Anoxic Freshwater Sediments Research Articles

Anaerobic biodegradation of phenanthrene and pyrene by sulfate-reducing cultures enriched from contaminated freshwater lake sediments

Our current understanding of the susceptibility of hazardous polycyclic aromatic hydrocarbons (PAHs) to anaerobic microbial degradation is very limited. In the present study, we obtained phenanthrene- and pyrene-degrading strictly anaerobic sulfate-reducing enrichments using contaminated freshwater lake sediments as the source material. The highly enriched phenanthrene-degrading culture, MMKS23, was dominated (98%) by a sulfate-reducing bacterium belonging to the genus Desulfovibrio. While Desulfovibrio sp. was also predominant (79%) in the pyrene-degrading enrichment culture, MMKS44, an anoxygenic purple non-sulfur bacterium, Rhodopseudomonas sp., constituted a significant fraction (18%) of the total microbial community. Phenanthrene or pyrene biodegradation by the enrichment cultures was coupled with sulfate reduction, as evident from near stoichiometric consumption of sulfate and accumulation of sulfide. Also, there was almost complete inhibition of substrate degradation in the presence of an inhibitor of sulfate reduction, i.e., 20 mM MoO42−, in the culture medium. After 180 days of incubation, about 79.40 μM phenanthrene was degraded in the MMKS23 culture, resulting in the consumption of 806.80 μM sulfate and accumulation of 625.80 μM sulfide. Anaerobic pyrene biodegradation by the MMKS44 culture was relatively slow. About 22.30 μM of the substrate was degraded after 180 days resulting in the depletion of 239 μM sulfate and accumulation of 196.90 μM sulfide. Biodegradation of phenanthrene by the enrichment yielded a metabolite, phenanthrene-2-carboxylic acid, suggesting that carboxylation could be a widespread initial step of phenanthrene activation under sulfate-reducing conditions. Overall, this novel study demonstrates the ability of sulfate-reducing bacteria (SRB), dwelling in contaminated freshwater sediments to anaerobically biodegrade three-ringed phenanthrene and highly recalcitrant four-ringed pyrene. Our findings suggest that SRB could play a crucial role in the natural attenuation of PAHs in anoxic freshwater sediments.

A rationale for higher ratios of CH4 to CO2 production in warmer anoxic freshwater sediments and soils

A rationale for higher ratios of <scp>CH₄</scp> to <scp>CO₂</scp> production in warmer anoxic freshwater sediments and soils

Source data of manuscript: A rationale for higher ratios of CH4 to CO2 production in warmer anoxic freshwater sediments and soils

Source data of manuscript: A rationale for higher ratios of CH4 to CO2 production in warmer anoxic freshwater sediments and soils

Microbial Succession of Anaerobic Chitin Degradation in Freshwater Sediments.

Chitin is massively produced by freshwater plankton species as a structural element of their exoskeleton or cell wall. At the same time, chitin does not accumulate in the predominantly anoxic sediments, underlining its importance as carbon and nitrogen sources for sedimentary microorganisms. We studied chitin degradation in littoral sediment of Lake Constance, Central Europe's third largest lake. Turnover of the chitin analog methyl-umbelliferyl-N,N-diacetylchitobioside (MUF-DC) was highest in the upper oxic sediment layer, with 5.4 nmol MUF-DC h-1 (g sediment [dry weight])-1 In the underlying anoxic sediment layers, chitin hydrolysis decreased with depth from 1.1 to 0.08 nmol MUF-DC h-1 (g sediment [dry weight])-1 Bacteria involved in chitin degradation were identified by 16S rRNA (gene) amplicon sequencing of anoxic microcosms incubated in the presence of chitin compared to microcosms amended either with N-acetylglucosamine as the monomer of chitin or no substrate. Chitin degradation was driven by a succession of bacteria responding specifically to chitin only. The early phase (0 to 9 days) was dominated by Chitinivibrio spp. (Fibrobacteres). The intermediate phase (9 to 21 days) was characterized by a higher diversity of chitin responders, including, besides Chitinivibrio spp., also members of the phyla Bacteroidetes, Proteobacteria, Spirochaetes, and Chloroflexi In the late phase (21 to 43 days), the Chitinivibrio populations broke down with a parallel strong increase of Ruminiclostridium spp. (formerly Clostridium cluster III, Firmicutes), which became the dominating chitin responders. Our study provides quantitative insights into anaerobic chitin degradation in lake sediments and linked this to a model of microbial succession associated with this activity.IMPORTANCE Chitin is the most abundant biopolymer in aquatic environments, with a direct impact on the carbon and nitrogen cycles. Despite its massive production as a structural element of crustaceans, insects, or algae, it does not accumulate in sediments. Little is known about its turnover in predominantly anoxic freshwater sediments and the responsible microorganisms. We proved that chitin is readily degraded under anoxic conditions and linked this to a succession of the members of the responsible microbial community over a 43-day period. While Fibrobacteres and Firmicutes members were driving the early and late phases of chitin degradation, respectively, a more diverse community was involved in chitin degradation in the intermediate phase. Entirely different microorganisms responded toward the chitin monomer N-acetylglucosamine, which underscores that soluble monomers are poor and misleading substrates to study polymer-utilizing microorganisms. Our study provides quantitative insights into the microbial ecology driving anaerobic chitin degradation in freshwater sediments.

