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Dimethylsulfoniopropionate (DMSP) is an abundant organic sulfur metabolite produced by many phytoplankton species and degraded by bacteria via two distinct pathways with climate-relevant implications. We assessed the diversity and abundance of bacteria possessing these pathways in the context of phytoplankton community composition over a 3-week time period spanning September-October, 2014 in Monterey Bay, CA. The dmdA gene from the DMSP demethylation pathway dominated the DMSP gene pool and was harboured mostly by members of the alphaproteobacterial SAR11 clade and secondarily by the Roseobacter group, particularly during the second half of the study. Novel members of the DMSP-degrading community emerged from dmdA sequences recovered from metagenome assemblies and single-cell sequencing, including largely uncharacterized gammaproteobacteria and alphaproteobacteria taxa. In the DMSP cleavage pathway, the SAR11 gene dddK was the most abundant early in the study, but was supplanted by dddP over time. SAR11 members, especially those harbouring genes for both DMSP degradation pathways, had a strong positive relationship with the abundance of dinoflagellates, and DMSP-degrading gammaproteobacteria co-occurred with haptophytes. This in situ study of the drivers of DMSP fate in a coastal ecosystem demonstrates for the first time correlations between specific groups of bacterial DMSP degraders and phytoplankton taxa.

The abundant marine organosulfur compound, dimethylsulfoniopropionate (DMSP) can be degraded to acrylate and dimethylsulfide (DMS), with some DMS further oxidized to dimethylsulfoxide (DMSO). Despite intensive study of DMSP and DMS in a variety of marine settings, the processes affecting acrylate and DMSO concentrations in marine waters are poorly known, particularly their loss from the dissolved phase through biological uptake. We measured the concentrations of dissolved acrylate (acrylated) and DMSO (DMSOd) in coastal and open-ocean waters of the northern Gulf of Mexico during non-bloom conditions and quantified the rates and kinetics of their biological uptake using 14C labeled substrates. Acrylated concentrations and uptake rates ranged from 0.8–2.1 nmol L−1 and 0.07–1.8 nmol L−1 d−1, respectively. Somewhat higher uptake rates were observed for DMSOd (0.27–3.9 nmol L−1 d−1) owing to higher DMSOd concentrations (5.5–14 nmol L−1). Both compounds were taken up by the microbial community with high affinity uptake systems, with similar Ks and Vmax values to those for other well-studied biological substrates including amino acids and monosaccharides. However, median turnover times were relatively slow, 4.8 d for acrylated and 7.4 d for DMSOd. The slow acrylated turnover points to low supply rates of this compound to the dissolved phase, a finding consistent with previous observations that the microbial DMSP lyase pathway accounts for only a small fraction of dissolved DMSP degradation (and therefore acrylate production) in the Gulf of Mexico.

Biogenic dimethylsulfide (DMS) and its main precursors, dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO), are potential scavengers of reactive oxygen species in marine algae, and these dimethylated sulfur compounds (DSC) could take part in the algal antioxidant system. In this study, a link between the DSC production and the antioxidant capacity (AOC) of Acropora aspera reef coral was investigated under a range of environmental factors (temperature, light, salinity, and air exposure) that can lead to oxidative stress in the coral holobiont. Enhanced DMS(P)(O) production occurred under experimental conditions, indicating that DSC are potential biomarkers of stress level in coral tissue. Differences in concentrations and partitioning as a response to different treatments suggest that DSC production and turnover undergo different biochemical pathways depending on the type and severity of environmental stress. Osmotic pressure and light depletion led to an upregulation of the coral AOC that was correlated with a significant increase in DMSO:DSC ratio. These results, combined with a positive correlation between the AOC and DMSO concentrations under these two treatments, suggest that the DMSP-based antioxidant system is involved in the overall antioxidant regulation of the coral holobiont. Enhanced DMS production coupled with an increased DMS:DSC ratio under increased temperature indicated that thermal stress triggers DMS formation in coral tissue. Considering the role that DMS can have in both climate regulation and the DMSP-based antioxidant system, our findings highlight the need to further examine the fate of DSC in coral reef environments under scenarios of increasing sea surface temperatures.

