Stable Sulfur Isotope Fractionation Research Articles

Energy flux couples sulfur isotope fractionation to proteomic and metabolite profiles in Desulfovibrio vulgaris.

Microbial sulfate reduction is central to the global carbon cycle and the redox evolution of Earth's surface. Tracking the activity of sulfate reducing microorganisms over space and time relies on a nuanced understanding of stable sulfur isotope fractionation in the context of the biochemical machinery of the metabolism. Here, we link the magnitude of stable sulfur isotopic fractionation to proteomic and metabolite profiles under different cellular energetic regimes. When energy availability is limited, cell-specific sulfate respiration rates and net sulfur isotope fractionation inversely covary. Beyond net S isotope fractionation values, we also quantified shifts in protein expression, abundances and isotopic composition of intracellular S metabolites, and lipid structures and lipid/water H isotope fractionation values. These coupled approaches reveal which protein abundances shift directly as a function of energy flux, those that vary minimally, and those that may vary independent of energy flux and likely do not contribute to shifts in S-isotope fractionation. By coupling the bulk S-isotope observations with quantitative proteomics, we provide novel constraints for metabolic isotope models. Together, these results lay the foundation for more predictive metabolic fractionation models, alongside interpretations of environmental sulfur and sulfate reducer lipid-H isotope data.

Modeling of biogeochemical processes in a barrier island freshwater lens (Spiekeroog, Germany)

Freshwater lenses present valuable water resources on barrier islands. Yet, the biogeochemical processes that control the groundwater quality of these freshwater lenses and how they are affected by the prevailing groundwater dynamics is largely unexplored. In this study we investigated the biogeochemistry of a barrier island freshwater lens with a focus on understanding and quantifying organic matter mineralization, sulfur cycling, and chemical fluxes to the land-ocean interface. We analyzed a comprehensive set of hydrogeochemical field data from Spiekeroog Island (Germany), including stable sulfur isotope signatures of dissolved sulfur species, with a reactive transport modeling approach. Tritium-Helium groundwater ages were used to constrain the hydrogeochemistry as a function of residence time. Our results revealed that freshwater lenses can act as archives for anthropogenic pollution, conserving the high sulfur loads associated with historic atmospheric deposition. We observed two distinct (hydro)biogeochemical patterns, which we attribute to a heterogeneous distribution of reactive organic matter. Those patterns were well replicated by two separate reactive transport models that considered the variations in organic matter reactivity. Simulation and field results demonstrated that net sulfur cycling is dominated by microbial sulfate reduction and subsequent iron sulfide precipitation. In the absence of dissolved oxidants, we attribute the observed high stable sulfur isotope fractionation between dissolved sulfate and sulfide of up to 67‰ to low (<20 pmol mL−1 d−1) microbial sulfate reduction rates. We show that older groundwater becomes progressively enriched in ammonium and phosphate due to the mineralization of organic matter, and we speculate that lens-derived nutrient fluxes may be important for the benthic zone at local groundwater discharge sites, at least seasonally in spring and summer.

Elemental sulfur in northern South China Sea sediments and its significance

Elemental sulfur (ES) is one of the intermediates in the inorganic sulfur cycle and thus plays a key role in the fractionation of stable sulfur isotopes in different reservoirs and the marine environment. In this study, solid ES is discovered in sediments near the Jiulong Methane Reef in the northern South China Sea by scanning electron microscopy and Raman spectroscopy. Combining the morphology and distribution of ES, pyrite concentrations, and sulfur isotopes, we conclude that: (1) solid ES coexists with pyrite microcrystals and sulfide (oxyhydr)oxides as well as clay minerals, and they are mainly distributed on the surface of mineral aggregates; (2) ES mainly occurs within and near the sulfate-methane transition zone (SMTZ) despite little morphological diversity; (3) ES formation might be related to hydrogen sulfide oxidation and is therefore linked with fluctuations in the SMTZ. Within the SMTZ, hydrogen sulfide is produced and pyrite precipitates because of enhanced anaerobic oxidation of methane coupled with dissimilatory sulfate reduction. This enhances the efficiency of the inorganic sulfur cycle and provides favorable conditions for ES formation. The discovery of solid ES in sediments near the Jiulong Methane Reef suggests an important relationship with SMTZ fluctuations that could have implications for the evolution of methane hydrate in the South China Sea.

