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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. [ABSTRACT FROM AUTHOR]

Geogenic arsenic (As)-rich groundwater poses a significant environmental challenge worldwide, yet our understanding of the interplay between dissolved organic matter (DOM) transformation and arsenic mobilization during microbial sulfate reduction remains limited. This study involved microcosm experiments using As-rich aquifer sediments from the Singe Tsangpo River basin (STR) and Jianghan Plain (JHP), respectively. The findings revealed that microbial sulfate reduction remarkably increased arsenic mobilization in both STR and JHP sediments compared to that in unamended sediments. Moreover, the mobilization of As during microbial sulfate reduction coincided with increases in the fluorescence intensity of two humic-like substances, C2 and C3 (R = 0.87/0.87 and R = 0.73/0.66 in the STR and JHP sediments, respectively; p < 0.05), suggesting competitive desorption between DOM and As during incubation. Moreover, the transformations in the DOM molecular characteristics showed significant increases in CHOS molecular and low-O/C-value molecular intensities corresponding to the enhancement of microbial sulfate reduction and the possible occurrence of methanogenesis processes, which suggests a substantial bioproduction contribution to DOM components that is conducive to As mobilization during the microbial sulfate reduction. The present results thus provide new insights into the co-evolution between As mobilization and DOM transformations in alluvial aquifer systems under strong microbial sulfate reduction conditions. [ABSTRACT FROM AUTHOR]

Microbial sulfate reduction is generally limited in the deep sea compared to shallower marine environments, but cold seeps and hydrothermal systems are considered an exception. Here, we report sulfate reduction rates and geochemical data from marine sediments and hydrothermal vent fields along the Arctic Mid Ocean Ridges (AMOR), to assess the significance of basalt-hosted hydrothermal activity on sulfate reduction in a distal deep marine setting. We find that cored marine sediments do not display evidence for sulfate reduction, apart from low rates in sediments from the Knipovich Ridge. This likely reflects the overall limited availability of reactive organic matter and low sedimentation rates along the AMOR, except for areas in the vicinity of Svalbard and Bear Island. In contrast, hydrothermal samples from the Seven Sisters, Jan Mayen and Loki's Castle vent fields all demonstrate active microbial sulfate reduction. Rates increase from a few 10s to 100s of pmol SO42- cm-3 d-1 in active hightemperature hydrothermal chimneys, to 10s of nmol SO42- cm-3 d-1 in lowtemperature barite chimneys and up to 110 nmol cm-3 d-1 in diffuse venting hydrothermal sediments in the Barite field at Loki's Castle. Pore fluid and sediment geochemical data suggest that these high rates are sustained by organic compounds from microbial mats and vent fauna as well as methane supplied by high-temperature hydrothermal fluids. However, significant variation was observed between replicate hydrothermal samples and observation of high rates in seemingly inactive barite chimneys suggests that other electron donors may be important as well. Sediment sulfur isotope signatures concur with measured rates in the Barite field and indicate that microbial sulfate reduction has occurred in the hydrothermal sediments since the recent geological past Our findings indicate that basalt-hosted vent fields provide sufficient electron donors to support microbial sulfate reduction in high- and low-temperature hydrothermal areas in settings that otherwise show very low sulfate reduction rates. [ABSTRACT FROM AUTHOR]

Microbial sulfate reduction is generally limited in the deep sea compared to shallower marine environments, but cold seeps and hydrothermal systems are considered an exception. Here, we report sulfate reduction rates and geochemical data from marine sediments and hydrothermal vent fields along the Arctic Mid Ocean Ridges (AMOR), to assess the significance of basalt-hosted hydrothermal activity on sulfate reduction in a distal deep marine setting. We find that cored marine sediments do not display evidence for sulfate reduction, apart from low rates in sediments from the Knipovich Ridge. This likely reflects the overall limited availability of reactive organic matter and low sedimentation rates along the AMOR, except for areas in the vicinity of Svalbard and Bear Island. In contrast, hydrothermal samples from the Seven Sisters, Jan Mayen and Loki’s Castle vent fields all demonstrate active microbial sulfate reduction. Rates increase from a few 10s to 100s of pmol SO42- cm-3 d-1 in active high-temperature hydrothermal chimneys, to 10s of nmol SO42- cm-3 d-1 in low-temperature barite chimneys and up to 110 nmol cm-3 d-1 in diffuse venting hydrothermal sediments in the Barite field at Loki’s Castle. Pore fluid and sediment geochemical data suggest that these high rates are sustained by organic compounds from microbial mats and vent fauna as well as methane supplied by high-temperature hydrothermal fluids. However, significant variation was observed between replicate hydrothermal samples and observation of high rates in seemingly inactive barite chimneys suggests that other electron donors may be important as well. Sediment sulfur isotope signatures concur with measured rates in the Barite field and indicate that microbial sulfate reduction has occurred in the hydrothermal sediments since the recent geological past. Our findings indicate that basalt-hosted vent fields provide sufficient electron donors to support microbial sulfate reduction in high- and low-temperature hydrothermal areas in settings that otherwise show very low sulfate reduction rates.

In freshwater systems, contributions of chemosynthetic products by sulfur‐oxidizing bacteria in sediments as nutritional resources in benthic food webs remain unclear, even though chemosynthetic products might be an important nutritional resource for benthic food webs in deep‐sea hydrothermal vents and shallow marine systems. To study geochemical aspects of this trophic pathway, we sampled sediment cores and benthic animals at two sites (90 and 50 m water depths) in the largest freshwater (mesotrophic) lake in Japan: Lake Biwa. Stable carbon, nitrogen, and sulfur isotopes of the sediments and animals were measured to elucidate the sulfur nutritional resources for the benthic food web precisely by calculating the contributions of the incorporation of sulfide‐derived sulfur to the biomass and of the biogeochemical sulfur cycle supporting the sulfur nutritional resource. The recovered sediment cores showed increases in 34S‐depleted sulfide at 5 cm sediment depth and showed low sulfide concentration with high δ34S in deeper layers, suggesting an association of microbial activities with sulfate reduction and sulfide oxidation in the sediments. The sulfur‐oxidizing bacteria may contribute to benthic animal biomass. Calculations based on the biomass, sulfur content, and contribution to sulfide‐derived sulfur of each animal comprising the benthic food web revealed that 58%–67% of the total biomass sulfur in the benthic food web of Lake Biwa is occupied by sulfide‐derived sulfur. Such a large contribution implies that the chemosynthetic products of sulfur‐oxidizing bacteria are important nutritional resources supporting benthic food webs in the lake ecosystems, at least in terms of sulfur. The results present a new trophic pathway for sulfur that has been overlooked in lake ecosystems with low‐sulfate concentrations. [ABSTRACT FROM AUTHOR]

