The ecological significance of sulfur in the energy dynamics of salt marsh and coastal marine sediments

Sulfur is an important element in the metabolism of salt marshes and subtidal, coastal marine sediments because of its role as an electron acceptor, carrier, and donor. Sulfate is the major electron acceptor for respiration in anoxic marine sediments. Anoxic respiration becomes increasingly important in sediments as total respiration increases, and so sulfate reduction accounts for a higher percentage of total sediment respiration in sediments where total respiration is greater. Thus, sulfate accounts for 25% of total sediment respiration in nearshore sediments (200 m water depth or less) where total respiration rates are 0.1 to 0.3gCm^–1 day^–1 , for 50% to 70% in nearshore sediments with higher rates of total respiration (0.3 to 3gCm^–2 day^–1), and for 70% to 90% in salt marsh sediments where total sediment respiration rates are 2.5 to 5.5gcm^–2 day^–1 .During sulfate reduction, large amounts of energy from the respired organic matter are conserved in inorganic reduced sulfur compounds such as soluble sulfides, thiosulfate, elemental sulfur, iron monosulfides, and pyrite. Only a small percentage of the reduced sulfur formed during sulfate reduction is accreted in marine sediments and salt marshes. When these reduced sulfur compounds are oxidized, energy is released. Chemolithoautotrophic bacteria which catalyze these oxidations can use the energy of oxidation with efficiencies (the ratio of energy fixed in organic biomass to energy released in sulfur oxidation) of up to 21–37% to fix CO₂ and produce new organic biomass.Chemolithoautotrophic bacterial production may represent a significant new formation of organic matter in some marine sediments. In some sediments, chemolithoautotrophic bacterial production may even equal or exceed organoheterotrophic bacterial production. The combined cycle of anaerobic decomposition through sulfate reduction, energy conservation as reduced sulfur compounds; and chemolithoautotrophic production of new organic carbon serves to take relatively low-quality organic matter from throughout the sediments and concentrate the energy as living biomass in a discrete zone near the sediment surface where it can be readily grazed by animals.Contribution from a symposium on the role of sulfur in ecosystem processes held August 10, 1983, at the annual meeting of the A.I.B.S., Grand Forks, ND; Myron Mitchell, convenor.     

Reduction of Sulfur Compounds in the Sediments of a Eutrophic Lake Basin

Concentrations of various sulfur compounds (SO₄²⁻, H₂S, S⁰, acid-volatile sulfide, and total sulfur) were determined in the profundal sediments and overlying water column of a shallow eutrophic lake. Low concentrations of sulfate relative to those of acid-volatile sulfide and total sulfur and a decrease in total sulfur with sediment depth implied that the contribution of dissimilatory sulfur reduction to H₂S production was relatively minor. Addition of 1.0 mM Na₂³⁵SO₄ to upper sediments in laboratory experiments resulted in the production of H₂³⁵S with no apparent lag. Kinetic experiments with ³⁵S demonstrated an apparent K_m of 0.068 mmol of SO₄²⁻ reduced per liter of sediment per day, whereas tracer experiments with ³⁵S indicated an average turnover time of the sediment sulfate pool of 1.5 h. Total sulfate reduction in a sediment depth profile to 15 cm was 15.3 mmol of sulfate reduced per m² per day, which corresponds to a mineralization of 30% of the particulate organic matter entering the sediment. Reduction of ³⁵S⁰ occurred at a slower rate. These results demonstrated that high rates of sulfate reduction occur in these sediments despite low concentrations of oxidized inorganic compounds and that this reduction can be important in the anaerobic mineralization of organic carbon.     

Partitioning Effects during Terminal Carbon and Electron Flow in Sediments of a Low-Salinity Meltwater Pond near Bratina Island, McMurdo Ice Shelf, Antarctica