Tropidoatractidae fam. nov., a Deep Branching Lineage of Metopida (Armophorea, Ciliophora) Found in Diverse Habitats and Possessing Prokaryotic Symbionts

We report a discovery of a novel family of anaerobic ciliates, Tropidoatractidae fam. nov. Phylogenetic analyses based on the 18S rRNA gene show that the family Tropidoatractidae corresponds to the previously reported clade of environmental sequences closely related to the lineage consisting of orders Metopida and Clevelandellida. The family comprises two genera, Tropidoatractus and Palmarella, and five species, two of which are newly described herein. Tropidoatractidae are cosmopolitan Metopida with sparse somatic and oral ciliature, deep, cup-like buccal cavity, and hyaline cortex with interkinetal ridges. Moreover, all species occur in two morphotypes, slender and stout. They inhabit microoxic or anoxic freshwater, brackish, and marine sediments and possess anaerobic mitochondrion-related organelles and various prokaryotic symbionts. The discovery of Tropidoatractidae provides valuable information about the evolution of Armophorea and gives us insights to the diversity and ecological preferences of anaerobic ciliates in general.

Large but variable methane production in anoxic freshwater sediment upon addition of allochthonous and autochthonous organic matter.

An important question in the context of climate change is to understand how CH4 production is regulated in anoxic sediments of lakes and reservoirs. The type of organic carbon (OC) present in lakes is a key factor controlling CH4 production at anoxic conditions, but the studies investigating the methanogenic potential of the main OC types are fragmented. We incubated different types of allochthonous OC (alloOC; terrestrial plant leaves) and autochthonous OC (autoOC; phytoplankton and two aquatic plants species) in an anoxic sediment during 130 d. We tested if (1) the supply of fresh alloOC and autoOC to an anoxic refractory sediment would fuel CH4 production and if (2) autoOC would decompose faster than alloOC. The addition of fresh OC greatly increased CH4 production and the δ13C‐CH4 partitioning indicated that CH4 originated exclusively from the fresh OC. The large CH4 production in an anoxic sediment fueled by alloOC is a new finding which indicates that all systems with anoxic conditions and high sedimentation rates have the potential to be CH4 emitters. The autoOC decomposed faster than alloOC, but the total CH4 production was not higher for all autoOC types, one aquatic plant species having values as low as the terrestrial leaves, and the other one having values as high as phytoplankton. Our study is the first to report such variability, suggesting that the extent to which C fixed by aquatic plants is emitted as greenhouse gases or buried as OC in sediment could more generally differ between aquatic vegetation types.

Spatial change of reservoir nitrite-dependent methane-oxidizing microorganisms

Nitrite-dependent anaerobic methane oxidation (n-damo), catalyzed by microorganisms affiliated with bacterial phylum NC10, can have an important contribution to the reduction of the methane emission from anoxic freshwater sediment to the atmosphere. However, information on the variation of sediment n-damo organisms in reservoirs is still lacking. The present study monitored the spatial change of sediment n-damo organisms in the oligotrophic freshwater Xinfengjiang Reservoir (South China). Sediment samples were obtained from six different sampling locations and two sediment depths (0–5 cm, 5–10 cm). Sediment n-damo bacterial abundance was found to vary with sampling location and layer depth, which was likely influenced by pH and nitrogen level. The presence of the n-damo pmoA gene was found in all these samples. A remarkable shift occurred in the diversity and composition of sediment n-damo pmoA gene sequences. A variety of distinctively different n-damo pmoA clusters existed in reservoir sediments. The pmoA sequences affiliated with Candidatus Methylomirabilis oxyfera formed the largest group, while a significant proportion of the obtained n-damo pmoA gene sequences showed no close relationship to those from any known NC10 species. In addition, the present n-damo process was found in reservoir sediment, which could be enhanced by nitrite nitrogen amendment.

