Microbial sedimentary imprint on the deep Dead Sea sediment

Although hypersaline environments are often thought of as terminal habitats, they play an active role in the interconversion of organic and inorganic carbon, a process that is interfaced microbiologically with the reduction of sulfate to sulfide. In both normal marine and hypersaline marine environments, carbon cycle transformations result in changing pools of dissolved Σ CO2 (CO2 + HCO -3 + CO 2-3 ), solid carbonates, and organic carbon. In the marine environment, sulfur cycle transformations result in changing pools of dissolved sulfate and sulfide, solid sulfides and sulfur, and organic sulfur, while in marine-derived hypersaline environments, solid sulfate (gypsum) may provide an additional large pool of potential oxidant of organic matter. Given the simplicity of food chains in hypersaline environments relative to those in normal marine habitats, the high concentrations of sulfate, and the possibly limited activity of anaerobic bacteria in extremely saline sediments, it might be expected that rates and kinds of interconversions between reduced and oxidized pools of carbon and sulfur in hypersaline environments may differ from those in normal marine sediments. The following discussion outlines aspects of carbon and sulfur cycles from sediment and porewaters of hyper-saline environments, using evidence from rates and types of transformations, organic and inorganic geochemistry, and stable isotope fractionations.KeywordsTotal AlkalinityHalophilic BacteriumSulfur CycleHypersaline EnvironmentSulfate Reduction RateThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Bacterial predation limits microbial sulfate-reduction in a coastal acid sulfate soil (CASS) ecosystem

Microbiological, geochemical, and isotopic analyses of sediment and water samples from the unconsolidated Yegua formation in east-central Texas were used to assess microbial processes in the terrestrial subsurface. Previous geochemical studies suggested that sulfide oxidation at shallow depths may provide sulfate for sulfate-reducing bacteria (SRB) in deeper aquifer formations. The present study further examines this possibility, and provides a more detailed evaluation of the relationship between microbial activity, lithology, and the geochemical environment on meter-to-millimeter scales. Sediment of varied lithology (sands, silts, clays, lignite) was collected from two boreholes, to depths of 30 m. Our findings suggest that pyrite oxidation strongly influences the geochemical environment in shallow sediments (∼5 m), and produces acidic waters (pH 3.8) that are rich in sulfate (28 mM) and ferrous iron (0.3 mM). Sulfur and iron-oxidizing bacteria are readily detected in shallow sediments; they likely play an indirect role in pyrite oxidation. In consistent fashion, there is a relative paucity of pyrite in shallow sediments and a low 34S/32S-sulfate ratio (0.2‰) (reflecting contributions from 34S-depleted sulfides) in shallow regions. Pyrite oxidation likely provides a sulfate source for both oxic and anoxic aquifers in the region. A variety of assays and direct-imaging techniques of 35S-sulfide production in sediment cores indicates that sulfate reduction occurs in both the oxidizing and reducing portions of the sediment profile, with a high degree of spatial variability. Narrow zones of activity were detected in sands that were juxtaposed to clay or lignite-rich sediments. The fermentation of organic matter in the lignite-rich laminae provides small molecular weight organic acids to support sulfate reduction in neighboring sands. Consequently, sulfur cycling in shallow sediments, and sulfate transport represent important mechanisms for commensal interaction among subsurface microorganisms by providing electron donors for chemoautotrophic bacteria and electron acceptors for SRB. The activity of SRB is linked to the availability of suitable electron donors from spatially distinct zones.

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.

Study of thermochemical sulfate reduction mechanism using compound specific sulfur isotope analysis