Purpose: Increased temperatures can promote phosphorus (P) mobilization in black-odor sediments. It is generally believed that the temperature-induced release of P from sediments is mainly controlled by iron reduction, while the role of sulfate reduction is often neglected. In this study, the effect and mechanism of sulfate reduction on P release in response to increasing temperatures were investigated. Materials and methods: In January 2021, a total of nine sediment cores were collected in the Zhongzi River, a typical black-odorous river, using a gravity sampler. When all samples had been transported to the laboratory, three sediment cores were placed in three different tanks for incubation, and the water temperature in the tanks was controlled at 5 ℃, 15 ℃, or 25 ℃ through circulating water flumes. Following a 15-day incubation, the diffusive gradients in thin films (DGT) were combined, and the high-resolution dialysis (HR-Peeper), P sequential extraction, and the 16S rRNA amplicon sequencing techniques were used to investigate the relationship between temperature changes and variations in sulfate reduction rates and P migration. Results and discussion: A rise in the temperature from 5 to 25 ℃, the concentrations of labile sulfide (S(-II)) and soluble reactive P (SRP) in porewater, and the vertical distribution trends of these two variables were significantly and positively correlated (r ≥ 0.45, p ≤ 0.05). Meanwhile, the largest decrease in organic P (OP) and iron-bound phosphorus (Fe–P) was observed in sediments. Further analysis found that the relative abundance of sulfate-reducing bacteria (SRB) increased from 4.35 to 5.43%, while that of iron-reducing bacteria (IRB) decreased from 4.99 to 3.68%, indicating that SRB inhibited the growth of IRB and that microbial sulfate reduction (MSR) became the primary pathway of OP mineralization. Although the growth of IRB was inhibited, the Fe–P and Fe(III) oxide content in sediments were significantly reduced, indicating that chemical iron reduction (CIR) had occurred. Conclusions: Increasing temperatures promoted the propagation of SRB and inhibited the growth of IRB in black-odor sediments, which not only made MSR become the primary pathway of OP mineralization but also changed the main pathway of iron reduction so that microbial iron reduction (MIR) was replaced by S(-II)-induced CIR, leading to high mobilization of P in black-odor sediments. [ABSTRACT FROM AUTHOR]

The possibilities of simultaneous removal of sulfates and heavy metals (Cu, Ni, Zn) from acid mine drainage have been investigated in two-section bioelectrochemical system (BES). The used BES is based on the microbial sulfate reduction (MSR) process in the anode zone and abiotic reduction processes in the cathodic zone. In the present study, the model acid mine drainage with high sulfate (around 4.5 g/l) and heavy metals (Cu2+, Ni2+ and Zn2+) content was performed. As a separator in the laboratory, BES used an anionic exchange membrane (AEM), and for electron donor in the process of microbial sulfate reduction in the bioanode zone - waste ethanol stillage from the distillery industry was employed. In this study, the possibility of sulfates removal from the cathodic zone was established by their forced migration through AEM to the anode zone. Simultaneously, as a result of the MSR process, the sulfate ions passed through AEM are reduced to H2S in the anode zone. The produced H2S, having its role as a mediator in electron transfer, is oxidized on the anode surface to S0 and other forms of sulfur. The applicability of waste ethanol stillage as a cheap and affordable organic substrate for the MSR process has also been established. Heavy metals (Cu2+, Ni2+ and Zn2+) occur in the cathode chamber of BES in different degrees of the removal. As a microbial fuel cell (MFC) operating for 120 hours, the reduction rate of Cu2+ reaches 94.6% (in waste ethanol stillage) and 98.6% (in the case of Postgate culture medium). On the other hand, in terms of Ni2+ and Zn2+, no significant decrease in their concentrations in the liquid phase is found. In the case of microbial electrolysis cell (MEC) mode reduction of Cu2+- 99.9%, Ni2+- 65.9% and Zn2+- 64.0% was achieved. For 96 hours, the removal of sulfates in MEC mode reached 69.9% in comparison with MFC mode - 35.2%. [ABSTRACT FROM AUTHOR]

Mm-scale and/or µm-scale in situ inorganic geochemical analyses of organic-rich, laminated fine-grained sediments increasingly offer decadal or even seasonal perspectives of ecological or environmental dynamics. Organic geochemical studies, in contrast, seldom explicitly consider mm-scale variability, so that it remains unclear to what extent organic geochemical proxy-based reconstructions are a product of signal-averaging of higher-resolution variability. Here we assess the 2–8 mm-scale variability in organic geochemical proxies for depositional environment and organic source, utilising an organic-rich, low maturity and well-laminated shale from the Eocene Shahejie Formation, Dongying Depression, Bohai Bay Basin. We identify mm-scale (approximately decadal-scale) compositional variability that is comparable in magnitude to vertical variability at the m-scale (approximately millennial-scale), test multiple relationships between key biomarker and mineral proxies at the mm-scale, and assess likely implications for palaeowater conditions, anaerobic heterotrophic microbial activity, early diagenetic dolomitization, and organic matter preservation. Our results suggest that the decadal-scale oscillation of organic carbon content is a result of varying degrees of anaerobic heterotrophic microbial reworking with periodic euxinia in the deep photic zone. In contrast to previous models based on analysis of bulk samples, our results suggest that anaerobic respiration during intervals of more intense/persistent euxinia is likely accompanied by more organic matter loss. Variable low-light euxinia seems to be a modifier to the net OM content. The analysis of bulk samples may hide important relationships at shorter temporal scales which are key to reconstructing palaeoenvironments and the operation of key controls on, for example, the carbon cycle. [ABSTRACT FROM AUTHOR]

Cold seep sediments are dominated by intensive microbial sulfate reduction coupled to anaerobic oxidation of methane. However, the contribution proportion between this process and the role of organic matter has remained enigmatic. Here, pore water data combined with PROFILE model, fluxes of sulfate and methane concentration calculated from Fick's first law, and δ34SSO4 and δ18OSO4 of pore water sulfate were studied to reconstruct co-occurring microbial organoclastic sulfate reduction and anaerobic oxidation of methane coupled with sulfate reduction in methane seep sediments collected from South China Sea. The sulfate concentration profiles of C9 and C14 in Qiongdongnan Basin generally show quasilinear depletion with depth. Reaction-transport modeling provided close fits to concentration data. δ18OSO4 and δ34SSO4 increase fastest with sediment depth above 400 cmbsf and slowest below that depth. The values of methane flux are always lower than those of total sulfate reduction of sulfate diffusive flux at GC-10, GC-9, GC-11 and HD319 sites in Taixinan Basin. Besides, positions of sulfate methane transition zone in all study sites are approximately ~400 to 800 centimeters below seafloor. These results showed that microbial sulfate reduction in sediments is mainly controlled by intense anaerobic oxidation of methane, but there is a certain relationship with organic matter metabolism process. This emphasizes that traditional redox order of bacterial respiration is highly simplified, where, in sediments such as these seeps, all of these microbial sulfate reduction processes can occur together with complex couplings between them.