Microbiological Reduction of Sb(V) in Anoxic Freshwater Sediments

Microbiological reduction of millimolar concentrations of Sb(V) to Sb(III) was observed in anoxic sediments from two freshwater settings: (1) a Sb- and As-contaminated mine site (Stibnite Mine) in central Idaho and 2) an uncontaminated suburban lake (Searsville Lake) in the San Francisco Bay Area. Rates of Sb(V) reduction in anoxic sediment microcosms and enrichment cultures were enhanced by amendment with lactate or acetate as electron donors but not by H2, and no reduction occurred in sterilized controls. Addition of 2-(14)C-acetate to Stibnite Mine microcosms resulted in the production of (14)CO2 coupled to Sb(V) reduction, suggesting that this process proceeds by a dissimilatory respiratory pathway in those sediments. Antimony(V) reduction in Searsville Lake sediments was not coupled to acetate mineralization and may be associated with Sb-resistance. The microcosms and enrichment cultures also reduced sulfate, and the precipitation of insoluble Sb(III)-sulfide complexes was a major sink for reduced Sb. The reduction of Sb(V) by Stibnite Mine sediments was inhibited by As(V), suggesting that As(V) is a preferred electron acceptor for the indigenous community. These findings indicate a novel pathway for anaerobic microbiological respiration and suggest that communities capable of reducing high concentrations of Sb(V) commonly occur naturally in the environment.

Stratification of Archaea in the Deep Sediments of a Freshwater Meromictic Lake: Vertical Shift from Methanogenic to Uncultured Archaeal Lineages

As for lineages of known methanogens, several lineages of uncultured archaea were recurrently retrieved in freshwater sediments. However, knowledge is missing about how these lineages might be affected and structured according to depth. In the present study, the vertical changes of archaeal communities were characterized in the deep sediment of the freshwater meromictic Lake Pavin. For that purpose, an integrated molecular approach was performed to gain information on the structure, composition, abundance and vertical stratification of archaeal communities thriving in anoxic freshwater sediments along a gradient of sediments encompassing 130 years of sedimentation. Huge changes occurred in the structure and composition of archaeal assemblages along the sediment core. Methanogenic taxa (i.e. Methanosaeta and Methanomicrobiales) were progressively replaced by uncultured archaeal lineages (i.e. Marine Benthic Group-D (MBG-D) and Miscellaneous Crenarchaeal Group (MCG)) which are suspected to be involved in the methane cycle.

Tolumonas osonensis sp. nov., isolated from anoxic freshwater sediment, and emended description of the genus Tolumonas

A polyphasic taxonomic study was performed on a strain of an unknown Gram-negative, non-motile, saccharolytic, facultatively anaerobic bacterium, strain OCF 7(T), isolated from anoxic freshwater sediment. The strain grew optimally at 22 °C and pH 7.5, and was able to grow under strictly anaerobic conditions. Major fermentation products from glucose metabolism were formate, acetate, ethanol and lactate. Comparative 16S rRNA gene sequence analysis indicated that strain OCF 7(T) was phylogenetically related to the type strain of Tolumonas auensis (97.2 % similarity) within the family Aeromonadaceae of the Gammaproteobacteria. However, OCF 7(T) did not produce toluene from phenylacetate, phenylalanine, phenoxyacetate, phenylsuccinate or phenylbutyrate in the presence of glucose. Phenol was not produced from tyrosine or phenoxyacetate in the presence of glucose. Dominant fatty acids of this micro-organism included C(16 : 0), C(18 : 1)ω7c and C(16 : 1)ω7c (and/or iso-C(15 : 0) 2-OH). Major polar lipids were phosphatidylglycerol and phosphatidylethanolamine, and the respiratory quinone was menaquinone MK-8. The genomic DNA G+C content of strain OCF 7(T) was 52.1 mol%. Based on phylogenetic and phenotypic evidence, strain OCF 7(T) should be classified as a representative of a novel species of Tolumonas, for which the name Tolumonas osonensis sp. nov. is proposed; the type strain is OCF 7(T) ( = DSM 22975(T) = ATCC BAA-1908(T)). An emended description of the genus Tolumonas is also given.