A study of anaerobic sediments below cyanobacterial mats of a low-salinity meltwater pond called Orange Pond on the McMurdo Ice Shelf at temperatures simulating those in the summer season (<5°C) revealed that both sulfate reduction and methane production were important terminal anaerobic processes. Addition of [2-¹⁴C]acetate to sediment samples resulted in the passage of label mainly to CO₂. Acetate addition (0 to 27 mM) had little effect on methanogenesis (a 1.1-fold increase), and while the rate of acetate dissimilation was greater than the rate of methane production (6.4 nmol cm⁻³ h⁻¹ compared to 2.5 to 6 nmol cm⁻³ h⁻¹), the portion of methane production attributed to acetate cleavage was <2%. Substantial increases in the methane production rate were observed with H₂ (2.4-fold), and H₂ uptake was totally accounted for by methane production under physiological conditions. Formate also stimulated methane production (twofold), presumably through H₂ release mediated through hydrogen lyase. Addition of sulfate up to 50-fold the natural levels in the sediment (interstitial concentration, ~0.3 mM) did not substantially inhibit methanogenesis, but the process was inhibited by 50-fold chloride (36 mM). No net rate of methane oxidation was observed when sediments were incubated anaerobically, and denitrification rates were substantially lower than rates for sulfate reduction and methanogenesis. The results indicate that carbon flow from acetate is coupled mainly to sulfate reduction and that methane is largely generated from H₂ and CO₂ where chloride, but not sulfate, has a modulating role. Rates of methanogenesis at in situ temperatures were four- to fivefold less than maximal rates found at 20°C.     

Dynamics of Bacterial Sulfate Reduction in a Eutrophic Lake

Bacterial sulfate reduction in the surface sediment and the water column of Lake Mendota, Madison, Wis., was studied by using radioactive sulfate (³⁵SO₄²⁻). High rates of sulfate reduction were observed at the sediment surface, where the sulfate pool (0.2 mM SO₄²⁻) had a turnover time of 10 to 24 h. Daily sulfate reduction rates in Lake Mendota sediment varied from 50 to 600 nmol of SO₄²⁻ cm⁻³, depending on temperature and sampling date. Rates of sulfate reduction in the water column were 10³ times lower than that for the surface sediment and, on an areal basis, accounted for less than 18% of the total sulfate reduction in the hypolimnion during summer stratification. Rates of bacterial sulfate reduction in the sediment were not sulfate limited at sulfate concentrations greater than 0.1 mM in short-term experiments. Although sulfate reduction seemed to be sulfate limited below 0.1 mM, Michaelis-Menten kinetics were not observed. The optimum temperature (36 to 37°C) for sulfate reduction in the sediment was considerably higher than in situ temperatures (1 to 13°C). The response of sulfate reduction to the addition of various electron donors metabolized by sulfate-reducing bacteria in pure culture was investigated. The degree of stimulation was in this order: H₂ > n-butanol > n-propanol > ethanol > glucose. Acetate and lactate caused no stimulation.     

Stimulation of sulfate reduction rates in Mediterranean fish farm sediments inhabited by the seagrass <Emphasis Type="Italic">Posidonia oceanica</Emphasis>

Sulfate reduction rates and biogeochemical parameters of fish farm sediments across the Mediterranean were investigated in the order to evaluate the potential effects of organic matter inputs on habitat quality for the common seagrass Posidonia oceanica. Four study sites were selected in Spain, Italy, Greece and Cyprus to represent the Mediterranean basin. P. oceanica was found in immediate vicinity of all the farms, which were located at physically exposed sites about 1 km from the shore lines. Organic matter accumulation, sulfate reduction rates and sulfur pools were measured in depth profiles along transects from the farms in both bare and vegetated sediments. Results show that although the organic matter accumulation was minor at the sites (POC < 2.8% DW), the sulfate reduction rates were high, in particular at the largest farm in Italy (up to 212 mmol m⁻² d⁻¹), similar to rates found at shallower, temperate fish farm sites, where higher sedimentation rates can be expected. Sulfate reducing bacteria in these low-organic, carbonate-rich Mediterranean sediments respond strongly to organic matter loadings and cause habitat degradation. Sulfate reduction rates measured in the P. oceanica sediments were among the highest recorded (7.8–42.0 mmol m⁻² d⁻¹) similar to rates found in degrading meadows impacted by organic matter loadings. As sulfate reduction rates were correlated with the sedimentation rates along the transects rather than organic matter pools this suggests mineralization processes were controlled by organic matter loading in fish farm sediments. The vegetated sediments near the net cages were more reduced due to accumulation of sulfides compared to control sites, which is a possible contributing factor to the observed seagrass decline in the farm surroundings. It is recommended that Mediterranean fish farms are placed in areas with rapid dispersal of particulate waste products to minimize organic matter loading of the sediments and thereby preserve habitat quality for benthic fauna and flora.     