Triple-oxygen isotope (δ 18O and Δ ′ 17 O) analysis of sulfate is becoming a common tool to assess several biotic and abiotic sulfur-cycle processes, both today and in the geologic past. Multi-step sulfur redox reactions often involve intermediate sulfoxyanions such as sulfite, sulfoxylate, and thiosulfate, which may rapidly exchange oxygen atoms with surrounding water. Process-based reconstructions therefore require knowledge of equilibrium oxygen-isotope fractionation factors (18 α and 17 α) between water and each individual sulfoxyanion. Despite this importance, there currently exist only limited experimental 18 α data and no 17 α estimates due to the difficulty of isolating and analyzing short-lived intermediate species. To address this, we theoretically estimate 18 α and 17 α for a suite of sulfoxyanions—including several sulfate, sulfite, sulfoxylate, and thiosulfate species—using quantum computational chemistry. We determine fractionation factors for sulfoxyanion "water droplets" using the B3LYP/6-31G+(d,p) method; we additionally calculate higher-order method (CCSD/aug-cc-pVTZ and MP2/aug-cc-pVTZ) scaling factors, and we qualitatively estimate the importance of anharmonic zero-point energy (ZPE) corrections using a suite of gaseous sulfoxy compounds. Methodological scaling factors greatly impact 18 α predictions, whereas ZPE corrections are likely small (i.e., ⩽ 1 ‰) at Earth-surface temperatures; existing experimental data best agree with 18 α predictions when including redox state-specific CCSD/aug-cc-pVTZ scaling factors. Theoretical pH- and temperature-specific bulk-solution (i.e., abundance-weighted average of all species) 18 α values yield root-mean-square errors for sulfate/water, sulfite/water, and thiosulfate/water equilibrium of 4.5‰ (n = 18 experimental conditions), 3.7‰ (n = 27), and 2.2‰ (n = 3), respectively. However, sulfate- and sulfite-system agreement improves considerably when comparing experimental results only to SO 3 (OH)−/H 2 O (RMSE = 1.6‰) and SO 2 (OH)−/H 2 O (RMSE = 2.2‰) predictions, rather than bulk solutions. This is particularly true for the sulfite system at high and low pH, when SO 2 (OH)− is not the dominant species. We discuss potential experimental and theoretical biases that may lead to this apparent improvement. By combining 18 α and 17 α predictions, we additionally estimate that sulfate, sulfite, sulfoxylate, and thiosulfate species can exhibit Δ ′ 17 O values as much as 0.199‰, 0.205‰, 0.101‰, and 0.186‰ more negative than equilibrated water at Earth-surface temperatures (reference line slope = 0.5305). This theoretical framework provides a foundation to interpret experimental and observational triple-oxygen isotope results of several sulfur-cycle processes including pyrite oxidation, microbial metabolisms (e.g., sulfate reduction, thiosulfate disproportionation), and hydrothermal anhydrite precipitation. We highlight this with several examples. [ABSTRACT FROM AUTHOR]

It is not well understood how, in the immediate aftermath of the Sturtian Snowball Earth, marine sulfur cycling resulted in a global distribution of sedimentary pyrite with δ34S values higher than coeval seawater. Here, we analyze the quadruple sulfur isotope systematics of organic-bound sulfur (OS) from the lowermost post-Sturtian Datangpo Formation, South China, and identify two generations of OS formation, each sampling an isotopically distinct sulfate reservoir (δ34S ≈ 26‰ and 52–93‰) that differentially impacted its respective, co-occurring pyrite. Combining several lines of geochemical evidence, we argue that the first OS generation was the product of a sulfate-impoverished meltwater-influenced setting, with OS preservation being the result of resistance to acid hydrolysis. However, the second OS generation was sourced from H 2 S produced in sediments during early diagenesis via microbial reduction of a 34S-enriched sulfate pool derived from overlying euxinic or ferruginous seawater. This is the first ancient marine data set where all observed pyrite is more enriched in 34S than its associated OS. Our proposed origin may be applied to global superheavy pyrite (SHP) immediately after the Sturtian and is comparable to processes linked to freshwater-to-marine transitions during rising sea level in the wake of recent glaciation. [ABSTRACT FROM AUTHOR]

The hypersaline Dead Sea and its sediments are natural laboratories for studying extremophile microorganism habitat response to environmental change. In modern times, increased freshwater runoff to the lake surface waters resulted in stratification and dilution of the upper water column followed by microbial blooms. However, whether these events facilitated a microbial response in the deep lake and sediments is obscure. Here we investigate archived evidence of microbial processes and changing regional hydroclimate conditions by reconstructing deep Dead Sea chemical compositions from pore fluid major ion concentration and stable S, O, and C isotopes, together with lipid biomarkers preserved in the hypersaline deep Dead Sea ICDP‐drilled core sediments dating to the early Holocene (ca. 10,000 years BP). Following a significant negative lake water balance resulting in salt layer deposits at the start of the Holocene, there was a general period of positive net water balance at 9500–8300 years BP. The pore fluid isotopic composition of sulfate exhibit evidence of intensified microbial sulfate reduction, where both δ34S and δ18O of sulfate show a sharp increase from estimated base values of 15.0‰ and 13.9‰ to 40.2‰ and 20.4‰, respectively, and a δ34S vs. δ18O slope of 0.26. The presence of the n‐C17 alkane biomarker in the sediments suggests an increase of cyanobacteria or phytoplankton contribution to the bulk organic matter that reached the deepest parts of the Dead Sea. Although hydrologically disconnected, both the Mediterranean Sea and the Dead Sea microbial ecosystems responded to increased freshwater runoff during the early Holocene, with the former depositing the organic‐rich sapropel 1 layer due to anoxic water column conditions. In the Dead Sea prolonged positive net water balance facilitated primary production and algal blooms in the upper waters and intensified microbial sulfate reduction in the hypolimnion and/or at the sediment–brine interface. [ABSTRACT FROM AUTHOR]

Sedimentary records of superheavy pyrites in Phanerozoic and Proterozoic successions (i.e., extremely positive δ34Spyrite values together with higher δ34Spyrite than coeval δ34SCAS) are mostly interpreted as resulting either from secondary postdepositional processes or from multiple redox reactions between sulfate and sulfide in stratified sulfate‐poor environments. We report here the first observation of strongly positive δ34S values for both dissolved sulfate and sulfide (average δ34Sdiss.sulfate value of 34.6‰ and δ34Sdiss.sulfide values of 36.7‰) compared to the present‐day seawater δ34Sdiss.sulfate (~21‰), with a negative apparent fractionation between sulfate and sulfide (∆34Sdiss.sulfate‐diss.sulfide ~ −2.1 ± 1.4‰), in the sulfate‐poor (<3 mm) modern thalassohaline lacustrine system Dziani Dzaha (Mayotte, Indian Ocean). Overall, surface sediments faithfully record the water column isotopic signatures including a mainly negative ∆34Ssed.sulfate‐sed.sulfide (−4.98 ± 4.5‰), corresponding to the definition of superheavy pyrite documented in the rock record. We propose that in the Dziani Dzaha this superheavy pyrite signature results from a two‐stage evolution of the sulfur biogeochemical cycle. In a first stage, the sulfur cycle would have been dominated by sulfate from initially sulfate‐rich marine waters. Overtime, Raleigh distillation by microbial sulfate reduction coupled with sulfide burial in the sediment would have progressively enriched in 34S the water column residual sulfate. In a second still active stage, quantitative sulfate reduction not only occurs below the halocline during stratified periods but also in the whole water column during fully anoxic episodes. Sulfates are then regenerated by partial oxidation of sulfides as the oxic–anoxic interface moves downward. These results demonstrate that the atypical superheavy pyrite isotope signature does not necessarily require postdepositional or secondary oxidative processes and can result from primary processes in restricted sulfate‐poor and highly productive environments analogous to the Dziani Dzaha. [ABSTRACT FROM AUTHOR]