Hydrothermal alteration and serpentinization of ultramafic rocks at oceanic core complexes involve extensive element exchange between fluid and rock and a wide range of biogeochemical processes. These processes influence the global sulfur cycle due to both biogenic and abiogenic removal of seawater sulfate. Hence, ocean floor serpentinization connects the hydrosphere, biosphere, and lithosphere. This work presents a study of the sulfur geochemistry of highly altered mafic and ultramafic samples from the Atlantis Massif located at 30°N along the Mid‐Atlantic Ridge. The analyzed samples were drilled during International Ocean Discovery Program Expedition 357 and collected during Alvin dives in 2000, 2003, and 2005. Multiple sulfur isotope analyses of sulfide and sulfate phases indicate that several processes took place during progressive hydrothermal alteration: (1) incorporation of seawater sulfate, (2) thermochemical sulfate reduction during interaction with high‐temperature (high‐T; 350–400 °C), low‐pH fluids and input of H2S derived from leaching gabbroic intrusions, (3) microbial sulfate reduction, and (4) oxidation of sulfides at high water‐rock ratios. Petrological examinations show that high‐T fluids mostly postdated the bulk serpentinization and that these fluid pulses were relatively localized (<1 dm scale) resulting in a highly heterogeneous mineralogy. Locally, high‐T fluid influx took place subsequent to microbial sulfate reduction and oxidation as indicated by geochemical modeling. Overall, this study documents the complex interplay of magmatic processes, fluid‐rock interaction, and microbial activity that take place during the formation of oceanic core complexes and where mantle rocks are exposed to seawater.

Microbial mat communities possess extensive taxonomic and functional diversity, which drive high metabolic rates and rapid cycling of major elements. Modern microbial mats occurring in hypersaline environments are considered as analogs to extinct geobiological formations dating back to ∼ 3.5 Gyr ago. Despite efforts to understand the diversity and metabolic potential of hypersaline microbial mats in Shark Bay, Western Australia, there has yet to be molecular analyses at the transcriptional level in these microbial communities. In this study, we generated metatranscriptomes for the first time from actively growing mats comparing the type of mat, as well as the influence of diel and seasonal cycles. We observed that the overall gene transcription is strongly influenced by microbial community structure and seasonality. The most transcribed genes were associated with tackling the low nutrient conditions by the uptake of fatty acids, phosphorus, iron, and nickel from the environment as well as with protective mechanisms against elevated salinity conditions and to prevent build-up of ammonium produced by nitrate reducing microorganisms. A range of pathways involved in carbon, nitrogen, and sulfur cycles were identified in mat metatranscriptomes, with anoxygenic photosynthesis and chemoautotrophy using the Arnon–Buchanan cycle inferred as major pathways involved in the carbon cycle. Furthermore, enrichment of active anaerobic pathways (e.g., sulfate reduction, methanogenesis, Wood–Ljungdahl) in smooth mats corroborates previous metagenomic studies and further advocates the potential of these communities as modern analogs of ancient microbialites.

Abstract. Hydrocarbon seepage at the deep seafloor fuels flourishing chemosynthetic communities. These seeps impact the functionality of the benthic ecosystem beyond hotspots of gas emission, altering the abundance, diversity, and activity of microbiota and fauna and affecting geochemical processes. However, these chemosynthetic ecotones (chemotones) are far less explored than the foci of seepage. To better understand the functionality of chemotones, we (i) mapped seabed morphology at the periphery of gas seeps in the deep eastern Mediterranean Sea, using video analyses and synthetic aperture sonar; (ii) sampled chemotone sediments and described burrowing using computerized tomography; (iii) explored nutrient concentrations; (iv) quantified microbial abundance, activity, and N2 fixation rates in selected samples; and (v) extracted DNA and explored microbial diversity and function using amplicon sequencing and metagenomics. Our results show that gas seepage creates burrowing intensity gradients at seep ecotones, with the ghost shrimp Calliax lobata primarily responsible for burrowing, which influences nitrogen and sulfur cycling through microbial activity. Burrow walls form a unique habitat, where macromolecules are degraded by Bacteroidota, and their fermentation products fuel sulfate reduction by Desulfobacterota and Nitrospirota. These, in turn, support chemosynthetic Campylobacterota and giant sulfur bacteria Thiomargarita, which can aid C. lobata nutrition. These interactions may support enhanced productivity at seep ecotones.

