Rapid Sulfate Reduction Research Articles

The role of S−3 ion in thermochemical sulphate reduction: Geological and geochemical implications

Thermochemical sulphate reduction (TSR) plays a crucial role in the global sulphur cycle in the Earth's crust, and may affect current and past sulphur isotopic records. However, the extrapolation of experimental reaction rates measured at high temperature (above 200 °C) towards lower temperatures, as well as the interpretation of the sulphur isotopic fractionation recorded in natural samples, require an accurate description of the elementary steps controlling these reactions. We addressed this question through dedicated experiments. Based on in situ Raman spectroscopy measurements, we show that the trisulphur ion S−3 is the dominant intermediate sulphur valence species involved in abiogenic sulphate reduction processes initiated by H2S, over a wide range of temperature (100-350 °C) and solution compositions, whatever the electron donor considered. The in situ spectroscopic data reported here unambiguously demonstrate the presence of S−3 at temperatures as low as 100 °C. The presence of S−3 is critical to achieve rapid sulphate reduction, especially at low temperature. We propose that any dissolved constituent which decreases the dielectric constant of water, or which yields favourable S−3 coordination, will stabilise the trisulphur ion (thus promoting TSR) at T and pH conditions that are less extreme than previously thought. The importance of S−3 in these processes should also be taken into account when discussing the mass-independent sulphur isotopic compositions recorded in natural and/or experimental TSR-related samples.

Methane Migration and Its Influence on Sulfate Reduction in the Good Weather Ridge Region, South China Sea Continental Margin Sediments

Bacteria sulfate reduction is a major pathway for organic carbon oxidation in marine sediments. Upward diffusion of methane from gas hydrate deep in the sedimentary strata might be another important source of carbon for sulfate reducing bacteria and subsequently induce higher rates of sulfate reduction in sediments. Since abundant gas may migrate upward to the surface as a result of tectonic activity occurring in the accretionary wedge, this study investigates the effect of methane migration on the sulfate reduction process in continental margin sediments offshore southwestern Taiwan. Piston and gravity core samples were taken in order to evaluate vertical and spatial variations of sulfate and methane. Pore water sulfate, sulfide, methane, sediment pyrite, and organic carbon were extracted and analyzed. Rapid sulfate reduction, high pyrite contents in sediments and higher concentrations of dissolved sulfide and methane were found in the study area with relatively low organic carbon concentrations. Up to 300 µmole g -1 of pyrite, 10 mM dissolved sulfide and 9 mM methane were found in the study region. Sulfate depletion was as shallow as ~1 m beneath the sediment/water interface. At similar water depths, however, low methane and pyrite were found in some areas with almost no sulfate reduction.

Rates of carbonate cementation associated with sulphate reduction in DSDP/ODP sediments: implications for the formation of concretions

DSDP/ODP porewater profiles in organic carbon-bearing (<5% org. C) sediments commonly show decreases in Ca 2+ concentrations and increases in alkalinity over depths where sulphate is being removed by microbial reduction. These Ca 2+ depletion profiles represent the combined effect of diffusion, advection and reaction (addition by ion exchange and removal by precipitation mainly as CaCO 3 and/or dolomite). A diagenetic model has been used to estimate the rate constant ( k) for Ca 2+ removal by precipitation during sulphate depletion over depths of 15–150 m, assuming first order kinetics. The rate constants for Ca 2+ removal range from 10 −14 to 10 −11 s −1 in 19 DSDP/ODP sediments, which span a range of bottom water temperatures (0–10 °C), lithologies (calcareous to clastic) and sedimentation rates (0.001–0.4 cm year −1). Values of k correlate with sedimentation rate ( ω) such that log k=1.16 log ω−10.3, indicating that faster rates of Ca 2+ removal occur at higher sedimentation rates where there are also higher degrees of saturation with respect to CaCO 3 and dolomite. Depth-integrated masses of Ca 2+ removed (<100 μmol cm −2) during sulphate depletion over these depth ranges are equivalent to a dispersed phase of approximately 1.5 wt.% CaCO 3 or 3 wt.% dolomite in a compacted sediment. The complete occlusion of sediment porosity observed in concretions with isotopic signatures suggesting carbonate sourced from sulphate reduction therefore requires more time (a depositional hiatus), more rapid sulphate reduction (possibly by anaerobic methane oxidation) and/or the continued transport of isotopically light carbonate to the concretion site after sulphate reduction has ceased.