Summary Bacterioplankton consume a large proportion of pho- tosynthetically fixed carbon in the ocean and control its biogeochemical fate. We used an experimental metatranscriptomics approach to compare bacterial activities that route energy and nutrients during a phytoplankton bloom compared with non-bloom con- ditions. mRNAs were sequenced from duplicate bloom and control microcosms 1 day after a phy- toplankton biomass peak, and transcript copies per litre of seawater were calculated using an internal mRNA standard. Transcriptome analysis revealed a potential novel mechanism for enhanced efficiency during carbon-limited growth, mediated through membrane-bound pyrophosphatases (V-type H(+)- translocating; hppA); bloom bacterioplankton partici- pated less in this metabolic energy scavenging than non-bloom bacterioplankton, with possible implica- tions for differences in growth yields on organic sub- strates. Bloom bacterioplankton transcribed more copies of genes predicted to increase cell surface adhesiveness, mediated by changes in bacterial sig- nalling molecules related to biofilm formation and motility; these may be important in microbial aggre- gate formation. Bloom bacterioplankton also tran- scribed more copies of genes for organic acid utilization, suggesting an increased importance of this compound class in the bioreactive organic matter released during phytoplankton blooms. Transcription patterns were surprisingly faithful within a taxon regardless of treatment, suggesting that phylogeny broadly predicts the ecological roles of bacterial groups across 'boom' and 'bust' environmental backgrounds.

Microbes play critical roles in the biogeochemical cycling of nitrogen and sulfur in aquatic environments. Here we investigated the interaction between the naturally occurring organic sulfur compound methanethiol (MeSH) and the final step of the denitrification pathway, the reduction of nitrous oxide (N2 O) to dinitrogen (N2 ) gas, in sediment slurries from the temperate Douro and Ave estuaries (NW Portugal) and in pure cultures of the marine bacterium Ruegeria pomeroyi. Sediment slurries and cell suspensions were amended with a range of concentrations of either MeSH (0-120 µM) or methionine (0-5 mM), a known precursor of MeSH. MeSH or methionine additions caused N2 O to accumulate and this accumulation was linearly related to MeSH concentrations in both coastal sediments (R(2) = 0.7-0.9, P

Summary The fraction of dissolved dimethylsulfoniopropionate (DMSPd) converted by marine bacterioplankton into the climate-active gas dimethylsulfide (DMS) varies widely in the ocean, with the factors that determine this value still largely unknown. One current hypothesis is that the ratio of DMS formation : DMSP demethylation is determined by DMSP availability, with ‘availability’ in both an absolute sense (i.e. concentration in seawater) and in a relative sense (i.e. proportionally to other labile organic S compounds) proposed as the critical factor. We investigated these models during an experimentally induced phytoplankton bloom using a taxon-specific microarray targeting DMSP-related gene transcription in members of the Roseobacter clade, a group hypothesized to play an important role in the surface ocean sulfur cycle and well represented by genome sequences. The array consisted of 1578 probes to 431 genes and was designed to target diverse Roseobacter communities in natural seawater by using hierarchical probe design based on 13 genome sequences. The prevailing pattern of Roseobacter gene transcription showed relative depletion of DMSP-related transcripts during the peak of the bloom, despite increasing absolute concentrations and flux of DMSP-related compounds. DMSPd thus appeared to be assimilated by Roseobacter populations in proportion to its relative abundance in the organic matter pool (the ‘relative sense’ hypothesis), rather than assimilated in preference to other labile organic sulfur or carbon compounds produced during the bloom. The relative investment of the Roseobacter community in DMSP demethylation was not useful for predicting the formation of DMS, however, suggesting a complex regulatory process that may involve multiple taxa and alternative fates of DMSPd.

Dimethylsulfoxide (DMSO) is an abundant but poorly understood methylated sulfur compound in the marine environment. One potentially significant loss pathway for DMSO is through its biological reduction to dimethylsulfide (DMS), which has been documented in a number of organisms, most notably bacteria. Here we present the first detailed study of DMSO reduction by several marine phytoplankton in axenic cultures. Reduction of DMSO was observed in four algal classes, with in vivo reduction rates ranging from 0.006 to 1.5 mmol [L cell volume]21 s21 at 1.0 mmol L21 DMSO. Corresponding turnover times for measured intracellular DMSO pools varied from hours to days. Michaelis–Menton kinetic parameters were estimated for Isochrysis galbana, Thalassiosira pseudonana, and Amphidinium carterae. The half-saturation constant (Km) and maximal rate (Vmax) for DMSO reduction ranged between 0.96 and 2.7 mmol [L cell volume]21 and 17–118 nmol [L cell volume]21 s21, respectively. Our results suggest that DMSO reduction is a universal activity in marine phytoplankton, even in algae with no detectable dimethylsulfoniopropionate (DMSP). Although reduction of DMSO by marine eukaryotes may not contribute significantly to removal of DMSO from the dissolved phase, this reduction is likely to be a major source of DMS in species lacking detectable DMSP lyase activity. The ability of marine phytoplankton to reduce DMSO to DMS should allow algae to cycle these compounds as part of an antioxidant system.