Measurements of Seasonal Rates and Annual Budgets of Organic Carbon Fluxes in an Antarctic Coastal Environment at Signy Island, South Orkney Islands, Suggest a Broad Balance between Production and Decomposition

We report here the first comprehensive seasonal study of benthic microbial activity in an Antarctic coastal environment. Measurements were made from December 1990 to February 1992 of oxygen uptake and sulfate reduction by inshore coastal sediments at Signy Island, South Orkney Islands, Antarctica. From these measurements the rate of benthic mineralization of organic matter was calculated. In addition, both the deposition rate of organic matter to the bottom sediment and the organic carbon content of the bottom sediment were measured during the same period. Organic matter input to the sediment was small under winter ice cover, and the benthic respiratory activity and the organic content of the surface sediment declined during this period as available organic matter was depleted. On an annual basis, about 32% of benthic organic matter mineralization was anoxic, but the proportion of anoxic compared with oxic mineralization increased during the winter as organic matter was increasingly buried by the amphipod infauna. Fresh organic input occurred as the sea ice melted and ice algae biomass sedimented onto the bottom, and input was sustained during the spring after ice breakup by continued primary production in the water column. The benthic respiratory rate and benthic organic matter content correspondingly increased towards the end of winter with the input of this fresh organic matter. The rates of oxygen uptake during the southern summer (80 to 90 mmol of O₂ m^-2 day^-1) were as high as those reported for other sediments at much higher environmental temperatures, and the annual mineralization of organic matter was equally high (12 mol of C m^-2 year^-1). Seasonal variations of benthic activity in this antarctic coastal sediment were regulated by the input and availability of organic matter and not by seasonal water temperature, which was relatively constant at between -1.8 and 0.5°C. We conclude that despite the low environmental temperature, organic matter degradation broadly balanced organic matter production, although there may be significant interrannual variations in the sources of the organic matter inputs.     

Sulfate Reduction in Peat from a New Jersey Pinelands Cedar Swamp

Microbial sulfate reduction rates in acidic peat from a New Jersey Pine Barrens cedar swamp in 1986 were similar to sulfate reduction rates in freshwater lake sediments. The rates ranged from a low of 1.0 nmol cm⁻³ day⁻¹ in February at 7.5- to 10.0-cm depth to 173.4 nmol cm⁻³ day⁻¹ in July at 5.0- to 7.5-cm depth. The presence of living Sphagnum moss at the surface generally resulted in reduced rates of sulfate reduction. Pore water sulfate concentrations and water table height also apparently affected the sulfate reduction rate. Concentrations of sulfate in pore water were nearly always higher than those in surface water and groundwater, ranging from 26 to 522 μM. The elevated pore water sulfate levels did not result from the evapotranspiratory concentration of infiltrating stream water or groundwater, but probably resulted from oxidation of reduced sulfur compounds, hydrolysis of ester sulfates present in the peat, or both. The total sulfur content of peat that had no living moss at the surface was 164.64 ± 1.5 and 195.8 ± 21.7 μmol g (dry weight)⁻¹ for peat collected from 2.5 to 5.0 and 7.5 to 10.0 cm, respectively. Organosulfur compounds accounted for 84 to 88% of the total sulfur that was present in the peat. C-bonded sulfur accounted for 91 to 94% of the organic sulfur, with ester sulfate being only a minor constituent. Reduced inorganic sulfur species in peat from 2.5 to 7.5 cm were dominated by H₂S-FeS (68%), while pyritic sulfide was the predominant inorganic sulfur species in the peat from depths of 7.5 to 10.0 cm (75%).     