Organic carbon sequestration in sedimentary environments controls oxygen and carbon dioxide concentrations in the atmosphere. While minerals play an important role in the preservation of organic carbon, there is a lack of understanding about the formation and stability of organo-mineral interactions in anoxic environments, especially those involving authigenic iron sulfide minerals. In this study, we quantified organic carbon and nitrogen sequestered in biogenic iron sulfide minerals co-precipitated with sulfate-reducing bacteria (SRB) in freshwater and marine conditions in long-term laboratory experiments. The amounts of C and N associated with biogenic iron sulfide minerals increased with increasing cell biomass concentrations available in the media. C and N levels stabilized over the first 2 months of incubation and remained stable for up to 1 year. Crystalline mackinawite (FeS) formed in all experimental conditions and transformed to greigite only in some experimental conditions. We did not find evidence that this mineral transformation affected C and N levels, neither could we identify the factors that controlled greigite formation. Pyrite did not form in our experimental conditions. While C concentrations in minerals correlated with concentrations of reduced sulfate in both the freshwater and marine media, removal of OC by iron sulfide minerals was more efficient in freshwater than marine conditions. Removal of OC by iron sulfide minerals was also more efficient when cells were present (SRB biomass) in comparison with abiotic incubations with organic mixtures (e.g., tryptone, yeast extract, and casamino acids). Our study highlights the potential for biogenic iron sulfide minerals to quantitatively contribute to organic carbon preservation in anoxic environments. [ABSTRACT FROM AUTHOR]

Organic carbon sequestration in sedimentary environments controls oxygen and carbon dioxide concentrations in the atmosphere. While minerals play an important role in the preservation of organic carbon, there is a lack of understanding about the formation and stability of organo-mineral interactions in anoxic environments, especially those involving authigenic iron sulfide minerals. In this study, we quantified organic carbon and nitrogen sequestered in biogenic iron sulfide minerals co-precipitated with sulfate-reducing bacteria (SRB) in freshwater and marine conditions in long-term laboratory experiments. The amounts of C and N associated with biogenic iron sulfide minerals increased with increasing cell biomass concentrations available in the media. C and N levels stabilized over the first 2 months of incubation and remained stable for up to 1 year. Crystalline mackinawite (FeS) formed in all experimental conditions and transformed to greigite only in some experimental conditions. We did not find evidence that this mineral transformation affected C and N levels, neither could we identify the factors that controlled greigite formation. Pyrite did not form in our experimental conditions. While C concentrations in minerals correlated with concentrations of reduced sulfate in both the freshwater and marine media, removal of OC by iron sulfide minerals was more efficient in freshwater than marine conditions. Removal of OC by iron sulfide minerals was also more efficient when cells were present (SRB biomass) in comparison with abiotic incubations with organic mixtures (e.g., tryptone, yeast extract, and casamino acids). Our study highlights the potential for biogenic iron sulfide minerals to quantitatively contribute to organic carbon preservation in anoxic environments.

The sulfur isotope composition of pyrite in marine sedimentary rocks is often difficult to interpret due to a lack of precise isotopic constraints for coeval sulfate. This study examines pyrite and barite in the Late Devonian Canol Formation (Selwyn Basin, Canada), which provides an archive of δ34S and δ18O values during diagenesis. Scanning electron microscopy (SEM) has been combined with microscale secondary ion mass spectrometry (SIMS) analysis (n = 1,032) of pyrite (δ34S) and barite (δ34S and δ18O) on samples collected from nine stratigraphic sections of the Canol Formation. Two paragenetic stages of pyrite and barite formation have been distinguished, both replaced by barium carbonate and feldspar. The δ34Sbarite and δ18Obarite values from all sections overlap, between +37.1‰ and +67.9‰ (median = +45.7‰) and +8.8‰ and +23.9‰ (median = +20.0‰), respectively. Barite morphologies and isotopic values are consistent with precipitation from diagenetically modified porewater sulfate (sulfate resupply << sulfate depletion) during early diagenesis. The two pyrite generations (Py-1 and Py-2) preserve distinct textures and end-member isotopic records. There is a large offset from coeval Late Devonian seawater sulfate in the δ34Spyrite values of framboidal pyrite (-29.4‰ to -9.3‰), consistent with dissimilatory microbial sulfate reduction (MSR) during early diagenesis. The Py-2 is in textural equilibrium with barite generation 2 (Brt-2) and records a broad range of more positive δ34SPy-2 values (+9.4‰ to + 44.5‰). The distinctive highly positive δ34Spyrite values developed from sulfate limited conditions around the sulfate methane transition zone (SMTZ). We propose that a combination of factors, including low sulfate concentrations, MSR, and sulfate reduction coupled to anaerobic oxidation of methane (SR-AOM), led to the formation of highly positive δ34Spyrite and δ34Sbarite values in the Canol Formation. The presence of highly positive δ34Spyrite values in other Late Devonian sedimentary units indicate that diagenetic pyrite formation at the SMTZ may be a more general feature of other Lower Paleozoic basins.

An imbalance in pyrite weathering and burial is a primary mechanism responsible for oxygenation of the atmosphere and oceans, but key processes governing the terrestrial sulfur cycle remain nebulous. Here, we investigate components of the terrestrial sulfur cycle in a highly productive, glacier‐fed catchment, and use a global mass balance model to constrain riverine sulfur fluxes. Chemistry of stream water and plant debris in the Jostedal watershed, Norway suggests sulfur isotope discrimination is occurring in the porewater. Global models also corroborate additional, previously overlooked pyrite burial with a modest isotope fractionation (<20‰), similar to values reported from freshwater ecosystems. Collectively, our results indicate that a significant amount of sulfate produced by weathering remains trapped in terrestrial environments. This terrestrial sulfur sink might have waxed and waned over geologic time in response to major biogeochemical events such as terrestrial afforestation. Plain Language Summary: The amount of oxygen in Earth's surface environments is governed by the balance between formation of sulfur‐bearing pyrite and its exposure to the atmosphere and consequent destruction (rusting). Pyrite commonly forms during bacterial activity in oxygen‐free conditions, for example, in bogs, soil, and sediment. Here, we study the destruction and formation of pyrite by examining the chemistry of stream water, meltwater of snow and glacial ice, rain water, sediment, rock, soil, and plants within a glacial meltwater stream environment. Sulfur isotope compositions indicate that a large amount of pyrite forms by bacterial activity in this river system. Modeling of the global sulfur budget also supports our findings that, on the global scale, a significant amount of pyrite has formed in such systems throughout Earth's history, affecting the balance between formation and destruction of pyrite, thus playing a key role in climate and environmental changes. Key Points: Glacier‐driven weathering (both pyrite and silicate) and sulfur cycle were examined in a meltwater systemδ34S of the natural reservoirs indicates microbial sulfate reduction and burial of reduced sulfur occurring in the watershedResults of global sulfur cycle model confirm a significant flux of terrestrial sulfur burial driven by microbial reduction [ABSTRACT FROM AUTHOR]

Diagenetic alteration of magnetic minerals, driven by closely linked C-S-Fe cycles, is highly likely to complicate the paleomagnetic record. In addition to the anomalous diagenetic paleomagnetic signatures caused by ferromagnetic greigite growth, pyrite sulfur isotope compositions are often "heavy" (i.e., δ34S pyr > 0). However, the dependencies and mechanistic origins of these signatures remain controversial. This study presents a high-resolution δ34S pyr record of a long sediment core collected from the South Yellow Sea, China. Ferromagnetic greigite is prominently identified in two coastal deposits within this core. The δ34S pyr values of these coastal deposits are isotopically (super) heavy, ranging from −10.6 to 22.8‰ and from −14.5 to 26.5‰, with mean and 1σ values of 5.9 ± 10.3‰ (n = 15) and 12.2 ± 9.8‰ (n = 33), respectively. Additionally, magnetic parameters show positive trends with δ34S pyr values throughout the sediment core. These positive trends, along with the enrichment of ferrous iron and sedimentary microtextural evidence of the authigenic growth sequence of framboidal pyrite, siderite, euhedral pyrite, and greigite, indicate that anaerobic oxidation of methane (AOM) is a fundamental factor for ferromagnetic greigite formation in coastal sediments with sulfate limitation. We estimate the delay time of greigite formation relative to the depositional age of surrounding sediments to be a few hundred years due to the rapid sedimentation rates and shallow burial depths of the sulfate-methane transition zone (SMTZ) in coastal deposits. Conversely, the deep burial of SMTZ likely suggests that a longer delay time is prevailing for greigite formation in hemipelagic sediments. This study highlights the role of AOM in controlling the formations of greigite and coeval (super) heavy pyrite. • We found positive trends between δ34S pyr and magnetic parameters. • Isotopically heavy pyrite and coeval greigite are observed in coastal sediments. • Heavy pyrite and coeval ferromagnetic greigite may have similar origin mechanisms. • Anaerobic oxidation of methane is dominating factor for heavy pyrite and greigite. [ABSTRACT FROM AUTHOR]