Syntrophic degradation of cadaverine by a defined methanogenic coculture.

A novel, strictly anaerobic, cadaverine-oxidizing, defined coculture was isolated from an anoxic freshwater sediment sample. The coculture oxidized cadaverine (1,5-diaminopentane) with sulfate as the electron acceptor. The sulfate-reducing partner could be replaced by a hydrogenotrophic methanogenic partner. The defined coculture fermented cadaverine to acetate, butyrate, and glutarate plus sulfide or methane. The key enzymes involved in cadaverine degradation were identified in cell extracts. A pathway of cadaverine fermentation via 5-aminovaleraldehyde and crotonyl-coenzyme A with subsequent dismutation to acetate and butyrate is suggested. Comparative 16S rRNA gene analysis indicated that the fermenting part of the coculture belongs to the subphylum Firmicutes but that this part is distant from any described genus. The closest known relative was Clostridium aminobutyricum, with 95% similarity.

Identification of acetate‐utilizing Bacteria and Archaea in methanogenic profundal sediments of Lake Kinneret (Israel) by stable isotope probing of rRNA

Acetate is an important intermediate in the decomposition of organic matter in anoxic freshwater sediments. Here, we identified distinct microorganisms active in its oxidation and transformation to methane in the anoxic methanogenic layers of Lake Kinneret (Israel) profundal sediment by rRNA-based stable isotope probing (RNA-SIP). After 18 days of incubation with amended [U-(13)C]acetate we found that archaeal 16S rRNA was (13)C-labelled to a far greater extent than bacterial rRNA. We identified acetoclastic methanogens related to Methanosaeta concilii as being most active in the degradation and assimilation of acetate. Oxidation of the acetate-methyl group played only a minor role, but nevertheless 'heavy'(13)C-labelled bacterial rRNA templates were identified. 'Heavy' bacteria were mainly affiliated with the Betaproteobacteria (mostly Rhodocyclales and Nitrosomonadales), the Nitrospira phylum (related to 'Magnetobacterium bavaricum' and Thermodesulfovibrio yellowstonii), and also with the candidate phylum 'Endomicrobia'. However, the mode of energy gain that allowed for the assimilation of (13)C-acetate by these bacterial groups remains unknown. It may have involved syntrophic oxidation of acetate, reduction of chlorinated compounds, reduction of humic substances, fermentation of organic compounds, or even predation of (13)C-labelled Methanosaeta spp. In summary, this SIP experiment shows that acetate carbon was predominantly consumed by acetoclastic methanogens in profundal Lake Kinneret sediment, while it was also utilized by a small and heterogeneous community of bacteria.

The influence of Glyceria maxima and nitrate input on the composition and nitrate metabolism of the dissimilatory nitrate-reducing bacterial community

The influence of nitrate addition and the presence of Glyceria maxima (reed sweetgrass) on the composition and nitrate metabolism of the dissimilatory nitrate-reducing bacterial community was investigated. Anoxic freshwater sediment was incubated in pots with or without G. maxima and with or without addition of nitrate. After incubation the sediments were sampled. Dilution series of these sediment samples were incubated in deep agar tubes and almost all colonies from the most diluted tubes were isolated and purified. When the nitrate concentration was low, 80% of the nitrate-reducing community in the rhizosphere of G. maxima consisted of NO2−-accumulating or NH4+-producing Bacillus strains. In bulk sediment with low nitrate concentrations, denitrifying Pseudomonas and Acinetobacter strains were dominant. The difference in the composition of the nitrate-reducing community between sediment with or without plants disappeared when nitrate was added. Denitrifying pseudomonads then made up 78–86% of the nitrate-reducing community. This shows that when the nitrate concentration was low, the presence of G. maxima greatly influenced the nitrate-reducing community. However, when nitrate was present and available, the composition and nitrate metabolism of the dominant nitrate-reducing community was probably not affected by G. maxima.