Sulfur isotope evolution in sulfide ores from Western Alps: Assessing the influence of subduction-related metamorphism

Sulfides entering subduction zones can play an important role in the release of sulfur and metals to the mantle wedge and contribute to the formation of volcanic arc-associated ores. Fractionation of stable sulfur isotopes recorded by sulfides during metamorphism can provide evidence of fluid-rock interactions during metamorphism and give insights on sulfur mobilization. A detailed microtextural and geochemical study was performed on mineralized samples from two ocean floor-related sulfide deposits (Servette and Beth-Ghinivert) in high-pressure units of the Italian Western Alps, which underwent different metamorphic evolutions. The combination of microtextural investigations with δ34S values from in situ ion probe analyses within individual pyrite and chalcopyrite grains allowed evaluation of the effectiveness of metamorphism in modifying the isotopic record and mobilizing sulfur and metals and have insights on fluid circulation within the slab. Textures and isotopic compositions inherited from the protolith are recorded at Beth-Ghinivert, where limited metamorphic recrystallization is attributed to limited interaction with metamorphic fluids. Isotopic modification by metamorphic processes occurred only at the submillimeter scale at Servette, where local interactions with infiltrating hydrothermal fluid are recorded by metamorphic grains. Notwithstanding the differences recorded by the two deposits, neither underwent intensive isotopic reequilibration or records evidence of intense fluid-rock interaction and S mobilization during metamorphism. Therefore, subducted sulfide deposits dominated by pyrite and chalcopyrite are unlikely to release significant quantities of sulfur to the mantle wedge and to arc magmatism sources at metamorphic grades below the lower eclogite facies.

Sulfate reducing activity and sulfur isotope fractionation by natural microbial communities in sediments of a hypersaline soda lake (Mono Lake, California)

Hypersaline soda lakes are extreme environments for microbial life. In the geological past, similar environments produced evaporite mineral deposits and sulfide-rich shales that are used to reconstruct the sulfur isotopic composition of seawater through time. The relationship between microbial sulfur metabolism and associated sulfur isotope fractionation is currently not well constrained for hypersaline or hyperalkaline environments. In this study, sulfate reduction rates (SRRs) and sulfur isotope fractionations (ε) were measured using flow-through reactors containing intact sediment slices collected from three sites and two depth intervals (0–2 and 2–4 cm) along the littoral zone of Mono Lake, a hypersaline soda lake located in California, USA. Incubation temperature (10, 20, 30, 40 and 50 °C) and inflow sulfate concentration (1, 2 and 3 mM) were varied. The resulting SRRs ranged up to 62 nmol cm − 3 h − 1 , with the highest rates measured at 40 °C. The rates were systematically lower in the deeper (2–4 cm) than in the shallower (0–2 cm) depth intervals. When inflow solutions were amended with lactate (10 mM), SRRs increased 2- to 5-fold, with a maximum value of 176 nmol cm − 3 h − 1 . Stable sulfur isotope fractionation was relatively small: average ε values varied between 5 and 21‰ with an overall mean value for the site of 12‰. The general inverse relationship between ε and SRR predicted by the standard isotope fractionation model for dissimilatory sulfate reduction was not observed. In fact, at SRRs below approximately 30 nmol cm − 3 h − 1 , ε values correlated positively with SRR. We interpret this to reflect a gradual increase in cellular energy available for the synthesis of adenosine-5′-phosphosulfate (APS) as metabolic activity increases, leading to greater isotopic selectivity. At higher SRRs, isotope fractionation follows the standard fractionation model and the positive trend shifts towards a weak negative one. Relatively low isotope fractionations at low rates of sulfate reduction may be a characteristic of halophilic sulfate reducers.