Superhigh-organic‑sulfur (SHOS) coals (coals with organic sulfur content >4 wt%) are unique coal deposits found at a few notable locations in the world. Specific peat accumulation and preservation conditions must be met to form SHOS coals. Organic sulfur is a major constituent of such coals, and it may have various sources depending on the prevailing paleomire conditions. Understanding such paleomire conditions sheds light on the formation mechanisms of SHOS coals. This investigation decodes the paleomire conditions of the Paleogene SHOS coals from Meghalaya, India, using sulfur isotopic compositions (δ 34S) of organic sulfur (δ 34S OS) and pyritic sulfur (δ 34S Py) along with organic petrography, pyrite morphology and trace element ratios. Thirty coal samples were collected from the Jaintia Hills in the east, Khasi Hills in the middle, and Garo Hills in the west of Meghalaya. The organic sulfur content in the Garo, Khasi, and Jaintia coals varies from 1.0 to 3.3 wt%, 1.4 to 13.8 wt%, and 1.0 to 7.2 wt%, respectively. Further, after separation from pyritic sulfur and sulfate sulfur phases, the organic sulfur content ranges from 54.4 to 69.2%, 63.8 to 79.9%, and 59.3 to 73.8%, in the Garo, Khasi, and Jaintia Hills, respectively, suggesting the SHOS nature of these coal samples. The δ 34S Py varies from −29.3 ‰ to +5.7 ‰, −21.3 ‰ to +27.3 ‰, and −12.1 ‰ to −4.3 ‰, in the Jaintia, Khasi, and Garo Hills, respectively, while the δ 34S OS fluctuates from −4.6 ‰ to +3.7 ‰, −9.3 ‰ to +7.8 ‰, and − 9.0 ‰ to −5.0 ‰, respectively. The δ 34S values of pyrite and organic sulfur (OS) in Jaintia coals are 34S depleted compared to seawater sulfate (+22 ‰), leading to fractionations in the range of −51.3 ‰ to −16.3 ‰ (mean − 31.6 ‰) and − 26.6 ‰ to −18.3 ‰ (mean − 23.1 ‰) for pyritic and organic sulfur (OS), respectively. Pyrite in Khasi coals show a relatively heavier δ 34S composition averaging at −20.5 ‰, whereas organic sulfur (OS) isotope compositions range from −31.3 ‰ to −14.2 ‰ with a mean of −22.6 ‰. Pyrite and OS in the Garo coals are depleted compared to seawater sulfate. Isotope variations in the Jaintia, Khasi, and Garo coals indicate microbial sulfate reduction (MSR) of seawater sulfate. Large isotopic fractionations between Eocene seawater sulfate and pyritic sulfur (Δ 34S SO4Eocene – pyrite = up to −51.3 ‰; mean − 31.6 ‰) in Jaintia coals indicate their possible formation in the water column/near the sediment-seawater interface (open system) and also hint toward dissimilatory sulfate reduction pathways that prevailed under anoxic redox conditions. However, mean values of Δ 34S SO4Eocene – pyrite (−20.5 ‰) in the Khasi coals imply pyrite formation deeper in the sediments (more closed system) under dysoxic conditions. The dominance of OS over pyritic sulfur, framboidal pyrite, and its microcrystal size distributions in Jaintia coals may suggest syngenetic pyrite formation in open water reducing/anoxic conditions under paralic environments. Elevated Sr/Ba and U/Th values in these coals further confirm the anoxic conditions. Nevertheless, the presence of euhedral pyrite with the alleviated pyrite framboids in the Khasi coals and their complete absence in the Garo coals may suggest dysoxic-suboxic and suboxic-oxic depositional conditions, respectively. The isotopic signatures of the Garo coals suggest sulfur contribution from the parent paleobiota and MSR under a freshwater-oxic environment. Insignificant fractionations between δ 34S Py and δ 34S OS indicate limited iron and sulfate availability for additional sulfur cycling and disproportionation reactions, typical of oxic conditions. The absence of framboidal pyrite, elevated sulfate concentration, and mean Sr/Ba and U/Th values of 0.5 and 0.3, respectively, further suggest the freshwater peat deposition in the Garo Hills under limnotelmatic to telmatic freshwater conditions. Moreover, high inertinite content (I mmf = 9.77–33.16 vol%), possibly induced by atmospheric peat exposure, supports the interpretation of suboxic-oxic paleomire conditions in Garo Hills. Gradually decreasing mineral matter content from Jaintia (mean 13.6 vol%) to Garo coals (mean 7.4 vol%) additionally projects a transition from mesotrophic brackish to freshwater limnotelmatic environment, complementing the shift in the paleomire condition from eastern (Jaintia) to western (Garo) Meghalayan Hills. [Display omitted] • Pathways, timing, and mechanism of the OM sulfurization using δ34S Py and δ34S TOS. • Trace elements and their ratios depict redox conditions. • Application of pyrite morphology and framboidal pyrite microcrystal size distribution. • Use of organic petrography to infer paleomire conditions. • Origin of pyrite and paleoredox condition of mire. [ABSTRACT FROM AUTHOR]

The Arctic Ocean has experienced environmental fluctuations during the Pleistocene glacial-interglacial cycles. Since the last deglaciation, the reinundation of the Bering Strait and regional transgression have led to dramatic changes in the western Arctic Ocean, affecting coastal landscapes, depositional processes, marine ecosystem, and, inherently, biogeochemical element cycles. Here, we investigate the records of sulfur, carbon, and iron cycles in the Chukchi Sea of the western Arctic, with a primary focus on microbial sulfate reduction and subsequent pyrite formation. Variations in the pyrite abundance and sulfur isotopic composition, derived from a 10-m sediment core, demonstrate that dynamic changes in the factors governing pyrite formation – organic electron donor, reactive iron, and sulfate – are closely linked to the regional environmental conditions. In the early Holocene sediment, pyrite precipitation was impeded by the lack of organic matter, even in the presence of abundant sulfate and reactive iron. In the overlying sediment, pyrite contents increased due to vigorous microbial sulfate reduction fueled by enhanced availability of organic substrate and additional methane from the sulfate-methane transition zone. In general, the increased supply of organic matter to the Chukchi Shelf sediments can be attributed to the resumed inflow of nutrient-rich Pacific Water through the flooded Bering Strait. The increase in pyrite content, however, does not correlate exactly with either the increase in TOC or the opening of the Bering Strait, both of which precede the increased pyrite formation. These lags reflect the balance between marine primary production and terrestrial carbon sources, as well as the balance between the Pacific Inflow and the Beaufort Gyre. While a comparable increase in pyrite and TOC contents during the Holocene deglaciation has also been reported in the Black and Baltic Sea sediments, their distinct sulfur isotope records, contingent upon the availability of sulfate and reactive iron, highlight that pyrite can serve as a valuable proxy for tracking past climate and environmental change, especially in barren sedimentary records such as those found in the Arctic Ocean. • Pyrite sulfur geochemistry for Holocene environmental changes in the Chukchi Sea. • Environmental controls on microbial sulfate reduction and pyrite precipitation. • Impacts of the Bering Strait flooding and inflow of Pacific Water on S-C-Fe cycling. • Pyrite sulfur isotopes as a proxy for biogeochemical cycles in the Arctic Ocean. [ABSTRACT FROM AUTHOR]