Reversibility of radiocaesium sorption on illite

Adsorption of trace amounts of radiocaesium on NH 4-, K-, and Na-saturated Fithian illite and subsequent desorption by 1 M NH 4 showed that a substantial amount of radiocaesium (44%, 46%, and 91% for NH 4-, K-, and Na-illite, respectively) cannot be desorbed after only 5 min of adsorption. Our results suggest that this instantaneous fixation is caused by the collapse of the frayed edges of the clay mineral and the relatively high concentration of radiocaesium building up in solution in the batch desorption experiments. Consequently, commonly applied high-NH 4 extractions underestimate truly exchangeable amounts of radiocaesium in soils and sediments containing illitic clay minerals. The rate of desorption of trace amounts of radiocaesium from the solids using high NH 4 or Cs concentrations has a half-life of about 2 yr, reflecting radiocaesium desorption from (partially) collapsed interlayers. Extraction of radiocaesium from illite after 5 min of contact time with a Cs-selective adsorbent or a 1 × 10 -6 M CsCl solution shows that 100% of the bound radiocaesium is readily available. The desorption rate in the presence of a Cs-selective adsorbent has a half-life of about 0.2 yr. Desorption of radiocaesium from illite using different ammonium concentrations shows that radiocaesium partitioning follows reversible ion-exchange theory if the NH 4 concentration is below 1 × 10 -4 M, and sufficient time (weeks) is allowed for the reaction to proceed. Thus, radiocaesium sorption reversibility in the natural environment is much higher than generally assumed, and equilibrium solid/liquid partitioning may be assumed for the long-term modelling of radiocaesium mobility in the natural environment. In the particular case of anoxic freshwater sediments with very high NH 4 concentrations in the pore waters (up to several mmol.L -1), collapse of the frayed edges of illite may occur, influencing radiocaesium partitioning. If collapse occurs before radiocaesium adsorbs to illite, high caesium sorption reversibility as measured by high-NH 4 extraction can be expected because further collapse of the frayed edges during the extraction procedure will be limited. This effect has indeed been observed earlier in the extraction of radiocaesium from anoxic freshwater sediments with high-NH 4 solutions and was as yet unexplained.

Microbial Mercury Transformation in Anoxic Freshwater Sedi ments under Iron-Reducing and Other Electron-Accepting Conditions

Microbial Mercury Transformation in Anoxic Freshwater Sedi ments under Iron-Reducing and Other Electron-Accepting Conditions

Thiol methylation potential in anoxic, low-pH wetland sediments and its relationship with dimethylsulfide production and organic carbon cycling

Dimethylsulfide (CH 3SCH 3) is formed in anoxic freshwater sediments by biological methylation of methanethiol (CH 3SH). We measured thiol methylation potential in low-pH, Sphagnum peat sediments from Alaska and Alabama by adding ethanethiol (CH 3CH 2SH) to peat slurries and quantifying the rate of ethylmethylsulfide (CH 3CH 2SCH 3) formation. Thiol methylation potential ranged from 12 to 154 nM h −1 and was significantly related to dimethylsulfide accumulation rates ( P=0.0007; r 2=0.48). Addition of methanol or syringic acid stimulated thiol methylation potential and dimethylsulfide accumulation rate, suggesting that these compounds could be methyl donors. Addition of acetate or its metabolic precursors (glucose or Sphagnum plant material) inhibited thiol methylation potential, but not carbon dioxide or methane production. Inhibition of methanogenesis with either 2-bromoethanesulfonic acid or KNO 3 consistently inhibited thiol methylation potential and dimethylsulfide accumulation. These results suggest that methanogens play a role in thiol methylation and therefore dimethylsulfide formation.

Microbial mercury transformation in anoxic freshwater sediments under iron-reducing and other electron-accepting conditions.

Potential rates of microbial methylation of inorganic mercury (added as HgCl2) and degradation of methyl mercury (MeHg) (added as CH3HgCl) were investigated in anoxic sediments from the Mobile Alabama River Basin (MARB) dominated by different terminal electron-accepting processes (TEAPs). Potential rates of methylation were comparable under methanogenic and sulfate-reducing conditions but suppressed under iron-reducing conditions, in slurries of freshwater wetland sediment In contrast, MeHg degradation rates were similar under all three TEAPs. Microbial Hg methylation and MeHg degradation were also investigated in surface sediment from three riverine sites, two of which had iron reduction and one sulfate reduction, as the dominant TEAP (as determined by 14C-acetate metabolism and other biogeochemical measurements). Methylation was active in sulfate-reducing sediments of a tributary creek and suppressed in iron-reducing, sandy sediments from the open river, whereas MeHg degradation was active at all three sites. Although iron-reducing conditions often suppressed methylation, some methylation activity was observed in two out of three replicates from iron-reducing sediments collected near a dam. Given that MeHg degradation was consistently observed under all TEAPs, our results suggest that the net flux of MeHg from iron-reducing surface sediments may be suppressed (due to inhibition of gross MeHg production) compared to sediments supporting other TEAPs.