Documenting anaerobic microbial metabolisms in hypersaline perennially ice‐covered lakes in Antarctica further refines the environmental limits to life and may reveal rare biogeochemical mechanisms and/or novel microbial catalysts of elemental cycling. We assessed rates of sulfate reduction, methanogenesis, and anaerobic oxidation of methane using radiotracers and generated 16S rRNA gene libraries from the microbial communities inhabiting the deep calcium‐chloride‐rich brine and sediments of Lake Vanda, McMurdo Dry Valleys, Antarctica. Sulfate reduction rates were observed in surface sediments but not in the brine overlying the sediments. Methane formation through the methylotrophic, acetoclastic, and hydrogenotrophic pathways was quantified using 14C‐labeled methylamine, acetate, and CO2, respectively, and methanogenesis was detected in both the brine and the sediments. Hydrogenotrophic methanogenesis rates were the highest of all substrates tested in the sediments, while methylotrophic methanogenesis was highest in the brines. Anaerobic oxidation of methane was below the limit of detection in both the brines and sediments. The major taxa of Bacteria and Archaea detected were most similar to organisms previously observed in hypersaline environments and included examples related to known sulfate‐reducing bacteria other than Deltaproteobacteria (surprisingly, sulfate‐reducing Deltaproteobacteria were not observed in this study), and both methanogenic and methanotrophic Archaea. These data indicate an active microbial community in the anoxic brine of Lake Vanda that while similar in terms of community structure and metabolism to other brine habitats, is uniquely evolved to survive in this extreme environment.

Organic matter composition, microbial biomass and microbial activity in gravel-bed constructed wetlands treating farm dairy wastewaters

The fate of CO2 derived from thermochemical sulfate reduction (TSR) and effect of TSR on carbonate porosity and permeability, Sichuan Basin, China

Acid gas generation by thermochemical sulfate reduction (TSR) evolves within a complex web of petroleum-water-rock-gas interactions in reservoirs under high temperature conditions of more than ca. 100°C. The interactions lead to the formation of toxic and corrosive hydrogen sulfide (free H2S gas and dissolved H2S). Such interactions are caused by the instability of hydrocarbons in the presence of water and a reactive reservoir rock matrix containing water-soluble anhydrite. The mass conversions of the inorganic water-rock-gas interactions which are triggered by the redox degradation of hydrocarbons establish a certain, thermodynamically defined state of chemical equilibrium. Any approach to geochemically model “acid gas generation” and “H2S-risk distribution” in petroleum systems should be based on a conceptual model that adequately reproduces the intimately interconnected and interdependent nature of all isochronous hydrogeochemical reactions, whether they are kinetically controlled or establish equilibrium species distributions. Such approaches rely (1) on the thermodynamical calculation of chemical equilibrium species distribution, (2) on the coupling of kinetically controlled oil degradation and sulfate reduction by oil-derived reductants to the equilibrium calculations, and (3) on the calculation of diffusive mass transport through the free pore water network and the irreducible water film. The key to model TSR, “acid gas generation, and “H2S-risk distribution” is not to consider and model any single, isolated reaction like the kinetically controlled sulfate reduction which depends on the thermal history. The actual key to model TSR, the fate and behavior of sulfidic sulfur, and a realistic “H2S-risk distribution” in petroleum reservoirs is an overall reproduction of the hydrogeochemical reactive transport processes which temporally and spatially evolve in a complex network of oil/petroleum-water-rock-gas interactions under reservoir conditions. Consequently, we perform 3D hydrogeochemical, multi-component and multi-species reactive mass transport modeling for a semi-generic case study by using the PHAST computer code (provided by the U.S. Geological Survey) and take the following boundary conditions into account: gas reservoir; carbonate (dolomite plus calcite) reservoir rocks; anhydrite seal; 140°C; 600 bar total pressure; kinetic rate constant for sulfate reduction by CH4 = 1.08 x 10-16 mol s-1 l-1; mass transport is restricted to diffusion; modeled time span is 10 Ma

Microbial mediation is the only demonstrated mechanism to precipitate dolomite under Earth surface conditions. A link between microbial activity and dolomite formation in the sabkha of Abu Dhabi has, until now, not been evaluated, even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences. Such an evaluation is the purpose of this study, which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of Abu Dhabi, along a transect from the intertidal to the supratidal zone. This investigation revealed a close association between microbial mats and dolomite, suggesting that microbes are involved in the mineralization process. Observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats. In current models, microbial dolomite precipitation is linked to an active microbial activity that sustains high pH and alkalinity and decreased sulphate concentrations in pore waters. Such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone. By contrast, these models cannot be applied to the supratidal zone, where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity. As no abiotic mechanism is known to form dolomite at Earth surface conditions, two different hypotheses can reconcile this result. In a first scenario, all of the dolomite present in the supratidal zone formed in the past, when the mats were active at the surface. In a second scenario, dolomite formation continues within the buried and inactive mats. In order to explain dolomite formation in the absence of active microbial metabolisms, a revised microbial model is proposed in which the mineral-template properties of exopolymeric substances play a crucial role.