We present pore fluid geochemistry, including major ion and trace metal concentrations and the isotopic composition of pore fluid calcium and sulfate, from the uppermost meter of sediments from the Gulf of Aqaba (Northeast Red Sea) and the Iberian Margin (North Atlantic Ocean). In both the locations, we observe strong correlations among calcium, magnesium, strontium, and sulfate concentrations as well as the sulfur isotopic composition of sulfate and alkalinity, suggestive of active changes in the redox state and pH that should lead to carbonate mineral precipitation and dissolution. The calcium isotope composition of pore fluid calcium (δ44Ca) is, however, relatively invariant in our measured profiles, suggesting that carbonate mineral precipitation is not occurring within the boundary layer at these sites. We explore several reasons why the pore fluid δ44Ca might not be changing in the studied profiles, despite changes in other major ions and their isotopic composition, including mixing between the surface and deep precipitation of carbonate minerals below the boundary layer, the possibility that active iron and manganese cycling inhibits carbonate mineral precipitation, and that mineral precipitation may be slow enough to preclude calcium isotope fractionation during carbonate mineral precipitation. Our results suggest that active carbonate dissolution and precipitation, particularly in the diffusive boundary layer, may elicit a more complex response in the pore fluid δ44Ca than previously thought.

In this study, we present quadruple sulfur isotope values (QSI: 32S,33S,34S,36S) measured in sediments from two sulfur-rich Mars analogue environments: i) the glacially-fed hydrothermal pools in Iceland (Kerlingarfjöll and Kverkfjöll), and ii) the Lost Hammer hypersaline spring from Axel Heiberg Island, Nunavut, Canada. The localities host different physical and geochemical characteristics, including aqueous geochemistry, volcanic input, temperature, pH and salinity. The δ34S values of sulfur compounds from the Lost Hammer hypersaline spring exhibit large fractionations typical of microbial sulfate reduction (MSR) with or without additional oxidative sulfur cycling and microbial sulfur disproportionation (MSD) (34ε SO4-CRS from −49.5 to −43.5‰), contrary to the small S isotope fractionations reported for the Icelandic hydrothermal sites (34ε SO4-CRS from −9.9 to −0.7‰). Lost Hammer minor S isotope values (Δ33S and Δ36S), interpreted within the context of a sulfur cycling box model, are consistent with a biogeochemical S cycle including both MSR and MSD. In contrast, the small range in δ34S values within the Iceland hydrothermal pools are consistent with a large volcanic H 2 S flux and minimal biological S cycling. The minor S isotope values recorded in the hydrothermal pools, however, indicate further biogeochemical sulfur cycling. Our results demonstrate that contrasting physical and chemical characteristics between sites support different microbial S cycling processes, as recorded in the QSI sedimentary values. The QSI data and the derived models support the strong potential for QSI values to be used as biosignatures in the search for life in Martian S-rich environments. These results also suggests that extreme, metabolic energy-limited environments with low abiotic sulfur fluxes could be more likely to produce unequivocal biological QSI signals than those with more moderate conditions or abundant available energy. This finding carries significant implications for targeting sites on Mars for in situ measurements or future sample return missions. [ABSTRACT FROM AUTHOR]

Microbially mediated sulfate reduction is a ubiquitous process in many subsurface systems. Isotopic fractionation is characteristic of this anaerobic process, since sulfate-reducing bacteria (SRB) favor the reduction of the lighter sulfate isotopologue (S32O42−) over the heavier isotopologue (S34O42−). Detection of isotopic shifts has been utilized as a proxy for the onset of sulfate reduction in subsurface systems such as oil reservoirs and aquifers undergoing heavy metal and radionuclide bioremediation. Reactive transport modeling (RTM) of kinetic sulfur isotope fractionation has been applied to field and laboratory studies. We developed a benchmark problem set for the simulation of kinetic sulfur isotope fractionation during microbially mediated sulfate reduction. The benchmark problem set is comprised of three problem levels and is based on a large-scale laboratory column experimental study of organic carbon amended sulfate reduction in soils from a uranium-contaminated aquifer. Pertinent processes impacting sulfur isotopic composition such as microbial sulfate reduction and iron-sulfide reactions are included in the problem set. This benchmark also explores the different mathematical formulations in the representation of kinetic sulfur isotope fractionation as employed in the different RTMs. Participating RTM codes are the following: CrunchTope, TOUGHREACT, PHREEQC, and PHT3D. Across all problem levels, simulation results from all RTMs demonstrate reasonable agreement. [ABSTRACT FROM AUTHOR]

In the ethanol stillage biomethanation system a microbial fuel cell (MFC) is integrated in the anode area when combined with a process of aerobic photosynthesis of microalgae in the cathode zone. The dynamics of the main technological parameters in the area of the bioanode and biocathode of MFC has been established. Elimination of H2S from the composition of biogas has been achieved in the integration of the fuel products by achieving a higher degree of mineralization of the organic substrate under the load of fuel products compared to the modes of operation without electric load. The dynamics of the cathode potential during the different phases of photosynthesis has been established in the biocathode zone. Maximum values for power and current densities were achieved -- 27.4 mW m-2 and 68 mA m-2, respectively, during the Log phase of the cultivation of oxygen microalgae. [ABSTRACT FROM AUTHOR]

Here we present the carbon isotopic composition of dissolved inorganic carbon (DIC) and the sulfur isotopic composition of sulfate, along with changes in sulfate concentrations, of the pore fluid collected from a series of sediment cores located along a depth transect on the Iberian Margin. We use these data to explore the coupling of microbial sulfate reduction (MSR) to organic carbon oxidation in the uppermost (up to nine meters) sediment. We argue that the combined use of the carbon and sulfur isotopic composition, of DIC and sulfate respectively, in sedimentary pore fluids, viewed through a δ13CDIC vs. δ34SSO4 cross plot, reveals significant insight into the nature of carbon-sulfur coupling in marine sedimentary pore fluids on continental margins. Our data show systemic changes in the carbon and sulfur isotopic composition of DIC and sulfate (respectively) where, at all sites, the carbon isotopic composition of the DIC decreases before the sulfur isotopic composition of sulfate increases. We compare our results to global data and show that this behavior persists over a range of sediment types, locations and water depths. We use a reactive-transport model to show how changes in the amount of DIC in seawater, the carbon isotopic composition of organic matter, the amount of organic carbon oxidation by early diagenetic reactions, and the presence and source of methane influence the carbon and sulfur isotopic composition of sedimentary pore fluids and the shape of the δ13CDIC vs. δ34SSO4 cross plot. The δ13C of the DIC released during sulfate reduction and sulfate-driven anaerobic oxidation of methane is a major control on the minimum δ13CDIC value in the δ13CDIC vs. δ34SSO4 cross plot, with the δ13C of the organic carbon being important during both MSR and combined sulfate reduction, sulfate-driven AOM and methanogenesis.