Dimethylsulfoxide (DMSO) is an abundant form of methylated sulfur in marine systems and it is known to be produced from dimethylsulfide (DMS). Using radiolabeled 35S-DMS and gas chromatography techniques, we quantified the dissolved DMSO (DMSOd) produced from photo- and biological oxidation of dissolved DMS and compared the DMSOd production from these pathways with the net change in DMSOd concentrations in unfiltered seawater samples. The net change in DMSOd in light-exposed treatments exceeded DMSOd production from photo- plus biological oxidation of dissolved DMS. This indicated that DMSOd was produced by one or more light-driven processes likely associated with particulate material. Results from in situ incubation arrays showed that the relative importance of DMSOd production processes was dependent on irradiation depth, with the unidentified particle-associated process and dissolved DMS photooxidation the main DMSOd sources close to the surface (0–10 m) and biological oxidation of dissolved DMS the main process at depths at which the light level was low (.10 m). Deckboard and in situ incubations revealed that DMSOd production from the particle-associated process was stimulated by ultraviolet radiation. Higher particle-associated production of DMSOd in samples more prone to suffer light-induced stress supports the hypothesis that this process was related to phytoplanktonic biosynthesis and release of DMSO because of oxidative stress. Our results suggest that particle-associated DMSOd production is an important source of DMSOd in surface waters of the Ross Sea and also help to explain why DMSOd is periodically the main organosulfur compound detected in the upper water column.

12 pages, 5 figures, 2 tables, supplementary material http://onlinelibrary.wiley.com/doi/10.1111/j.1462-2920.2006.01102.x/suppinfo, Microbial consumption is one of the main processes, along with photolysis and ventilation, that remove the biogenic trace gas dimethylsulfide (DMS) from the surface ocean. Although a few isolates of marine bacteria have been studied for their ability to utilize DMS, little is known about the characteristics or phylogenetic affiliation of DMS consumers in seawater. We enriched coastal and open-ocean waters with different carbon sources to stimulate different bacterial communities (glucose-consuming bacteria, methyl group-consuming bacteria and DMS consumers) in order to test how this affected DMS consumption and to examine which organisms might be involved. Dimethylsulfide consumption was greatly stimulated in the DMS addition treatments whereas there was no stimulation in the other treatments. Analysis of microbial DNA by two different techniques (sequenced bands from DGGE gels and clone libraries) showed that bacteria grown specifically with the presence of DMS were closely related to the genus Methylophaga. We also followed the fate of consumed DMS in some of the enrichments. Dimethylsulfide was converted mostly to DMSO in glucose or methanol enrichments, whereas it was converted mostly to sulfate in DMS enrichments, the latter suggesting use of DMS as a carbon and energy source. Our results indicate that unlike the biochemical precursor of DMS, dimethylsulfoniopropionate (DMSP), which is consumed by a broad spectrum of marine microorganisms, DMS seems to be utilized as a carbon and electron source by specialists. This is consistent with the usual observation that DMSP turns over at much higher rates than DMS, This work was supported by a PhD fellowship from the Spanish Ministry of Education to M.V. Field work was supported by NSF through the Biocomplexity in the Environment program (OPP 0221748; P. Matrai, P.I.) and by the Spanish MEC through project CAOS (CTM2004-20022-E). The molecular work was funded by projects MICRODIFF (REN2001-2120/MAR), BASICS (EVK3-CT-2002-00078) and GENµMAR (CTM2004-02586/MAR)