Nutrient fluxes and sulfur cycling in the organic-rich sediment of Makirina Bay (Central Dalmatia, Croatia)

Fluxes of dissolved nutrients in the organic-rich sediment of Makirina Bay (Croatia) were estimated in in-situ incubation experiments and compared to those calculated from a diagenetic model based on the concentration vs. depth profiles in the pore water extracted from a sediment core at the same sampling site. Benthic fluxes observed in the in-situ experiment were strongly influenced by the decomposition of sedimentary organic matter by bacterial sulfate reduction and precipitation of authigenic pyrite and calcite. Assimilation of dissolved nitrate and ammonia within the sediment resulted in peculiar nitrate and ammonia concentration profiles. Differences between the benthic and diffusive fluxes indicated that the effects of bioturbation and irrigation, which were not taken into account in the model calculation of diffusive fluxes, should not be considered as negligible. They are, however, extremely difficult to estimate because of the heterogeneity of the sediment. Sulfate reduction in the pore water could not be detected, although it was obvious from pyrite precipitation and the isotopic compositions of different sedimentary sulfur species. It is suggested that reoxidation of sulfide and upward diffusion of sulfate from the sediment layer accumulated during past salt production can account for the scattered sulfate vs. depth profile. The distribution and stable isotope compositions of sedimentary sulfur species were studied in more detail. The influx of sulfate was explained by rapid sulfate reduction in the sediment. The sulfate reduction rate was highest in the upper part of the sediment column, as concluded from δ 34S values. In lower horizons, the concentration of organic sulfur increased, while concentrations of sulfide decreased. The mirror-imaged concentration vs. depth profiles of sulfide and organic sulfur, as well as their stable isotopic compositions, show that they were formed simultaneously, most probably in competitive processes.

Authigenic carbonates in methane seeps from the Norwegian sea: Mineralogy, geochemistry, and genesis

Authigenic carbonates in the caldera of an Arctic (72°N) submarine mud volcano with active CH4bearing fluid discharge are formed at the bottom surface during anaerobic microbial methane oxidation. The microbial community consists of specific methane-producing bacteria, which act as methanetrophic ones in conditions of excess methane, and sulfate reducers developing on hydrogen, which is an intermediate product of microbial CH4 oxidation. Isotopically light carbon (δ13Cav =−28.9%0) of carbon dioxide produced during CH4 oxidation is the main carbonate carbon source. Heavy oxygen isotope ratio (δ18Oav = 5%0) in carbonates is inherited from seawater sulfate. A rapid sulfate reduction (up to 12 mg S dm−3 day−1) results in total exhausting of sulfate ion in the upper sediment layer (10 cm). Because of this, carbonates can only be formed in surface sediments near the water-bottom interface. Authigenic carbonates occurring within sediments occur do notin situ. Salinity, as well as CO 3 2− /Ca and Mg/Ca ratios, correspond to the field of nonmagnesian calcium carbonate precipitation. Calcite is the dominant carbonate mineral in the methane seep caldera, where it occurs in the paragenetic association with barite. The radiocarbon age of carbonates is about 10000 yr.

Sulfate-reducing bacteria methylate mercury at variable rates in pure culture and in marine sediments.