A novel hydrogeochemical modeling approach is developed to unravel thermochemical sulfate reduction (TSR) in hydrocarbon reservoirs. Our numerical model couples a web of interconnected hydrogeochemical reactions to three-dimensional (3-D) and reservoir-wide diffusive mass transport. Our modeling approach simulates a semigeneric gas reservoir sealed by anhydrite. The calculated diagenetic processes fit the observations in reservoirs affected by TSR: formation of water, precipitation of calcite, metal (di-)sulfides, and elemental sulfur as replacements of dissolved anhydrite at the expense of CH4(g), as well as formation of hydrogen sulfide (H2S). By varying input parameters, the crucial factors controlling TSR have been identified. Our results highlight that reservoir-wide diffusive mass transport is one prerequisite for TSR. An increase in the rate constant of abiotic sulfate reduction (ASR) and in diffusive mass fluxes, as well as lack of precursor minerals for metal (di-)sulfide precipitation, can increase the souring intensity and accelerate H2S outgassing. In contrast, precipitation of elemental sulfur, which is stable according to the chemical thermodynamics, weakens H2S formation. Our modeling shows that TSR is complex and cannot be represented by the single reaction ASR and by simple correlations between the rate constant of ASR and the H2S gas content. The application of 3-D reactive transport modeling presented here, despite its semigeneric nature, provides a good example of how such an approach can be used ahead of drilling. Our modeling helps to investigate TSR in time and space to quantify the mass conversion of all reactants involved within this web and to predict the souring level.

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 400:37-53 (2010) - DOI: https://doi.org/10.3354/meps08400 Sulfur, carbon, and nitrogen cycling in faunated marine sediments impacted by repeated organic enrichment T. Valdemarsen*, E. Kristensen, M. Holmer Institute of Biology, University of Southern Denmark, 5230 Odense M, Denmark *Email: valdemarsen@biology.sdu.dk ABSTRACT: The effects of organic enrichment on sediment biogeochemistry were studied in sediment mesocosms with indigenous populations of infauna. Two types of sediments, differing in content of reactive Fe, were subjected to weekly pulses of labile organic matter (fish feed), corresponding to an average organic loading of ~350 C m–2 d–1 and ~50 mmol N m–2 d–1 for a period of 65 d. The aims of this experiment were 3-fold: (1) to study the metabolic capacity for organic C and N mineralization at the sediment–water interface; (2) to study the impact of organic enrichment on spatial and temporal sulfide accumulation; and (3) to estimate the importance of Fe-driven sulfide buffering for sulfide accumulation. Organic enrichment stimulated solute exchange between sediment and water within 10 d, resulting in 5 to 8 times stimulated O2 uptake and CO2 efflux, indicating a fast response time of surface-associated microbial consortia. Both 35S-SO42– radiotracer essays and sulfide microprofiles confirmed that added organic matter was primarily oxidized by sulfate reduction. However, despite high sulfate reduction of up to 70 mmol m–2 d–1, sulfide only accumulated to 1–2 mM in the upper 2 cm of enriched cores. In Fe-poor sediment, produced sulfide was reoxidized with O2 or NO3– at the sediment–water interface, whereas in Fe-rich sediment, sulfide was oxidized with O2, NO3–, or Fe oxides in a <2 mm thick suboxic zone or precipitated with dissolved Fe2+. By the end, up to 92 and 100% of added organic C and N, respectively, was mineralized, suggesting a high metabolic capacity for organic matter mineralization in faunated sediments. Furthermore, whereas sulfide precipitation was insignificant in low-Fe sediment, up to 63% of sulfide precipitated with Fe2+ in high-Fe sediment, suggesting a high capacity for sulfide mitigation in faunated Fe-rich sediment. KEY WORDS: Sulfide buffering · Benthic fluxes · Decomposition · Eutrophication · Fish feed · Sulfate reduction Full text in pdf format PreviousNextCite this article as: Valdemarsen T, Kristensen E, Holmer M (2010) Sulfur, carbon, and nitrogen cycling in faunated marine sediments impacted by repeated organic enrichment. Mar Ecol Prog Ser 400:37-53. https://doi.org/10.3354/meps08400 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 400. Online publication date: February 11, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.