Paleo-redox conditions during OAE 2 reflected in Demerara Rise sediment geochemistry (ODP Leg 207)

Cretaceous black shales deposited under severe oxygen-depletion are carriers of proxy signals for paleoenvironmental conditions. Using high-resolution patterns of iron and sulfur speciation, stable sulfur isotope discrimination, and trace element enrichment from black shale sequences of Sites 1258 and 1260 we identified alterations of the depositional environment during the Cenomanian/Turonian boundary Event (OAE 2) in the southern North Atlantic (ODP Leg 207, Demerara Rise). Changes in redox-conditions are suggested by high ratios of reactive to total iron which indicate that pyrite was formed both in the water column and within the sediment. This corresponds to euxinic paleoenvironmental conditions with at least temporarily free dissolved sulfide in the water column, a situation similar to the modern deep Black Sea. In addition, besides fixation of sulfide as iron sulfide, organic matter acted as an important sulfur trap during early diagenesis. Stable sulfur isotope fractionation went through a minimum within the OAE 2 interval indicating enhanced sulfur isotope discrimination during highest burial of organic matter (OM) potentially due to lower burial efficiency of reduced sulfur and/or a higher contributions from the oxidative part of the sulfur cycle (e.g., in the water column or the surface sediments). Elevated Fe/Al and Co/Al values within the Cenomanian/Turonian interval confirm euxinic conditions but, at the same time, require a zone where reducing but non-sulfidic conditions prevail, allowing reductive Fe and Co mobilization in oxygen-depleted nearshore sediments. The existence of an expanded oxygen-minimum-zone (OMZ) is demonstrated by extremely low Mn/Al ratios. A change in the trace metal (TM) inventory of seawater is postulated from a decline in seawater derived TM enrichment. Because hypoxic or even euxinic environments form an important sink for TM, the enlargement of euxinic depositional areas at the global onset of black shale deposition during OAE 2 have likely led to a drawdown of the seawater TM reservoir.

Stable sulfur isotope fractionation and discrimination between the sulfur atoms of thiosulfate during oxidation by Halothiobacillus neapolitanus

Growing cultures and nongrowing suspensions of Halothiobacillus neapolitanus selectively fractionated (32)S and (34)S during the oxidation of the sulfane- and sulfonate-sulfur atoms of thiosulfate. Sulfate was enriched in (32)S, with delta(34)S reaching -6.3 per thousand relative to the precursor sulfonate-sulfur of thiosulfate, which was progressively resynthesized from the thiosulfate-sulfane-sulfur during thiosulfate metabolism. Polythionates, principally trithionate, accumulated during thiosulfate oxidation and showed progressive increase in the relative (34)S content of their sulfonate groups, with delta(34)S values up to +20 per thousand, relative to the substrate sulfur. The origins of the sulfur in the sulfate and polythionate products of oxidation were tracked by the use thiosulfate labelled with (35)S in each of its sulfur atoms, enabling determination of the flow of the sulfur atoms into the oxidation products. The results confirm that highly significant fractionation of stable sulfur isotopes can be catalyzed by thiobacilli oxidizing thiosulfate, but that differences in the (34)S/(32)S ratios of the nonequivalent constituent sulfur atoms of the thiosulfate used as substrate mean that the oxidative fate of each atom needs separate determination. The data are very significant to the understanding of bacterial sulfur-compound oxidation and highly relevant to the origins of biogenic sulfate minerals.