A variety of filtration and passive dialysis protocols were tested with seawater for determination of the dissolved concentrations of dimethylsulfoniopropionate ( DMSPd), a compound originating in the cytosol of many marine phytoplankton. Commonly used sampling procedures, such as in-line filtration, syringe pressure filtration, and gravity filtration of relatively large-volume samples ( 5-50 mL) through glass fiber filters, caused release of DMSPd from particulate material as evidenced by increasing DMSPd concentrations with volume filtered. Exposure of filters to air at the end of filtration caused particularly severe DMSPd release. Dialysis bags or Slide-A-Lyzer dialysis cassettes equilibrated with seawater DMSPd within 4 h, even in polar waters ( -1.8 degrees C), and appeared to give accurate DMSPd concentrations in some circumstances. However, incubation of seawater in laboratory containers ( e. g., glass jars) during dialysis sometimes caused artifactual release of DMSPd. We therefore adopted a small-volume gravity drip filtration ( SVDF) procedure, the essential elements of which were: ( i) collection of 20 to 50 mL seawater directly from the primary sample container ( e. g., Niskin bottle) into a dry, all-plastic filtration tower; ( ii) use of a 47-mm-diameter Whatman GF/F filter; ( iii) rapid (< 3 min) filtration by gravity ( hydrostatic) pressure; ( iv) collection of only the first 3.5 mL filtrate for DMSPd analysis; and ( v) never exposing the filtered plankton to air. The SVDF procedure appeared to yield the same low DMSPd concentrations as dialysis samplers incubated in the ocean water column. Using the SVDF procedure, DMSPd was found to be < 2.8 nM over a broad range of ocean water types and particulate DMSP concentrations. The maximum DMSPd concentration observed in our study ( 2.8 nM) was far lower than the reported worldwide average DMSPd concentration of 16.9 nM, raising the possibility that past data collections may have been influenced by filtration artifacts.

8 pages, 4 figures, 2 tables A variety of bacterial phylogenetic groups assimilate dimethylsulfoniopropionate (DMSP), an organic sulfur compound that can satisfy most of the sulfur (S) demand of bacteria in the surface waters of the ocean. Marine Synechococcus are capable of utilizing some forms of dissolved organic matter, but it is unknown if Synechococcus also assimilate DMSP. To better understand the role of Synechococcus in the flux of DMSP, we used microautoradiography to follow the assimilation of 35S-DMSP and 35S-methanethiol, an intermediate in DMSP assimilation, by Synechococcus in the surface waters of the Gulf of Mexico and the northwest Atlantic Ocean. About 85% of Synechococcus cells assimilated S from DMSP and methanethiol in these environments, and Synechococcus assimilated more DMSP per cell than other bacteria. On average, Synechococcus accounted for roughly 20% of prokaryotic DMSP assimilation in the northwest Atlantic and the Gulf of Mexico. Tests with axenic cultures of Synechococcus revealed that two phycoerythrin-containing strains (WH8102 and WH7803) were capable of DMSP transport, although these strains did not produce dimethylsulfide (DMS). These data indicate that DMSP could provide a significant amount of S to Synechococcus and that Synechococcus are important consumers of DMSP in the ocean. © 2005, by the American Society of Limnology and Oceanography, Inc. This study was supported by grants from the NSF (OCE-9907471 and OPP-0221748) and DOE-BIOMP (DF-FG02-97 ER 62479). An NDSEG fellowship provided support for R.M.

The algal-derived compound dimethylsulfoniopropionate (DMSP), which is the precursor of the climatically active gas dimethylsulfide, is potentially an important source of carbon and sulfur to marine bacterioplankton. Currently, bacteria of the Roseobacter clade, a subgroup of a-proteobacteria, are hypothesized to be the key participants in the metabolism of DMSP. To test this hypothesis, we used a combination of microautoradiography and fluorescence in situ hybridization (Micro-FISH) to identify the bacteria assimilating 35 S DMSP in the Gulf of Mexico, the Gulf of Maine, and the Sargasso Sea. On average, half of the bacterial community assimilated DMSP in these environments. Members of the a-proteobacteria dominated DMSP assimilation, accounting for 35‐40% of bacteria assimilating DMSP. Cytophaga-like bacteria and g-proteobacteria each accounted for 15‐30% of DMSP-assimilating cells. The a-proteobacteria accounted for a greater fraction of the DMSP-assimilating community than expected based on their overall abundance, whereas Cytophaga-like bacteria were typically underrepresented in the DMSP-assimilating community. Members of the Roseobacter clade assimilated more DMSP on a per-cell basis than any other group, but they did not account for most of the DMSP assimilation, nor were they always present even when DMSP turnover was high. These results indicate that the biogeochemical flux of dissolved DMSP is mediated by a large and diverse group of heterotrophic bacteria.

