Biogeochemical Sulfur Cycle Research Articles

Experimental studies on dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) production by four marine microalgae

The production of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) by marine microalgae was investigated to elucidate more on the role of marine phytoplankton in ocean-atmosphere interactions in the global biogeochemical sulfur cycle. Axenic laboratory cultures of four marine microalgae-Isochrysis galbana 8701, Pavlova viridis, Platymonas sp. and Chlorella were tested for DMSP production and conversion into DMS. Among these four microalgae, Isochrysis galbana 8701 and Pavlova viridis are two species of Haptophyta, while Chlorella and Platymonas sp. belong to Chlorophyta. The results demonstrate that the four algae can produce various amounts of DMS(P), and their DMS(P) production was species specific. With similar cell size, more DMS was released by Haptophyta than that by Chlorophyta. DMS and dissolved DMSP (DMSPd) concentrations in algal cultures varied significantly during their life cycles. The highest release of DMS appeared in the senescent period for all the four algae. Variations in DMSP concentrations were in strong compliance with variations in algal cell densities during the growing period. A highly significant correlation was observed between the DMS and DMSPd concentrations in algal cultures, and there was a time lag for the variation trend of the DMS concentrations as compared with that of the DMSPd. The consistency of variation patterns of DMS and DMSPd implies that the DMSPd produced by phytoplankton cells has a marked effect on the production of DMS. In the present study, the authors’ results specify the significant contribution of the marine phytoplankton to DMS(P) production and the importance of biological control of DMS concentrations in oceanic water.

Diversity of Dissimilatory Sulfite Reductase Genes ( dsrAB ) in a Salt Marsh Impacted by Long-Term Acid Mine Drainage

Sulfate-reducing bacteria (SRB) play a major role in the coupled biogeochemical cycling of sulfur and chalcophilic metal(loid)s. By implication, they can exert a strong influence on the speciation and mobility of multiple metal(loid) contaminants. In this study, we combined DsrAB gene sequencing and sulfur isotopic profiling to identify the phylogeny and distribution of SRB and to assess their metabolic activity in salt marsh sediments exposed to acid mine drainage (AMD) for over 100 years. Recovered dsrAB sequences from three sites sampled along an AMD flow path indicated the dominance of a single Desulfovibrio species. Other major sequence clades were related most closely to Desulfosarcina, Desulfococcus, Desulfobulbus, and Desulfosporosinus species. The presence of metal sulfides with low delta(34)S values relative to delta(34)S values of pore water sulfate showed that sediment SRB populations were actively reducing sulfate under ambient conditions (pH of approximately 2), although possibly within less acidic microenvironments. Interestingly, delta(34)S values for pore water sulfate were lower than those for sulfate delivered during tidal inundation of marsh sediments. 16S rRNA gene sequence data from sediments and sulfur isotope data confirmed that sulfur-oxidizing bacteria drove the reoxidation of biogenic sulfide coupled to oxygen or nitrate reduction over a timescale of hours. Collectively, these findings imply a highly dynamic microbially mediated cycling of sulfate and sulfide, and thus the speciation and mobility of chalcophilic contaminant metal(loid)s, in AMD-impacted marsh sediments.

Oxidation of dimethylsulfide to tetrathionate by Methylophaga thiooxidans sp. nov.: a new link in the sulfur cycle

A new pathway of dimethylsulfide (DMS) metabolism was identified in a novel species of Gammaproteobacteria, Methylophaga thiooxidans sp. nov., in which tetrathionate (S(4)O(6)(2-)) was the end-product of DMS oxidation. Inhibitor evidence indicated that DMS degradation was initiated by demethylation, catalysed by a corrinoid demethylase. Thiosulfate was an intermediate, which was oxidized to tetrathionate by a cytochrome-linked thiosulfate dehydrogenase. Thiosulfate oxidation was coupled to ATP synthesis, and M. thiooxidans could also use exogenous thiosulfate as an energy source during chemolithoheterotrophic growth on DMS or methanol. Cultures grown on a variety of substrates oxidized thiosulfate, indicating that thiosulfate oxidation was constitutive. The observations have relevance to interactions among sulfur-metabolizing bacteria in the marine environment. The production of tetrathionate from an organosulfur precursor is previously undocumented and represents a potential step in the biogeochemical sulfur cycle, providing a 'shunt' across the cycle.