Sulfur isotope fractionation during growth of sulfate-reducing bacteria on various carbon sources

Stable sulfur isotope fractionation during microbial sulfate reduction is a potential tool to estimate sulfate reduction rates at field sites. However, little is known about the influence of the utilized carbon source on the magnitude of sulfur isotope fractionation. To investigate this effect, both a pure culture (strain PRTOL1) and enrichment cultures from a petroleum hydrocarbon (PHC)–contaminated aquifer were used and grown in batch cultures on various carbon sources with an initial sulfate concentration of 1 mmol/L. As sole carbon sources the PHC components naphthalene, 1,3,5-trimethylbenzene, and heating oil (enrichment culture) and the organic acids acetate, pyruvate, benzoate, and 3-phenylpropionate (enrichment culture and PRTOL1) were used. Sulfate reduction rates of all cultures ranged from 6 ± 1 nmol cm −3 d −1 (enrichment culture grown on 1,3,5-trimethylbenzene) to 280 ± 6 nmol cm −3 d −1 (enrichment culture grown on pyruvate). Cell-specific sulfate reduction rates ranged from 1.1 × 10 −14 mol cell −1 d −1 (PRTOL1 grown on pyruvate) to 1.5 × 10 −13 mol cell −1 d −1 (PRTOL1 grown on acetate). Sulfur isotope enrichment factors (ε) for the enrichment culture ranged from 16.1‰ (3-phenylpropionate) to 34.5‰ (1,3,5-trimethylbenzene) and for PRTOL1 from 30.0‰ (benzoate) to 36.0‰ (pyruvate). Cultures of PRTOL1 always showed higher ε values than the enrichment culture when grown on the same carbon source due to culture-specific properties. Higher ε values were obtained when the enrichment culture was grown on PHC components than on organic acids. No relationship between ε values and cell-specific sulfate reduction rate existed when all data were combined. When comparing the magnitude of ε values determined in this laboratory study with ε values measured at contaminated and uncontaminated field sites, it becomes evident that a multitude of factors influences ε values at field sites and complicates their interpretation. The results of this study help us assess some of the general parameters that govern the magnitude of ε in sulfate-reducing environments.

Microbial sulfate reduction in deep sediments of the Southwest Pacific (ODP Leg 181, Sites 1119–1125): evidence from stable sulfur isotope fractionation and pore water modeling

Interstitial water samples from seven ODP sites (Leg 181, Sites 1119–1125) of the southwestern Pacific Ocean have been analyzed for the stable sulfur isotopic composition of dissolved sulfate along with major and minor ions. Sulfate from the interstitial fluids (δ 34S values between +20.7 and +60‰ vs. the SO 2-based Vienna–Canyon Diablo troilite standard) was enriched in 34S with respect to modern sea water (δ 34S≈+20.6‰) indicating that microbial sulfate reduction takes place to different extents at all investigated sites. Microbial sulfate reduction (MSR) was found at all sites, the intensity depending on the availability of organic matter which is controlled by paleo-sedimentation conditions (sedimentation rate, presence of turbidites) and productivity. Microbial net sulfate reduction was additionally confirmed by modeling interstitial water sulfate profiles. Areal net sulfate reduction rates up to 14 mmol m −2 yr −1 have been calculated which were positively related to sedimentation rates. Total reduced inorganic sulfur (TRIS; essentially pyrite) as a product of microbial sulfate reduction was isotopically characterized in squeeze cake samples and gave δ 34S values between −51 and +9‰ indicating pyrite formation both close to the sediment–water interface and later diagenetic contributions.

Sulfur isotope fractionation during microbial sulfate reduction by toluene-degrading bacteria

Sulfate-reducing bacteria contribute considerably to the mineralization of petroleum hydrocarbons (PHC) in contaminated environments. Stable sulfur isotope fractionation during microbial sulfate reduction was investigated in microcosm experiments with different cultures of sulfate-reducing bacteria for various initial sulfate concentrations using toluene as the sole carbon source. Experiments were conducted with the marine strain Desulfobacula toluolica, the fresh water strain PRTOL1, and an enrichment culture from a PHC-contaminated aquifer. Sulfate reduction rates ranged from 7 ± 1 to 494 ± 9 nmol cm−3 d−1, whereas specific sulfate reduction rates (sSRR) ranged from 8.9 × 10−15 to 3.9 × 10−13 ± 9.2 × 10−14 mol cell−1 d−1. Calculated enrichment factors (ϵ) for the fractionation of stable sulfur isotopes during microbial sulfate reduction ranged from 19.8 ± 0.9 to 46.9 ± 2.1‰. In general, values of ϵ and sSRR obtained in our experiments were similar to those reported previously for sulfate-reducing bacteria incubated with readily available carbon sources under optimal growth conditions. Moreover, we found no obvious correlation between ϵ and sSRR values when data from all our microcosm experiments were combined or when we combined our data with several previously published data sets. In contrast, ϵ values determined in our enrichment culture experiments (average 23.5 ± 4.3‰) agreed well with ϵ values determined in a recent field study performed in situ in a PHC-contaminated aquifer. Thus, results from this laboratory study provide valuable information on stable sulfur isotope fractionation during microbial sulfate reduction under conditions that more closely resemble those in PHC-contaminated environments, i.e., for a variety of sulfate concentrations, including low sulfate concentrations, and for a an important PHC-constituent (toluene) used as sole carbon source.