Differences in methylmercury (CH(3)Hg) production normalized to the sulfate reduction rate (SRR) in various species of sulfate-reducing bacteria (SRB) were quantified in pure cultures and in marine sediment slurries in order to determine if SRB strains which differ phylogenetically methylate mercury (Hg) at similar rates. Cultures representing five genera of the SRB (Desulfovibrio desulfuricans, Desulfobulbus propionicus, Desulfococcus multivorans, Desulfobacter sp. strain BG-8, and Desulfobacterium sp. strain BG-33) were grown in a strictly anoxic, minimal medium that received a dose of inorganic Hg 120 h after inoculation. The mercury methylation rates (MMR) normalized per cell were up to 3 orders of magnitude higher in pure cultures of members of SRB groups capable of acetate utilization (e.g., the family Desulfobacteriaceae) than in pure cultures of members of groups that are not able to use acetate (e.g., the family Desulfovibrionaceae). Little or no Hg methylation was observed in cultures of Desulfobacterium or Desulfovibrio strains in the absence of sulfate, indicating that Hg methylation was coupled to respiration in these strains. Mercury methylation, sulfate reduction, and the identities of sulfate-reducing bacteria in marine sediment slurries were also studied. Sulfate-reducing consortia were identified by using group-specific oligonucleotide probes that targeted the 16S rRNA molecule. Acetate-amended slurries, which were dominated by members of the Desulfobacterium and Desulfobacter groups, exhibited a pronounced ability to methylate Hg when the MMR were normalized to the SRR, while lactate-amended and control slurries had normalized MMR that were not statistically different. Collectively, the results of pure-culture and amended-sediment experiments suggest that members of the family Desulfobacteriaceae have a greater potential to methylate Hg than members of the family Desulfovibrionaceae have when the MMR are normalized to the SRR. Hg methylation potential may be related to genetic composition and/or carbon metabolism in the SRB. Furthermore, we found that in marine sediments that are rich in organic matter and dissolved sulfide rapid CH(3)Hg accumulation is coupled to rapid sulfate reduction. The observations described above have broad implications for understanding the control of CH(3)Hg formation and for developing remediation strategies for Hg-contaminated sediments.

Organic carbon deposition and its control on iron sulfide formation of the southern East China Sea continental shelf sediments

Sulfate reduction rates and sedimentation rates were determined in sediments at ten stations on the southern East China Sea continental shelf. The area is characterized by relatively low organic carbon concentration with fast sedimentation rate. Rapid sulfate reduction rates, ranging from 1 to 4 mmol/m 2/day, were observed. Organic carbon concentration varied from 0.3 to 0.6 wt%. Sedimentation rate varied from 0.2 to 0.7 cm/yr. In addition, normal marine S/C ratios were observed. Up to 96% of pyrite-sulfur was reoxidized before its final burial. Sulfate reduction rate and pyrite-sulfur burial rate increased linearly with increasing organic carbon burial rate, indicating that organic carbon deposition controlled pyrite formation in the East China Sea continental shelf sediments. The organic carbon utilized by the sulfate reduction and its burial represented a significant but relatively small fraction of the primary production in the studied East China Sea region.

Porewater Distribution and Benthic Flux Measurements of Mercury and Methylmercury in the Gulf of Trieste (Northern Adriatic Sea)

The Gulf of Trieste is one of the most mercury-contaminated areas in the Mediterranean Sea. It is characterized by high mercury inputs from the Isonzo River whose tributary, the Idrijca River, drains the mercury mining area of Idrija in Slovenia where extraction activity has taken place for nearly 500 years. This appears, therefore, to be one of the most suitable sites for studying processes that affect Hg cycling in the marine environment and for determining whether sediments might act as secondary sources of mercury species in the water column. Porewater seasonal distributions of total dissolved Hg (HgT) and methylmercury (MeHg) were investigated. Usingin situbenthic chambers it was possible to determine benthic fluxes of HgTand MeHg at the water–sediment interface throughout the year. Benthic fluxes were also compared with diffusive fluxes calculated from porewater profiles. The results indicate that, following hypoxic conditions which occurred in late summer in the sea-bottom layer, highest benthic effluxes and porewater concentrations of Hg and MeHg appeared during autumn and winter. This was probably due to the transition from rapid sulphate reduction in late summer to cooler temperatures, higher oxygenation of the bottom water layer, and lower microbial activity which is well suited for Hg transformation, accumulation and flux. A tentative budget based on benthic flux measurements indicates that 75% of HgTis buried in the sediment whereas 25% of HgT, approximately 23% in methylated form, is annually recycled and released at the water–sediment interface.