Applying GeoChip Analysis to Disparate Microbial Communities

Microorganisms, the most diverse group of life, play integral roles in ecosystem functions and biogeochemical cycling of carbon, nitrogen, sulfur, phosphorus, and various metals. Microorganisms typically form complex communities whose structure, functions, interactions, and dynamics are critical to our lives. For instance, dramatic changes involving microbial communities from epidemics, plant or animal disease outbreaks, biological terrorist attacks, or atmospheric alterations from global climate change could prove a disaster for the environment and us. Although it is critical to understand which microorganisms are present and what they are doing, counting them is difficult, making it a major challenge to address such issues in quantitative ways and, thus, to make accurate predictions.

Biogeochemistry and cyanobacterial blooms: investigating the relationship in a shallow, polymictic, temperate lake

Environmental context.Effective mitigation of algal blooms, and their associated detrimental impacts on flora and fauna, requires an understanding of the factors leading to bloom development, including nutrients, light and hydrodynamics. We investigated a shallow, freshwater lake and demonstrate that there is sufficient bioavailable phosphorus to annually generate a large algal biomass. Extensive, seasonal phosphorus release from sediments is controlled by the interactions of the biogeochemical cycles of nitrogen, carbon, oxygen, iron and sulfur. Abstract.The shallow, polymictic Ornamental Lake in the Royal Botanic Gardens, Melbourne, Australia, has suffered significant blooms of toxic Anabaena then Microcystis species every summer over the last decade. Although the hydrodynamic conditions of the water column are conducive for algal growth, the prolific growth is controlled by the bioavailable phosphorus concentration. Springtime phosphorus fluxes of 0.1–0.2 mmol m–2 day–1 from the sediment contribute to bloom development. These rates are also observed in anoxic sediment core incubations. Diel stratification, combined with high oxygen consumption associated with organic carbon loading, favour P release. Release rates may be amplified by the effects of sulfate reduction on P sorption onto FeIII (oxyhydroxide) surfaces. Sulfate concentrations are at the threshold where methanogenesis is inhibited in anoxic conditions. Effective bloom mitigation will require a >100-fold reduction in P concentrations, which may be achieved through macrophyte planting and inducing greater water flow through the lake system.

Bio-Archive Core Storage and Subsampling Procedure for Subseafloor Molecular Biological Research

No abstract available. doi:10.2204/iodp.sd.8.05.2009

Microbial Architecture of Environmental Sulfur Processes: A Novel Syntrophic Sulfur-Metabolizing Consortia

Microbial oxidation of sulfur-rich mining waste materials drives acid mine drainage (AMD) and affects the global sulfur biogeochemical cycle. The generation of AMD is a complex, dynamic process that proceeds via multiple reaction pathways. The role of natural consortia of microbes in AMD generation, however, has received very little attention despite their widespread occurrence in mining environments. Through a combination of geochemical experimentation and modeling, scanning transmission X-ray microscopy, and fluorescent in situ hybridization, we show a novel interdependent metabolic arrangement of two ubiquitous and abundant AMD bacteria: chemoautotrophic sulfur-oxidizing Acidithiobacillus sp. and heterotrophic Acidiphilium sp. Highly reminiscent of anaerobic methane oxidation (AOM) consortia, these bacteria are spatially segregated within a planktonic macrostructure of extracellular polymeric substance in which they syntrophically couple sulfur oxidation and reduction reactions in a mutually beneficial arrangement that regenerates their respective sulfur substrates. As discussed here, the geochemical impacts of microbial metabolism are linked to the consortial organization and development of the pod structure, which affects cell-cell interactions and interactions with the surrounding geochemical microenvironment. If these pods are widespread in mine waters, echoing the now widespread discovery of AOM consortia, then AMD-driven CO(2) atmospheric fluxes from H(2)SO(4) carbonate weathering could be reduced by as much as 26 TgC/yr. This novel sulfur consortial discovery indicates that organized metabolically linked microbial partnerships are likely widespread and more significant in global elemental cycling than previously considered.