Sulfur and iron cycling in a coastal sediment: Radiotracer studies and seasonal dynamics

The seasonal variation in sulfate reduction ana the dynamics or sulfur ana iron geochemistry were studied throughout a year in sediment of Aarhus Bay, Denmark. A radiotracer method for measuring sulfate reduction rates was applied with incubation times down to 15 min and a depth resolution down to 2 mm in the oxidized surface layer of the sediment. The radiotracer data were analyzed by a mathematical model which showed that, due to partial, rapid reoxidation of radioactive sulfide during incubation, the actual reduction rates in this layer were probably underestimated 5-fold. In the deeper, sulfidic zone, measured rates appeared to be correct. Sulfate reduction followed the seasonal variation in temperature with maximum activity at 1–2 cm depth in late summer. In spite of its rapid production, free H₂S was detectable in the porewater only below the depth of free Fe²⁺ at 6–7 cm throughout the year. Following the massive sedimentation from a spring phytoplankton bloom, anaerobic degradation of phytoplankton detritus was strongly stimulated over several weeks. A transient reversed redox zonation developed with a thin, black zone on top of the brown, oxidized sediment layer due to intensive sulfate and iron reduction. Mineralization through sulfate reduction was equivalent to two thirds of the annual net sedimentation of organic matter.Author for correspondence     

Flowthrough Reactor Flasks for Study of Microbial Metabolism in Sediments

Flowthrough reactor flasks are described that allow continuous low-level nutrient input to mixed anoxic sediments without dilution of the sediment. The flasks were tested by simulating sulfate inputs into sediments collected from a freshwater eutrophic lake. After an initial 2-day adaptation within the reactor system, rates of methane production and sulfate consumption were constant for the duration of a 12-day incubation. A sulfate input rate of 0.15 mmol liter of sediment⁻¹ day⁻¹ resulted in an equivalent rate of sulfate removal, which was unaffected by inputs of acetate (1.0 mmol liter of sediment⁻¹ day⁻¹). The rate of methane production in control reactors, 0.18 mmol liter of sediment⁻¹ day⁻¹, was doubled by the addition of acetate, whereas sulfate consumption was only stimulated by additions of high concentrations of sulfate plus acetate (1.5 and 1.0 mmol liter of sediment⁻¹ day⁻¹, respectively). The reactor system appears to be effective in maintaining the balance between sulfate reduction and methane production in freshwater sediments and is potentially useful for study of the response of sediment populations to varying inputs of naturally occurring substrates, selected inhibitors, or xenobiotic compounds.     

Sulfhydrolase Activity in Sediments of Wintergreen Lake, Kalamazoo County, Michigan

The hydrolysis of p-nitrophenyl sulfate, p-nitrocatechol sulfate, and [³⁵S]sodium dodecyl sulfate was examined in anoxic sediments of Wintergreen Lake, Michigan. Significant levels of sulfhydrolase activity were observed in littoral, transition, and profundal sediment samples. Rates of sulfate formation suggest that the sulfhydrolase system would represent a major source of sulfate within these sediments. Sulfate formed by ester sulfate hydrolysis can support dissimilatory sulfate reduction as shown by the incorporation of ³⁵S from labeled sodium dodecyl sulfate into H₂³⁵S. Sulfhydrolase activity varied with sediment depth, was greatest in the littoral zone, and was sensitive to the presence of oxygen. Estimations of ester sulfate concentrations in sediments revealed large quantities of ester sulfate (~30% of total sulfur). Both total sulfur and ester sulfate concentrations varied with the sediment type and were two to three orders of magnitude greater than the inorganic sulfur concentration.     

Seasonality of sulfate reduction and pore water solutes in a marine fish farm sediment: the importance of temperature and sedimentary organic matter

Sulfate reduction and pore water solutes related to sulfur cycling and anaerobic processes (short chain fatty acids (SCFA), SO₄ ^2–, TCO₂, NH₄ ⁺, dissolved sulfides (H₂S) and CH₄) were examined during one year at a marine fish farm. Mineralization of fish farm waste products was rapid in this non-bioturbated, organic rich sediment. Stimulation of sulfate reduction rates (SRR) occurred primarily in the surface layers where the organic matter was deposited. Acetate was the most important (<99%) of the measured SCFA attaining high concentrations during summer months (up to 4.7 mM). The acetate profiles exhibited distinct seasonal cycles, where periods with high concentrations in the pore waters were found coincident with a high pool of particulate organic matter in the surface sediments and a low activity of the sulfate reducing bacteria (early spring and late summer). Periods with low acetate pools occurred when sulfate reduction rates were high in early summer and in winter were pools of particulate organic matter were decreasing. Methane production was observed concurrent with sulfate reduction in the microbial active surface layers in late summer. Subsurface peaks of SO₄ ^2–, TCO₂, NH₄ ⁺ and H₂S were evident in July and August due to rapid mineralization in these surface layers. With decreasing autumn water temperatures mineralization rates declined and subsurface peaks of these solutes disappeared. A strong relationship was found between pore water TCO₂, and NH₄ ⁺. Ratios between TCO₂, and NH₄ ⁺ were low compared to a control site, attaining minimum values in mid-summer. This indicated rapid nitrogen mineralization of nitrogen rich labile substrates in the fish farm sediment during the entire season.     