Controls on stable sulfur isotope fractionation during bacterial sulfate reduction in Arctic sediments

Sulfur isotope fractionation experiments during bacterial sulfate reduction were performed with recently isolated strains of cold-adapted sulfate-reducing bacteria from Arctic marine sediments with year-round temperatures below 2°C. The bacteria represent quantitatively important members of a high-latitude anaerobic microbial community. In the experiments, cell-specific sulfate reduction rates decreased with decreasing temperature and were only slightly higher than the inferred cell-specific sulfate reduction rates in their natural habitat. The experimentally determined isotopic fractionations varied by less than 5.8‰ with respect to temperature and sulfate reduction rate, whereas the difference in sulfur isotopic fractionation between bacteria with different carbon oxidation pathways was as large as 17.4‰. Incubation of sediment slurries from two Arctic localities across an experimental temperature gradient from −4°C to 39°C yielded an isotopic fractionation of 30‰ below 7.6°C, a fractionation of 14‰ and 15.5‰ between 7.6°C and 25°C, and fractionations of 5‰ and 8‰ above 25°C, respectively. In absence of significant differences in sulfate reduction rates in the high and low temperature range, respectively, we infer that different genera of sulfate-reducing bacteria dominate the sulfate-reducing bacterial community at different temperatures. In the Arctic sediments where these bacteria are abundant the isotopic differences between dissolved sulfate, pyrite, and acid-volatile sulfide are at least twice as large as the experimentally determined isotopic fractionations. On the basis of bacterial abundance and cell-specific sulfate reduction rates, these greater isotopic differences cannot be accounted for by significantly lower in situ bacterial sulfate reduction rates. Therefore, the remaining isotopic difference between sulfate and sulfide must derive from additional isotope effects that exist in the oxidative part of the sedimentary sulfur cycle.

Microbial processes of carbon and sulfur cycles in lake Mogil'noe

In the beginning of summer 1999, complex microbiological and biogeochemical investigations of meromictic Lake Mogil'noe (Kil'din Island, Barents Sea) were carried out. The analysis of the results shows clearly pronounced vertical zonality of the microbial processes occurring in the water column of the lake. To a depth of 8 m, the total number and activity of microorganisms was limited by the relatively low content of organic matter (OM). In the upper part of the hydrogen-sulfide zone of the lake (beginning at a depth of 8.25 m), the content of particulate OM and the microbial number sharply increased. In this zone, the daily production of OM during anaerobic photosynthesis at the expense of massive development of colored sulfur bacteria reached 620 mg C/m2, which was twofold greater than the daily production of phytoplankton photosynthesis and led to a considerable change in the isotopic composition (delta 13 C) of the particulate OM. In the same intermediate layer, the highest rates of sulfate reduction were recorded, and fractionation of stable sulfur isotopes occurred. Below 10 m was the third hydrochemical zone, characterized by maximum concentrations of H2S and CH4 and by a relatively high rate of autotrophic methanogenesis. The comparison of the results obtained with the results of investigations of previous years, performed in the end of summer, shows a decrease in the intensity of all microbial processes inspected. An exception was anoxygenic photosynthesis, which can utilize not only the de novo formed H2S but also the H2S accumulated in the lake during the winter period.