Role of methanogens and other bacteria in degradation of dimethyl sulfide and methanethiol in anoxic freshwater sediments.

The roles of several trophic groups of organisms (methanogens and sulfate- and nitrate-reducing bacteria) in the microbial degradation of methanethiol (MT) and dimethyl sulfide (DMS) were studied in freshwater sediments. The incubation of DMS- and MT-amended slurries revealed that methanogens are the dominant DMS and MT utilizers in sulfate-poor freshwater systems. In sediment slurries, which were depleted of sulfate, 75 micromol of DMS was stoichiometrically converted into 112 micromol of methane. The addition of methanol or MT to DMS-degrading slurries at concentrations similar to that of DMS reduced DMS degradation rates. This indicates that the methanogens in freshwater sediments, which degrade DMS, are also consumers of methanol and MT. To verify whether a competition between sulfate-reducing and methanogenic bacteria for DMS or MT takes place in sulfate-rich freshwater systems, the effects of sulfate and inhibitors, like bromoethanesulfonic acid, molybdate, and tungstate, on the degradation of MT and DMS were studied. The results for these sulfate-rich and sulfate-amended slurry incubations clearly demonstrated that besides methanogens, sulfate-reducing bacteria take part in MT and DMS degradation in freshwater sediments, provided that sulfate is available. The possible involvement of an interspecies hydrogen transfer in these processes is discussed. In general, our study provides evidence for methanogenesis as a major sink for MT and DMS in freshwater sediments.

Anaerobic versus aerobic degradation of dimethyl sulfide and methanethiol in anoxic freshwater sediments.

Degradation of dimethyl sulfide and methanethiol in slurries prepared from sediments of minerotrophic peatland ditches were studied under various conditions. Maximal aerobic dimethyl sulfide-degrading capacities (4.95 nmol per ml of sediment slurry. h-1), measured in bottles shaken under an air atmosphere, were 10-fold higher than the maximal anaerobic degrading capacities determined from bottles shaken under N2 or H2 atmosphere (0.37 and 0. 32 nmol per ml of sediment slurry. h-1, respectively). Incubations under experimental conditions which mimic the in situ conditions (i. e., not shaken and with an air headspace), however, revealed that aerobic degradation of dimethyl sulfide and methanethiol in freshwater sediments is low due to oxygen limitation. Inhibition studies with bromoethanesulfonic acid and sodium tungstate demonstrated that the degradation of dimethyl sulfide and methanethiol in these incubations originated mainly from methanogenic activity. Prolonged incubation under a H2 atmosphere resulted in lower dimethyl sulfide degradation rates. Kinetic analysis of the data resulted in apparent Km values (6 to 8 microM) for aerobic dimethyl sulfide degradation which are comparable to those reported for Thiobacillus spp., Hyphomicrobium spp., and other methylotrophs. Apparent Km values determined for anaerobic degradation of dimethyl sulfide (3 to 8 microM) were of the same order of magnitude. The low apparent Km values obtained explain the low dimethyl sulfide and methanethiol concentrations in freshwater sediments that we reported previously. Our observations point to methanogenesis as the major mechanism of dimethyl sulfide and methanethiol consumption in freshwater sediments.

Sulphur, organic carbon and iron relationships in estuarine and freshwater sediments: effects of sedimentation rate

Concentrations of S, organic C and Fe were investigated in profiles of sediments from two estuarine systems in the SW of Western Australia. In marine-affected sediments, inorganic S dominates total S and concentrations of total S correlate with Fe and not with organic C. In freshwater sediments, organic S dominates total S and concentrations of total S correlate with organic C and not with Fe. Molar Fe/S ratios in the estuarine sediments decrease with increasing salinity and approach unity for marine conditions. Net accumulation rates of S in sediments were estimated with a numerical computer model, calibrated with published data on profiles of marine sediments for diffusion of SO 2− 4, sedimentation rates and distributions of S. Measured depth-integrated reduction rates of SO 2− 4 in the marine-affected estuarine sediments approach those obtained for Fe-limited marine conditions at similar rates of sedimentation. Measured concentrations of inorganic S in anoxic freshwater sediments fit a numerically calculated relationship between inorganic S and sedimentation rate.