Stable isotope analysis of the Cretaceous sulfur cycle

We report the sulfur and oxygen isotope composition of sulfate (δ 34S SO4 and δ 18O SO4, respectively) in coexisting barite and carbonate-associated sulfate (CAS), which we use to explore temporal variability in the marine sulfur cycle through the middle Cretaceous. The δ 34S SO4 of marine barite tracks previously reported sulfur isotope data from the tropical Pacific. The δ 18O SO4 of marine barite exhibits more rapid and larger isotopic excursions than the δ 34S SO4 of marine barite; these excursions temporally coincide with Ocean Anoxic Events (OAEs). Neither the δ 34S SO4 nor the δ 18O SO4 measured in marine barite resembles the δ 34S SO4 or the δ 18O SO4 measured in coexisting CAS. Culling our data set for elemental parameters suggestive of carbonate recrystallization (low [Sr] and high Mn/Sr) improves our record of δ 18O SO4 in CAS in the Cretaceous. This suggests that the CAS proxy can be impacted by carbonate recrystallization in some marine sediments. A box model is used to explore the response of the δ 34S SO4 and δ 18O SO4 to different perturbations in the marine biogeochemical sulfur cycle. We conclude that the δ 34S SO4 in the middle Cretaceous is likely responding to a change in the isotopic composition of pyrite being buried, coupled possibly with a change in riverine input. On the other hand, the δ 18O SO4 is likely responding to rapid changes in the reoxidation pathway of sulfide, which we suggest may be due to anoxic versus euxinic conditions during different OAEs.

Anthropogenic and climate influences on biogeochemical dynamics and molecular‐level speciation of soil sulfur

The soil environment is a primary component of the global biogeochemical sulfur (S) cycle, acting as a source and sink of various S species and mediating oxidation state changes. However, ecological significance of the various S forms and the impacts of human intervention and climate on the amount and structural composition of these compounds are still poorly understood. We investigated the long-term influences of anthropogenically mediated transitions from natural to managed ecosystems on molecular-level speciation, biogeochemical dynamics, and the apparent temperature sensitivity of S moieties in temperate, subtropical, and tropical environments with mean annual temperature (MAT) ranging from 5 degrees C to 21 degrees C, using elemental analysis and X-ray absorption near-edge structure (XANES) spectroscopy. Land-use and land-cover changes led to the depletion of total soil S in all three ecoregions over a period of up to 103 years. The largest decline occurred from tropical forest agroecosystems (67% Kakamega and 76% Nandi, Kenya), compared to losses from temperate (36% at Lethbridge, Canada, and 40% at Pendleton, USA) and subtropical (48% at South Africa) grassland agroecosystems. The total S losses correlated significantly with MAT. Anthropogenic interventions profoundly altered the molecular-level composition and resulted in an apparent shift in oxidation states of organic S from native ecosystems composed primarily of S moieties in intermediate and highly reduced oxidation states toward managed agroecosystems dominated by organic S rich in strongly oxidized functionalities. The most prominent change occurred in thiols and sulfides, the proportion of which decreased by 46% (Lethbridge) and 57% (Pendleton) in temperate agroecosystems, by 46% in subtropical agroecosystems, and by 79% (Nandi) and 81% (Kakamega) in tropical agroecosystems. The proportion of organic S directly linked to O increased by 81%, 168%, 40%, 92%, and 85%, respectively. Among the various organic S functionalities, thiols and sulfides seem to have higher apparent temperature sensitivity, and thus these organic S moieties may become prone to losses due to land-use changes, even from the cooler regions of the world if MAT of these regions rise in the future.