Sulfate reduction and sediment metabolism in Tomales Bay,California

Sulfate reduction rates (SRR) in subtidal sediments of Tomales Bay, California, were variable by sediment type, season and depth. Higher rates were measured in near-surface muds during summer (up to 45 nmol cm^-3 h^-1), with lower rates in sandy sediments, in winter and deeper in the sediment. Calculations of annual, average SRR throughout the upper 20 cm of muddy subtidal sediments (about 30 mmol S m^-2 d^-1) were much larger than previously reported net estimates of SRR derived from both benthic alkalinity flux measurements and bay wide, budget stoichiometry (3.5 and 2.6 mmol m^-2 d^-1, respectively), indicating that most reduced sulfur in these upper, well-mixed sediments is re-oxidized. A portion of the net alkalinity flux across the sediment surface may be derived from sulfate reduction in deeper sediments, estimated from sulfate depletion profiles at 1.5 mmol m^-2 d^-1. A small net flux of CO₂ measured in benthic chambers despite a large SRR suggests that sediment sinks for CO₂ must also exist (e.g., benthic microalgae).     

Inorganic sulfur turnover in oligohaline estuarine sediments

Inorganic sulfur turnover was examined in oligohaline (salinity < 2 g kg^-1) Chesapeake Bay sediments during the summer. Cores incubated for < 3 hr exhibited higher sulfate reduction (SR) rates (13–58 mmol m^-2 d^-1) than those incubated for 3–8 hr (3–8 mmol m^-2 d^-1). SR rates (determined with³⁵SO ₄ ^2- ) increased with depth over the top few cm to a maximum at 5 cm, just beneath the boundary between brown and black sediment. SR rates decreased below 5 cm, probably due to sulfate limitation (sulfate < 25 μM). Kinetic experiments yielded an apparent half-saturating sulfate concentration (K_s) of 34 μM, ≈ 20-fold lower than that determined for sediments from the mesohaline region of the estuary. Sulfate loss from water overlying intact cores, predicted on the basis of measured SR rates, was not observed over a 28-hr incubation period. Reduction of³⁵SO ₄ ^2- during diffusion experiments with intact core segments from 0–4 and 5–9 cm horizons was less than predicted by non-steady state diagenetic models based on³⁵SO ₄ ^2- reduction in whole core injection experiments. The results indicate that net sulfate flux into sediments was an order of magnitude lower than the gross sulfur turnover rate. Solid phase reduced inorganic sulfur concentrations were only 2–3 times less than those in sediments from the mesohaline region of the Bay, despite the fact that oligohaline bottom water sulfate concentrations were 10-fold lower. Our results demonstrate the potential for rapid SR in low salinity estuarine sediments, which are inhabited by sulfate-reducing bacteria with a high affinity for sulfate, and in which sulfide oxidation processes replenish the pore water sulfate pool on a time scale of hours.     