Microbial sulfate reduction in oil reservoirs (biosouring) is often associated with secondary oil production where seawater containing high sulfate concentrations (~28 mM) is injected into a reservoir to maintain pressure and displace oil. The sulfide generated from biosouring can cause corrosion of infrastructure, health exposure risks, and higher production costs. Isotope monitoring is a promising approach for understanding microbial sulfur cycling in reservoirs, enabling early detection of biosouring, and understanding the impact of souring. Microbial sulfate reduction is known to result in large shifts in the sulfur and oxygen isotope compositions of the residual sulfate, which can be distinguished from other processes that may be occurring in oil reservoirs, such as precipitation of sulfate and sulfide minerals. Key to the success of this method is using the appropriate isotopic fractionation factors for the conditions and processes being monitored. For a set of batch incubation experiments using a mixed microbial culture with crude oil as the electron donor, we measured a sulfur fractionation factor for sulfate reduction of -30‰. We have incorporated this result into a simplified 1D reservoir reactive transport model to highlight how isotopes can help discriminate between biotic and abiotic processes affecting sulfate and sulfide concentrations. Modeling results suggest that monitoring sulfate isotopes can provide an early indication of souring for reservoirs with reactive iron minerals that can remove the produced sulfide, especially when sulfate reduction occurs in the mixing zone between formation waters (FW) containing elevated concentrations of volatile fatty acids (VFAs) and injection water (IW) containing elevated sulfate. In addition, we examine the role of reservoir thermal, geochemical, hydrological, operational and microbiological conditions in determining microbial souring dynamics and hence the anticipated isotopic signatures.

We present the results of an isotope-enabled reactive transport model of a sediment column undergoing active microbial sulfate reduction to explore the response of the sulfur and oxygen isotopic composition of sulfate under perturbations to steady state. In particular, we test how perturbations to steady state influence the cross plot of δ34S and δ18O for sulfate. The slope of the apparent linear phase (SALP) in the cross plot of δ34S and δ18O for sulfate has been used to infer the mechanism, or metabolic rate, of microbial metabolism, making it important that we understand how transient changes might influence this slope. Tested perturbations include changes in boundary conditions and changes in the rate of microbial sulfate reduction in the sediment. Our results suggest that perturbations to steady state influence the pore fluid concentration of sulfate and the δ34S and δ18O of sulfate but have a minimal effect on SALP. Furthermore, we demonstrate that a constant advective flux in the sediment column has no measurable effect on SALP. We conclude that changes in the SALP after a perturbation are not analytically resolvable after the first 5% of the total equilibration time. This suggests that in sedimentary environments the SALP can be interpreted in terms of microbial metabolism and not in terms of environmental parameters.

The isotopic composition of sulfate in the rock record has been frequently used to track the changes in the Earth's surface environments. By considering isotopic fractionation imparted by microbial sulfate reduction (MSR) and thermochemical sulfate reduction (TSR), in this study, we aim to develop a holistic understanding of the mixed effects of MSR and TSR on δ34S signals in sulfate‐rich carbonate systems. We report the occurrence of various types of sulfur‐bearing components from the Cambrian‐Ordovician carbonate system in the Tarim Basin, NW China, coupled with a well‐established diagenesis framework for these rocks. Our results indicate that most of the sulfur‐bearing species possess δ34S values slightly lower than both the source sulfate and the sulfide generated by TSR, yet these sulfur‐bearing species have substantially higher δ34S values than sulfide that resulted from MSR. Hence, a combination of sulfides sourced from MSR and TSR can adequately explain the sulfur isotope data in the studied interval. Building upon this hypothesis, we developed a new sulfur diagenesis model in order to quantify the accumulated H2S from the combined effects of MSR and TSR. Our new model can be used to explain the origin of sulfur‐bearing species in many other deep burial carbonate systems, including the Sichuan Basin, China, and the Gulf of Mexico, USA. We propose that greater attention should be paid to isotopic modulation through mixed diagenetic processes in order to gain a better mechanistic understanding of the primary geochemistry signals (e.g., δ34S) in marine carbonates. Key Points: A new sulfur cycling diagenesis model was built based on sulfur‐bearing species generated during progressive burialThis new model advanced the quantification of accumulated H2S from sulfate reductions in many global deep burial basinsThis new sulfur cycling model can be used to better define the origins of superheavy pyrite commonly present in the Neoproterozoic Era [ABSTRACT FROM AUTHOR]

Sulfate minerals are rare in the Archean rock record and largely restricted to the occurrence of barite (BaSO4). The origin of this barite remains controversially debated. The mass‐independent fractionation of sulfur isotopes in these and other Archean sedimentary rocks suggests that photolysis of volcanic aerosols in an oxygen‐poor atmosphere played an important role in their formation. Here, we report on the multiple sulfur isotopic composition of sedimentary anhydrite in the ca. 3.22 Ga Moodies Group of the Barberton Greenstone Belt, southern Africa. Anhydrite occurs, together with barite and pyrite, in regionally traceable beds that formed in fluvial settings. Variable abundances of barite versus anhydrite reflect changes in sulfate enrichment by evaporitic concentration across orders of magnitude in an arid, nearshore terrestrial environment, periodically replenished by influxes of seawater. The multiple S‐isotope compositions of anhydrite and pyrite are consistent with microbial sulfate reduction. S‐isotope signatures in barite suggest an additional oxidative sulfate source probably derived from continental weathering of sulfide possibly enhanced by microbial sulfur oxidation. Although depositional environments of Moodies sulfate minerals differ strongly from marine barite deposits, their sulfur isotopic composition is similar and most likely reflects a primary isotopic signature. The data indicate that a constant input of small portions of oxidized sulfur from the continents into the ocean may have contributed to the observed long‐term increase in Δ33Ssulfate values through the Paleoarchean. [ABSTRACT FROM AUTHOR]

Dissimilatory sulfate reduction is a microbial energy metabolism that can produce sulfur isotopic fractionations over a large range in magnitude. Calibrating sulfur isotopic fractionation in laboratory experiments allows for better interpretations of sulfur isotopes in modern sediments and ancient sedimentary rocks. The proteins involved in sulfate reduction are expressed in response to environmental conditions, and are collectively responsible for the net isotopic fractionation between sulfate and sulfide. We examined the role of DsrC, a key component of the sulfate reduction pathway, by comparing wildtype Desulfovibrio vulgaris DSM 644T to strain IPFG07, a mutant deficient in DsrC production. Both strains were cultivated in parallel chemostat reactors at identical turnover times and cell specific sulfate reduction rates. Under these conditions, sulfur isotopic fractionations between sulfate and sulfide of 17.3 ± 0.5‰ or 12.6 ± 0.5‰ were recorded for the wildtype or mutant, respectively. The enzymatic machinery that produced these different fractionations was revealed by quantitative proteomics. Results are consistent with a cellular-level response that throttled the supply of electrons and sulfur supply through the sulfate reduction pathway more in the mutant relative to the wildtype, independent of rate. We conclude that the smaller fractionation observed in the mutant strain is a consequence of sulfate reduction that proceeded at a rate that consumed a greater proportion of the strains overall capacity for sulfate reduction. These observations have consequences for models of sulfate reducer metabolism and how it yields different isotopic fractionations, notably, the role of DsrC in central energy metabolism.