Stable sulfur isotope fractionation during the reduction of thiosulfate by Dethiosulfovibrio russensis

Stable sulfur isotope fractionation was investigated during reduction of thiosulfate by growing batch cultures of Dethiosulfovibrio russensis at a cell-specific reduction rate of 2.4 +/- 0.72 fmol cell(-1) d(-1) (28 degrees C). Citrate was used as carbon and energy source. The hydrogen sulfide produced by this sulfur- and thiosulfate-reducing bacterium was depleted in 34S by 11% compared to total thiosulfate sulfur, in agreement with previous results observed for sulfate-reducing bacteria. This indicates the operation of a similar pathway for thiosulfate reduction in these phylogenetically different bacteria.

Sulfur isotope fractionation during experimental precipitation of iron(II) and manganese(II) sulfide at room temperature

Fractionation of stable sulfur isotopes ( 34S/ 32S) during the precipitation of iron(II) and manganese(II) sulfides from aqueous solutions at room temperature (≈20°C) was investigated using batch-type experiments. The sulfides were precipitated (1) by mixing of metal(II) chloride solutions with hydrogen bisulfide solutions, and (2) by the transformation of anhydrous metal(II) carbonates in hydrogen bisulfide solutions. It was found that the partitioning of 34S/ 32S upon metal sulfide precipitation depends on the synthesis procedure, an indication for the influence of reaction kinetics. The sulfur isotopic composition of sedimentary metal monosulfides is close to that of dissolved hydrogen bisulfide. Kinetic fractionation factors for method (1) were 0.9988±0.0007 ( α′(FeS–HS −)) and 1.0002±0.0007 ( α′(MnS–HS −)), and for method (2) 1.0004±0.0001 ( α′(FeS–HS −)) and 1.0012±0.0003 ( α′(MnS–HS −)). Therefore, sulfur isotope effects related to the precipitation of metal monosulfides are small when compared to important microbial reactions.

Stable sulfur isotope fractionation by anion exchange chromatography. production of compounds enriched in 34S

The separation of the 34S isotope was carried out by isotopic exchange between sulfurous acid in solution and bisulfite anions adsorbed on a quaternary ammonium (Dowex 1X8 and Dowex 2X8, 100-200 mesh) packed in six anion exchange columns. Each resin column packed with the anion exchange resin had a height of 115 cm and 2.2 cm in diameter. The columns were connected in series during the displacement of bisulfite bands. For the experiments, a band of bisulfite was fixed to the anion resin, initially in the hydroxyl ion form, and subsequently eluted with 0.2, 0.3, 0.4, or 0.6 mol L-1 HCl solution. The hydrochloric acid solution was kept under nitrogen at 245 kPa, in order to prevent the evolution of gases and the oxidation of the bisulfite band. The experiments with the Dowex 1X8 resin showed that the best results in terms of maximum enrichment (18.66 atom %34S) and total34S accumulated mass (161.7 mg, excess) were obtained by elution of the bisulfite band for 50 m with 0.2 and 0.3 mol L-1 HCl, respectively. The depleted portion of the band was periodically replaced with natural bisulfite, at 10 m intervals during elution.

Dissimilatory bacterial sulfate reduction in montana groundwaters

Abstract The origin of hydrogen sulfide in southeastern Montana groundwaters was investigated. Sulfate‐reducing bacteria were detected in 25 of 26 groundwater samples in numbers ranging from 2.0 × 101 to greater than 2.4 × 104 bacteria per 100 ml. Stable sulfur isotope fractionation studies indicated a biological role in sulfate reduction. However, sulfate‐reducing activity as determined by use of a radioactive sulfur isotope was observed in only 1 of 16 samples. It is postulated that bacterial dissimilatory sulfate reduction is responsible for a major portion of the sulfide produced in these groundwaters and that these bacteria are most likely active in the adsorbed state, possibly in subsurface microzones where environmental conditions are conducive to sulfate reduction.