Sulfur cycling in the water column of Little Rock Lake, Wisconsin

The S cycle in the water column of a small, soft-water lake was studied for 9 years as part of an experimental study of the effects of acid rain on lakes. The two basins of the lake were artificially separated, and one basin was experimentally acidified with sulfuric acid while the other served as a reference or control. Spatial and seasonal patterns of sulfate uptake by plankton (53–70 mmol m^–2 yr^–1), deposition of sulfur to sediments in settling seston (53 mmol m^–2 yr^–1), and sulfate diffusion (0–39 mmol m^–2 yr^–1) into sediments were examined. Measurements of inputs (12–108 mmol m^–2 yr^–1) and outputs (5.5–25 mmol m^–2 yr^–1) allowed construction of a mass balance that was then compared with rates of S accumulation in sediments cores (10–28 mmol m^–2 yr^–1) and measured fluxes of S into the sediments. Because of the low SO₄ ^2– concentrations (µmole L^–1) in the lake, annual uptake by plankton (53–70 mmol m^–2 yr^–1) represented a large fraction (>50%) of the SO₄ ^2– inventory in the lake. Despite this large flux through the plankton, only small seasonal fluctuations in SO₄ ^2– concentrations (µmole L^–1) were observed; rapid mineralization of organic matter (half-life <3 months) prevented sulfate depletion in the water column. The turnover time for sulfate in the water column is only 1.4 yr; much less than the 11-yr turnover time of a conservative ion in this seepage lake. Sulfate diffusion into and reduction in the sediments (0–160 µmole m^–2 d^–1) caused SO₄ ^2– depletion in the hypolimnion. Modeling of seasonal changes in lake-water SO₄ ^2– concentrations indicated that only 30–50% of the diffusive flux of sulfate to the sediments was permanently incorporated in solid phases, and about 15% of sulfur in settling seston was buried in the sediments. The utility of sulfur mass balances for seepage lakes would be enhanced if uncertainty about the deposition velocity for both sulfate aerosols and SO₂, uncertainty in calculation of a lake-wide rate of S accumulation in sediments, and uncertainty in the measured diffusive fluxes could be further constrained.     

Denitrification and Ammonia Formation in Anaerobic Coastal Sediments

Simultaneous determinations of nitrogen gas production, ammonia, and particulate organic nitrogen formation in the coastal sediments of Mangoku-Ura, Simoda Bay, and Tokyo Bay were made by using the ¹⁵N-label tracer method. The rate of nitrogen gas production in the sediment surface layer was about 10⁻² μg atom of N per g per h, irrespective of the location of the sediments examined. [¹⁵N]ammonia and -particulate organic nitrogen accounted for 20 to 70% of the three products, and after several hours of incubation, the major fraction of nondenitrified ¹⁵N in Mangoku-Ura and Simoda Bay sediments was recovered as ammonia. In Tokyo Bay sediments, particulate organic nitrogen was produced at a greater rate than was ammonia. The reduction rate data suggest that the pathway of nitrate reduction to ammonia is important in coastal sediments.     

Pathways and Microbiology of Thiosulfate Transformations and Sulfate Reduction in a Marine Sediment (Kattegat, Denmark)

Reductive and oxidative pathways of the sulfur cycle were studied in a marine sediment by parallel radiotracer experiments with ³⁵SO₄^2-, H₂³⁵S, and ³⁵S₂O₃^2- 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 >10⁶ cm^-3 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 H₂ 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% S₂O₃^2- and 35 to 15% SO₄^2-. 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.     

Lithotrophic and Heterotrophic Bacteria in Deep Subsurface Sediments and Their Relation to Sediment Properties

Subsurface sediments obtained from three cores drilled to depths of 260 m below the surface in South Carolina were analyzed for heterotrophic bacteria; N₂‐fixing microaerophiles; and nitrifying, sulfur‐oxidizing, and H₂‐oxidizing lithotrophic bacteria. In addition, pore waters were extracted for chemical analysis of inorganic nitrogen species, sulfate, dissolved organic carbon, pH, and Eh. Autotroph populations were generally less than 10³ most probable number (MPN) g^‐1 dry sediment with sulfur‐oxidizing bacteria, detected in 60% of the sediment samples, being the most frequently encountered group. Nitrifying bacteria were detected mainly in sediments from one borehole (P28), and their populations in those sediments were correlated with pore‐water ammonium concentrations. Populations of heterotrophic bacteria in 60% of the sediments were greater than 10⁶ colony forming units (CFU) g^‐1 dry sediment and were typically lower in sediments of high clay content and low pH. Microaerophilic N₂‐fixing bacteria were cultured from >50% and bacteria capable of growth on H₂ were cultured from 35% of the subsurface sediments examined. Sediment texture, which controls porosity, water potential, and hydraulic conductivity, appears to be a major factor influencing microbial populations in coastal plain subsurface sediments.     