Apparent quantum yields and rates of dimethylsulfide (DMS) photolysis were determined from Sargasso Sea seawater with the goal of assessing the extent to which photoreactions affect the unusually elevated upper ocean concentrations of DMS during the summer, the so-called DMS summer paradox. Apparent quantum yields determined with monochromatic radiation decrease exponentially with increasing wavelength and indicate that DMS photolysis is driven by ultraviolet (UV) radiation. The relative spectral partitioning differs between samples collected from the surface mixed layer (15 m) and from the chlorophyll a maximum (80 m), presumably because of differences in chromophoric dissolved organic matter (CDOM) quality (e.g., apparent degree of bleaching). Quantum yields are also temperature dependent, and an approximate doubling of photolysis rates occurs for a 208C increase in temperature. The significance of DMS photolysis to upper ocean sulfur budgets is explored using a multiyear (1992‐ 1994) DMS time series, concurrent irradiance determinations and temperature profiles, and estimates of CDOM absorption. Depth-integrated, mixed-layer DMS photolysis rates peak in the summer (15‐25 mmol m 22

A recently described unicellular chlorophytic alga isolated from meromictic Mono Lake, California, occupies a niche that spans two environments: the upper oxic mixolimnion and the deeper anoxic and highly reducing monimolimnion. This organism, Picocystis sp. strain ML, accounts for nearly 25% of the primary production during the winter bloom and more than 50% at other times of the year. In incubations, it is heavily grazed by the brine shrimp, Artemia monica. We assessed growth and photosynthetic parameters over broad ranges of irradiance, salinity, and pH and under oxic and anoxic conditions. Picocystis appears to be particularly adapted to low irradiance; we observed an order of magnitude increase in the cellular pigment concentrations, as well as marked increases in cellspecific photosynthetic parameters for cells acclimated to low-growth irradiance. Growth rates of 0.3‐1.5 d 21 were observed over a salinity range of 0‐260‰ and a pH range of 4‐12, with maximal growth at ;50 mmol photons m 22 s 21 , 40‰, and pH 6‐10. Growth and oxygenic photosynthesis were observed under anoxic conditions at rates comparable to those measured under oxic conditions. The ability of the organism to acclimate and grow under such a broad range of environmental conditions makes it an important component of the Mono Lake ecosystem and likely contributes to its dominance of the monimolimnion/mixolimnion interface.

The algal osmolyte, dimethylsulphoniopropionate (DMSP), is abundant in the surface oceans and is the major precursor of dimethyl sulphide (DMS), a gas involved in global climate regulation. Here, we report results from an in situ Lagrangian study that suggests a link between the microbially driven fluxes of dissolved DMSP (DMSPd) and specific members of the bacterioplankton community in a North Sea coccolithophore bloom. The bacterial population in the bloom was dominated by a single species related to the genus Roseobacter, which accounted for 24% of the bacterioplankton numbers and up to 50% of the biomass. The abundance of the Roseobacter cells showed significant paired correlation with DMSPd consumption and bacterioplankton production, whereas abundances of other bacteria did not. Consumed DMSPd (28 nM day(-1)) contributed 95% of the sulphur and up to 15% of the carbon demand of the total bacterial populations, suggesting the importance of DMSP as a substrate for the Roseobacter-dominated bacterioplankton. In dominating DMSPd flux, the Roseobacter species may exert a major control on DMS production. DMSPd turnover rate was 10 times that of DMS (2.7 nM day(-1)), indicating that DMSPd was probably the major source of DMS, but that most of the DMSPd was metabolized without DMS production. Our study suggests that single species of bacterioplankton may at times be important in metabolizing DMSP and regulating the generation of DMS in the sea.