Some Physiological Properties of Three Novel Fe(II)- and Sulfur-Oxidizing Strains Affiliated to <i>Alicyclobacillus</i>

This study focused on the soils of sofataric region and acid mine drainage from a copper mine. Based on cultivation, 8 and 6 strains that grow on Fe(II) and sulfur compounds, respectively, were obtained from samples from these two environments. Analysis of 16S rRNA genes of the 14 strains indicated that they were affiliated to Acidithiobacillus, Alicyclobacillus, Sulfobacillus and Leptospirillum. Physiological and phylogenetic studies indicated that three strains (TC-34, TC-71 and ZJ-6) might represent three novel members of Alicyclobacillus. These strains showed 94.8-97.1% 16S rRNA gene identity to other species of Alicyclobacillus. Otherwise, strain TC-34, TC-71 and ZJ-6 showed a range of phenotypic characteristics that differentiated them from previously recognized Alicyclobacillus species, including the growth temperature, assimilation of carbon sources and production of acids from a range of compounds. Chemoautotrophic growth using Fe2+, elemental sulfur and tetrathionate as sole energy source was observed. Especially strain TC-71 was obligately dependent on Fe(II) for growth and quickly oxidized Fe2+. It is concluded that the Fe(II)-oxidizers are metabolically diverse and represent novel Alicyclobacillus species. These are proposed to take part in biogeochemical cycling of iron and sulfur in the solfataric region and could be relevant for biomining.

Coral-Associated Bacteria and Their Role in the Biogeochemical Cycling of Sulfur

Marine bacteria play a central role in the degradation of dimethylsulfoniopropionate (DMSP) to dimethyl sulfide (DMS) and acrylic acid, DMS being critical to cloud formation and thereby cooling effects on the climate. High concentrations of DMSP and DMS have been reported in scleractinian coral tissues although, to date, there have been no investigations into the influence of these organic sulfur compounds on coral-associated bacteria. Two coral species, Montipora aequituberculata and Acropora millepora, were sampled and their bacterial communities were characterized by both culture-dependent and molecular techniques. Four genera, Roseobacter, Spongiobacter, Vibrio, and Alteromonas, which were isolated on media with either DMSP or DMS as the sole carbon source, comprised the majority of clones retrieved from coral mucus and tissue 16S rRNA gene clone libraries. Clones affiliated with Roseobacter sp. constituted 28% of the M. aequituberculata tissue libraries, while 59% of the clones from the A. millepora libraries were affiliated with sequences related to the Spongiobacter genus. Vibrio spp. were commonly isolated from DMS and acrylic acid enrichments and were also present in 16S rRNA gene libraries from coral mucus, suggesting that under "normal" environmental conditions, they are a natural component of coral-associated communities. Genes homologous to dddD, and dddL, previously implicated in DMSP degradation, were also characterized from isolated strains, confirming that bacteria associated with corals have the potential to metabolize this sulfur compound when present in coral tissues. Our results demonstrate that DMSP, DMS, and acrylic acid potentially act as nutrient sources for coral-associated bacteria and that these sulfur compounds are likely to play a role in structuring bacterial communities in corals, with important consequences for the health of both corals and coral reef ecosystems.

Dominant bacterioplankton populations in the meromictic Lake Suigetsu as determined by denaturing gradient gel electrophoresis of 16S rRNA gene fragments

Lake Suigetsu is a typical meromictic lake in Japan characterized by a permanent chemocline at a depth of between 3 and 8 m separating the oxic freshwater mixolimnion from anoxic saline sulfidogenic monimolimnion. Dominant bacterioplankton populations in Lake Suigetsu were investigated using PCR-denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments. The bacterial population was vertically stratified, and temporal shifts in the microbial communities were observed in both the oxic and anoxic layers of Lake Suigetsu during the sampling period. Several dominant DGGE bands were excised and sequenced. In the chemocline, green sulfur bacteria phylogenetically related to the genera Prosthecochloris, Pelodyctyon, and Chlorobium within the phylum Chlorobi were dominant; the colorless sulfur bacteria closely related to the genus Thiomicrospira were detected. These sulfur bacterial groups appear to be important in the biogeochemical cycling of sulfur and/or carbon in Lake Suigetsu. Bacterial sequences affiliated with the Bacteroidetes phylum were frequent among the dominant fragments in the DGGE profiles throughout the water column. Populations possessing a fermentative metabolism exist in Bacteroidetes, suggesting they may contribute to the degradation of organic matter in the anoxic environment of Lake Suigetsu.