Electron Donors Utilized by Sulfate-Reducing Bacteria in Eutrophic Lake Sediments

Mineralization rates of ¹⁴C-labeled substrates were determined in the presence and absence of Na₂MoO₄, an inhibitor of sulfate reduction, in the profundal sediments of a shallow eutrophic lake. Sulfate reduction was inhibited by Na₂MoO₄ at all concentrations tested (0.2 to 200 mM), whereas methane production was inhibited at Na₂MoO₄ concentrations greater than 20 mM. Initial mineralization rates of glucose were unaffected by Na₂MoO₄; however, Na₂MoO₄ decreased the mineralization rates of lactate (58%), propionate (52%), an amino acid mixture (85%), and acetate (14%). These decreases in the rates of mineralization were attributed to inhibition of sulfate reduction. Hydrogen stimulated the reduction of ³⁵SO₄²⁻ 2.5- to 2.8-fold, demonstrating potential hydrogen oxidation by sulfate-reducing bacteria. These results indicate that sulfate reducers utilize an array of substrates as electron donors and are of potential significance to the in situ mineralization of lactate, propionate, and free amino acids in these sediments.     

Diversity decoupled from sulfur isotope fractionation in a sulfate‐reducing microbial community

The extent of fractionation of sulfur isotopes by sulfate‐reducing microbes is dictated by genomic and environmental factors. A greater understanding of species‐specific fractionations may better inform interpretation of sulfur isotopes preserved in the rock record. To examine whether gene diversity influences net isotopic fractionation in situ, we assessed environmental chemistry, sulfate reduction rates, diversity of putative sulfur‐metabolizing organisms by 16S rRNA and dissimilatory sulfite reductase (dsrB) gene amplicon sequencing, and net fractionation of sulfur isotopes along a sediment transect of a hypersaline Arctic spring. In situ sulfate reduction rates yielded minimum cell‐specific sulfate reduction rates < 0.3 × 10^?15 moles cell^?1 day^?1. Neither 16S rRNA nor dsrB diversity indices correlated with relatively constant (38‰–45‰) net isotope fractionation (ε³⁴S_{sulfide‐sulfate}). Measured ε³⁴S values could be reproduced in a mechanistic fractionation model if 1%–2% of the microbial community (10%–60% of Deltaproteobacteria) were engaged in sulfate respiration, indicating heterogeneous respiratory activity within sulfate‐reducing populations. This model indicated enzymatic kinetic diversity of Apr was more likely to correlate with sulfur fractionation than DsrB. We propose that, above a threshold Shannon diversity value of 0.8 for dsrB, the influence of the specific composition of the microbial community responsible for generating an isotope signal is overprinted by the control exerted by environmental variables on microbial physiology.     

Chemoautotrophic Carbon Fixation Rates and Active Bacterial Communities in Intertidal Marine Sediments

Chemoautotrophy has been little studied in typical coastal marine sediments, but may be an important component of carbon recycling as intense anaerobic mineralization processes in these sediments lead to accumulation of high amounts of reduced compounds, such as sulfides and ammonium. We studied chemoautotrophy by measuring dark-fixation of ¹³C-bicarbonate into phospholipid derived fatty acid (PLFA) biomarkers at two coastal sediment sites with contrasting sulfur chemistry in the Eastern Scheldt estuary, the Netherlands. At one site where free sulfide accumulated in the pore water right to the top of the sediment, PLFA labeling was restricted to compounds typically found in sulfur and ammonium oxidizing bacteria. At the other site, with no detectable free sulfide in the pore water, a very different PLFA labeling pattern was found with high amounts of label in branched i- and a-PLFA besides the typical compounds for sulfur and ammonium oxidizing bacteria. This suggests that other types of chemoautotrophic bacteria were also active, most likely Deltaproteobacteria related to sulfate reducers. Maximum rates of chemoautotrophy were detected in first 1 to 2 centimeters of both sediments and chemosynthetic biomass production was high ranging from 3 to 36 mmol C m⁻² d⁻¹. Average dark carbon fixation to sediment oxygen uptake ratios were 0.22±0.07 mol C (mol O₂)⁻¹, which is in the range of the maximum growth yields reported for sulfur oxidizing bacteria indicating highly efficient growth. Chemoautotrophic biomass production was similar to carbon mineralization rates in the top of the free sulfide site, suggesting that chemoautotrophic bacteria could play a crucial role in the microbial food web and labeling in eukaryotic poly-unsaturated PLFA was indeed detectable. Our study shows that dark carbon fixation by chemoautotrophic bacteria is a major process in the carbon cycle of coastal sediments, and should therefore receive more attention in future studies on sediment biogeochemistry and microbial ecology.