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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 (ε34Ssulfide‐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. [ABSTRACT FROM AUTHOR]

Microbial sulfate reduction is associated with characteristic S isotope partitioning, which can aid in determining the rate of this ubiquitous reactive pathway in a wide variety of reducing environments. Here we introduce a new simple yet predictive model that expands the range of environmental conditions over which this functional relationship can be applied through the use of a modified Monod rate expression constrained by a novel set of experiments. A series of continuously-fed batch reactors containing an equivilant biomass of Desulfovibrio vulgaris , a sulfate reducing bacteria, were subjected to differing continuous mass addition rates of formate to precisely control the rate of sulfate reduction via electron donor limitation. Based on growth yield calculations, additional biomass accumulation was negligible. The isotopic composition (δ34S) of the residual sulfate pool was measured through time for each experiment. This approach resulted in five steady state reduction rates of 2.06, 1.22, 0.83, 0.52 and 0.28 μmol * h−1 that enriched the unreacted sulfate in 34S by unique characteristic enrichment factors (α obs) of 0.9976, 0.9962, 0.9938, 0.9924 and 0.9903, respectively. This relationship was used to calibrate a new coupled set of isotope-specific Monod rate laws that have been modified to incorporate (1) a minimum (α 1) and maximum (α 2) fractionation factor and (2) a rate-controlling electron donor factor (D F). Application of these parameters in the updated model reproduce most of our data and simulate realistic shifts in the observed fractionation factor (α obs) as a function of reduction rate in an electron donor limited system. The model was applied to our current dataset as well as three previous studies, which collectively provide a broad range in both rates and α obs , and support validation across substantially different conditions. We show that this approach offers a reasonable approximation to more detailed microbial reactive network models, while still maintaining sufficient simplicity and versatility to allow incorporation into multi-component reactive transport simulations. Thus, the current study provides a foundation for accurate simulation of the relationship between α obs and sulfate reduction rates in open, transient, and through-flowing systems. [ABSTRACT FROM AUTHOR]

Perturbations to the global carbon and sulfur cycles recurred episodically throughout the ~5-Myr-long Early Triassic, in the aftermath of the end-Permian mass extinction, the largest biocrisis in Earth's history. In this study, analyses of carbonate-associated sulfate (CAS) sulfur, CAS oxygen, and pyrite sulfur-isotope ratios in a continental shelf section from the southern Neo-Tethys Ocean (Spiti Valley, India) provide new insights into the Early Triassic marine sulfur cycle. Secular variation in CAS sulfur-isotope values at Spiti is similar to that in South China, suggesting that CAS was a robust recorder of a global seawater sulfate signal. The Spiti CAS and pyrite δ34S profiles show that the highest rates of pyrite burial coincided with cooler sea-surface temperatures. We infer that climatic cooling steepened equator-to-pole temperature gradients, invigorating thermohaline overturning circulation, and enhancing upwelling of nutrients that stimulated marine productivity and organic carbon sinking fluxes. Enhanced productivity fueled and sustained microbial respiration, increased oxygen demand, and, within the southern Neo-Tethys, caused the zone of microbial sulfate reduction to migrate upwards and become more connected to the water column. Microbial sulfate reduction, under these conditions, was no longer limited by organic matter or sulfate availability, leading to burial of more 34S-depleted pyrite and 34S- and 18O-enrichment of the oceanic sulfate pool. This environmental scenario suggests possible environmental stresses related to eutrophication during positive carbon-isotope excursions around the Griesbachian-Dienerian, Dienerian-Smithian, and Smithian-Spathian boundaries. Additionally, the difference between CAS and pyrite sulfur-isotope values, Δ34S CAS-pyr , slowly rose through the Early Triassic, reflecting a slow increase in seawater sulfate concentrations following a minimum close to the Permian-Triassic boundary. [ABSTRACT FROM AUTHOR]

• Field observations were combined with a reactive transport model approach. • Freshwater lenses can act as archives for anthropogenic sulfur emission. • S-cycling is dominated by slow SO 4 reduction (<20 pmol mL−1 d−1) and FeS formation. • High stable sulfur isotope fractionation of up to 67‰ related to MSR was detected. • Lens-derived nutrient fluxes may be important for the benthic zone at SGD sites. 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. [ABSTRACT FROM AUTHOR]

Abstract The impact of seasonal fluctuations linked to monsoon and irrigation generates redox oscillations in the subsurface, influencing the release of arsenic (As) in aquifers. Here, the biogeochemical control on As mobility was investigated in batch experiments using redox cycling bioreactors and As- and SO 4 2−-amended sediment. Redox potential (E h) oscillations between anoxic (−300–0 mV) and oxic condition (0–500 mV) were implemented by automatically modulating an admixture of N 2 /CO 2 or compressed air. A carbon source (cellobiose, a monomer of cellulose) was added at the beginning of each reducing cycle to stimulate the metabolism of the native microbial community. Results show that successive redox cycles can decrease arsenic mobility by up to 92% during reducing conditions. Anoxic conditions drive mainly the conversion of soluble As(V) to As(III) in contrast to oxic conditions. Phylogenetic analyses of 16S rRNA amplified from the sediments revealed the presence of sulfate and iron – reducing bacteria, confirming that sulfate and iron reduction are key factors for As immobilization from the aqueous phase. As and S K-edge X-ray absorption spectroscopy suggested the association of Fe-(oxyhydr)oxides and the importance of pyrite (FeS 2(s)), rather than poorly ordered mackinawite (FeS (s)), for As sequestration under oxidizing and reducing conditions, respectively. Finally, these findings suggest a role for elemental sulfur in mediating aqueous thioarsenates formation in As-contaminated groundwater of the Mekong delta. Graphical abstract Unlabelled Image Highlights • Microbial sulfate reduction was found to contribute to arsenic (As) sequestration. • During anoxic conditions, the conversion of soluble As(V) to As(III) was observed. • The formation of pyrite, rather than mackinawite was observed in redox cycles. • Absorption/desorption of aqueous As on Fe-(oxyhydr)oxides and pyrite was observed. [ABSTRACT FROM AUTHOR]

Dissimilatory sulfate reduction is a microbial energy metabolism that can produce sulfur isotopic fractionations over a large range in magnitude. Calibrating sulfur isotopic fractionation in laboratory experiments allows for better interpretations of sulfur isotopes in modern sediments and ancient sedimentary rocks. The proteins involved in sulfate reduction are expressed in response to environmental conditions, and are collectively responsible for the net isotopic fractionation between sulfate and sulfide. We examined the role of DsrC, a key component of the sulfate reduction pathway, by comparing wildtype Desulfovibrio vulgaris DSM 644T to strain IPFG07, a mutant deficient in DsrC production. Both strains were cultivated in parallel chemostat reactors at identical turnover times and cell specific sulfate reduction rates. Under these conditions, sulfur isotopic fractionations between sulfate and sulfide of 17.3 ± 0.5‰ or 12.6 ± 0.5‰ were recorded for the wildtype or mutant, respectively. The enzymatic machinery that produced these different fractionations was revealed by quantitative proteomics. Results are consistent with a cellular-level response that throttled the supply of electrons and sulfur supply through the sulfate reduction pathway more in the mutant relative to the wildtype, independent of rate. We conclude that the smaller fractionation observed in the mutant strain is a consequence of sulfate reduction that proceeded at a rate that consumed a greater proportion of the strains overall capacity for sulfate reduction. These observations have consequences for models of sulfate reducer metabolism and how it yields different isotopic fractionations, notably, the role of DsrC in central energy metabolism. [ABSTRACT FROM AUTHOR]