Sulphur isotope fractionation in modern microbial mats and the evolution of the sulphur cycle

THE sulphur cycle has evolved over the course of the Earth's history1,2. The early Earth's surface environment was reducing, containing little atmospheric oxygen3, and with seawater sulphate concentrations estimated at less than a few per cent of those found today. The accumulation of sulphate in the ocean to much higher concentrations was probably coincident with the initial accumulation of oxygen in the atmosphere and the consequent oxidative weathering of sulphide minerals on land4,5. Past changes in sulphate concentrations in ancient oceans have previously been assessed by comparing the systematics of sulphur isotope fractionation by sulphate-reducing bacteria6–9 with the isotopic composition of sedimentary sulphides1,2,5,10,11. But such interpretations have proven equivocal: the generally small 34S depletions in Archaean sulphides (deposited ∼2.5–3.8 billion years ago) have been separately argued to result both from rapid sulphate reduction in a sulphate-rich ocean5,12, and from sulphide formation in a sulphate-poor ocean1,2,11. Here we report large 34S depletions of 20–25%, observed during rapid sulphate reduction by sulphate-reducing bacteria in modern photosynthetic cyano-bacterial mats from Solar Lake, Sinai. We conclude that high sulphate concentrations give rise to highly 34S-depleted sulphides, and thus that appreciable concentrations of seawater sulphate did not accumulate until the initial accumulation of oxygen into the atmosphere in post-Archaean times.

Anaerobic microbial biogeochemistry in sediments from two Basins in the Gulf of Maine: Evidence for iron and manganese reduction

Rates of sulphate reduction, denitrification and glucose turnover were determined in surface sediments collected from Jeffreys and Wilkinson Basins in the Gulf of Maine. These data were compared to porewater profiles of iron and manganese. Three distinct anaerobic biogeochemical situations were evident: (1) sulphate reduction was dominant and active near the sediment-water interface (deeper portions of Jeffreys Basin); (2) denitrification was active in the upper few centimetres and was underlain by rapid sulphate reduction (slope of Jeffreys Basin); (3) denitrification was dominant (Wilkinson Basin). An apparent iron and manganese reduction region was evident in all cores. Since glucose turnover was active and both denitrification and sulphate reduction were low within this metal reduction zone it is possible that microbial metal reduction is a significant process within the basin sediments. High concentrations of dissolved iron occurred within sediments deeper than those supporting active sulphate reduction. Sufficient iron was remobilized during iron reduction to not only precipitate all of the sulphide generated during sulphate reduction but to also allow for dissolved iron to diffuse into deeper sediments. Therefore, large vertical regions containing dissolved iron are not necessarily regions of iron reduction and iron reduction is restricted to a narrow sediment layer.

Assay of sulfur as triphenylphosphine sulfide by high performance liquid chromatography: application to studies of sulfur bioproduction and sulfur in marine sediments

A gas chromatographic (GC) method for sulfur assay based on derivatization with triphenylphosphine (TPP)_to form triphenylphosphine sulfide (TPPS) was modified for use with high performance liquid chromatography (HPLC). Sulfur was extracted from samples with CHCl 3 and derivatized with TPP dissolved in HCHl 3 containing 5% (w/v) phenol. Phenol promoted the reaction of TPP with sulfur and inhibited its reaction with oxygen to form TPPO, thus allowing derivatization in air. Air must be excluded in the GC method using toluene to extract and derivatize sulfur. Sulfur was assayed in marine sediments and in mixed cultures of Desulfovibrio desulfuricans with photosynthetic bacteria. Clorobium thiosufatophitum and Chromatium vinosum. Rapid sulfate reduction prevented initial attempts at obtaining sulfur-producing consortia. Sulfur-producing associations with C. vinosum were secured if large inocula of the photosynthetic bacterium were used. The biotechnology of sulfur production is discussed.