected by employing physically meaningful boundary conditions, this divergence disappears. The modelling results, therefore, confirm the conclusion that any sulfate dependence inherent to the reduction kinetics does not appreciably affect sulfate pore water profiles, and that previous diagenetic studies using strong sulfate dependent models are erroneous.

Substrates for Sulfate Reduction and Methane Production in Intertidal Sediments

The activity of and potential substrates for methane-producing bacteria and sulfate-reducing bacteria were examined in marsh, estuary, and beach intertidal sediments. Slow rates of methane production were detected in all sediments, although rates of sulfate reduction were 100- to 1,000-fold higher. After sulfate was depleted in sediments, the rates of methane production sharply increased. The addition of methylamine stimulated methanogenesis in the presence of sulfate, and [C]methylamine was rapidly converted to CH(4) and CO(2) in freshly collected marsh sediment. Acetate, hydrogen, or methionine additions did not stimulate methanogenesis. [methyl-C]methionine and [2-C]acetate were converted to CO(2) and not to CH(4) in fresh sediment. No reduction of CO(2) to CH(4) occurred in fresh sediment. Molybdate, an inhibitor of sulfate reduction, inhibited [2-C]acetate metabolism by 98.5%. Fluoracetate, an inhibitor of acetate metabolism, inhibited sulfate reduction by 61%. These results suggest that acetate is a major electron donor for sulfate reduction in marine sediments. In the presence of high concentrations of sulfate, methane may be derived from novel substrates such as methylamine.

Diagenetic sulfate reduction in marine sediments

The importance of interstitial pore water as a vehicle for transport of dissolved ions between sediment at depth and the overlying ocean reservoir may be overestimated. The major cause of this overestimation is the assumption that the rate of ionic diffusion through the sediment is of a similar magnitude to the rate of reaction occurring within the sediment column. Examples are given to indicate that the above assumption may not be correct, at least for diagenetic sulfate reduction. Closed-system modeling, although approximate, is shown to be consistent, both qualitatively and isotopically, with measured changes that are observed for several sedimentary environments.

A comparison of methods for the quantification of bacterial sulfate reduction in coastal marine sediments

A radiotracer technique for measuring in situ rates of sulfate reduction in marine sediments is described. Microliter portions of labeled‐sulfate solution are injected into undisturbed sediment cores, and the amount of labeled sulfide produced is analyzed. This tracer method is experimentally compared with a method that mixes label into the sediment. Controls and methodological problems of both methods are discussed.