We measured the distribution of particulate and dissolved pools of the phytoplankton osmolyte dimethylsulfoniopropionate (DMSP) in the euphotic zone at a series of shelf (,40 m total water depth) and oceanic (.500 m depth) stations in the northern Gulf of Mexico. We also measured turnover rates of the dissolved DMSP pools (DMSPd) with tracer additions of 35 S-DMSPd and short-term (,1 h) incubations, with the aim of examining the relationship between DMSPd turnover and bacterial production. Particulate DMSP concentrations were relatively low (,25 nM) throughout the study area with about twofold higher mean concentration at the shelf sites (15 nM) compared with the oligotrophic oceanic sites (7 nM). DMSPd concentrations averaged 3.0 nM in shelf waters and 1.3 nM in oceanic waters. Concentrations of dimethylsulfide (DMS), a degradation product of DMSP, also were low throughout the Gulf, averaging 2.0 nM for all depths sampled and 2.5 nM in surface waters. Microbial assemblages metabolized 35 S-DMSPd with the sulfur being incorporated into biomass, volatile compounds (DMS and methanethiol), and other dissolved products. DMSPd turnover was relatively slow (mean of 3.8 nM d 21 ) in oligotrophic oceanic waters and averaged 10-fold higher (39 nM d 21 ) in mesotrophic shelf waters. DMS concentrations ranged from 0.2 to 5.1 nM in oceanic waters and appeared to be weakly related to DMSP turnover. In contrast, DMS concentrations in shelf waters fell within a narrow range (0.8‐2.8 nM) and showed no relationship at all with DMSPd turnover. DMSPd turnover rates were high enough to sustain the measured concentrations and estimated turnover of DMS, even if the conversion efficiency of DMSPd into DMS was only 10%. DMSPd turnover was significantly correlated with bacterial production (as measured by 3 H-thymidine incorporation) and we estimate that DMSPd turnover contributed a mean of 3.4% of the carbon and ;100% of the sulfur required for bacterial growth in Gulf of Mexico surface waters. In addition to its role as a precursor of DMS, DMSP deserves attention as an important substrate for bacterioplankton in the euphotic zone.

The uptake of nanomolar levels of methyl- 14 C-glycine betaine (GBT) was studied in surface water samples from the Gulf of Maine, Mobile Bay, and the Gulf of Mexico. At all locations, 14 C-GBT was rapidly taken up into particulate material (>0.2 μm). Formalin treatment, autoclaving, or prefiltration of water through 0.2-μm membranes completely inhibited 14 C-GBT uptake, indicating the process was biologically mediated, probably by bacteria. During the first several hours of uptake, >80% of the label found in particulates was recovered as untransformed 14 C-GBT. This fraction diminished over longer incubations but never fell below 16%, indicating some longer term storage of GBT in microorganisms. Respiration of the added label to 14 CO 2 was typically

The microbiota of freshwater and marine sediments serve similar roles in carbon degradation and nutrient regeneration. However, because of differences in the chemical environment between freshwater and marine systems, distinct physiological groups of bacteria dominate terminal carbon catabolism in each system. In general, the distribution and rates of microbial activities within a sediment are determined by availability of electron acceptors for respiration and metabolizable organic substrates. Sulfate ion is a primary factor in the distribution of microbial activities in anoxic sediments. At the high sulfate concentration found in seawater, sulfate reduction exceeds methanogenesis and is responsible for most of the organic carbon oxidation. The importance of methanogenesis in sediment metabolism increases as salinity and, hence, sulfate decreases. In freshwaters, methanogenesis is responsible for the bulk of terminal metabolism under anoxic conditions. The higher affinity of sulfate reducers for substrates that can be used by both groups (e.g. hydrogen, acetate, methanol), as well as the more favorable thermodynamic energy yields of sulfate respiration compared to methanogenesis, may account for the dominance of sulfate respiration under sulfate replete conditions. The sources of organic matter to marine and freshwater sediments can be qualitatively different. Complex structural polysaccharides and phenolic polymers (i.e. ligno-cellulose) may comprise a greater fraction of the organic input to freshwater systems. Organic compounds that act as osmoregulatory solutes in marine plants and animals may be unique substrates for bacteria of marine sediments. It is likely that these differences also result in distinct assemblages of microorganisms responsible for the breakdown of organic carbon. The quantity of organic matter present in the scdimcnts is also a major factor determining the magnitude and distribution of various microbial activities. Near the extremes of high and low organic loading, organic matter input may play a greater role than sulfate concentration in determining the relative importance of sulfate reduction or methanogenesis. In marine systems, methanogenesis occurs in the presence of sulfate ion, but only at the expense of substrates not utilized by sulfate reducers (“noncompetitive” substrates), such as methylamines and, possibly, methylated reduced sulfur compounds. Methanogenesis from noncompetitive substrates represents only a small fraction of the sulfate reduction or carbon catabolism observed in marine sediments. In marine systems with very high rates of organic matter deposition, sulfate can bc depleted to the extent that methanogenesis takes on quantitative significance. Although sulfate concentrations are usually insufficient to permit much sulfate reduction in lacustrine environments, in lakes with relatively low organic deposition, sulfate concentrations in the water (and that supplied to the sediments) can bc sufficient to allow for a more significant contribution of sulfate respiration to carbon catabolism.