Elevated sulfate reduction in metal‐contaminated freshwater lake sediments

Although sulfate‐reducing prokaryotes have long been studied as agents of metals bioremediation, impacts of long‐term metals exposure on biologically mediated sulfur cycling in natural systems remains poorly understood. The effects of long‐term exposure to metal stress on the freshwater sulfur cycle were studied, with a focus on biologic sulfate reduction using a combination of microbial and chemical methods. To examine the effects after decades of adaptation time, a field‐based experiment was conducted using multiple study sites in a natural system historically impacted by a nearby zinc smelter (Lake DePue, Illinois). Rates were highest at the most metals‐contaminated sites (∼35 μmol/cm3/day) and decreased with decreased pore water zinc and arsenic contamination levels, while other environmental characteristics (i.e., pH, nutrient concentrations and physical properties) showed little between‐site variation. Correlations were established using an artificial neural network to evaluate potentially non‐linear relationships between sulfate reduction rates (SRR) and measured environmental variables. SRR in Lake DePue were up to 50 times higher than rates previously reported for lake sediments and the chemical speciation of Zn was dominated by the presence of ZnS as shown by X‐ray Absorption Spectroscopy (XAS). These results suggest that long‐term metal stress of natural systems might alter the biogeochemical cycling of sulfur by contributing to higher rates of sulfate reduction.

Molecular analysis of the diversity of the sulfide : quinone reductase (sqr) gene in sediment environments

Our newly designed primers were evaluated for the molecular analysis of specific groups of the sqr gene encoding sulfide : quinone reductase (SQR) in sediment environments. Based on the phylogenetic analysis, we classified the sqr sequences into six groups. PCR primers specific for each group were developed. We successfully amplified sqr-like gene sequences related to groups 1, 2 and 4 from diverse sediments including a marine sediment (SW), a tidal flat (TS), a river sediment (RS) and a lake sediment (FW). We recovered a total of 82 unique phylotypes (based on a 95 % amino acid sequence similarity cutoff) from 243 individual sqr-like gene sequences. Phylotype richness varied widely among the groups of sqr-like gene sequences (group 1>group 2>group 4) and sediments (SW>TS>RS>FW). Most of the sqr-like gene sequences were affiliated with the Proteobacteria clade and were distantly related to the reference sqr gene sequences from cultivated strains (less than approximately 80 % amino acid sequence similarity). Unique sqr-like gene sequences were associated with individual sediment samples in groups 1 and 2. This molecular tool has also enabled us to detect sqr-like genes in a sulfur-oxidizing enrichment from marine sediments. Collectively, our results support the presence of previously unrecognized sqr gene-containing micro-organisms that play important roles in the global biogeochemical cycle of sulfur.

Sulfur isotope biogeochemistry of the Proterozoic McArthur Basin

Previous geochemical and biomarker studies of the late Paleo-Mesoproterozoic propose a stratified world, with strongly reducing (possibly sulfidic) deep-ocean conditions overlain by an oxygenated surface-ocean and atmosphere. To investigate such a scenario, we look to the structure of the biogeochemical sulfur cycle. We present sulfur ( 32S, 33S, 34S, and 36S in sulfides) isotope data from the McArthur Basin (Barney Creek, Reward, Velkerri, and McMinn formations) that allows for a direct evaluation of the surface biosphere. We are interested in investigating the types of information that can be gained by including 33S and 36S. When the 34S/ 32S fractionations are small, the inclusion of 33S and 36S provides little additional information, but does provide ancillary evidence for relative isotopic homogeneity (with the internal consistency of 33S/ 32S and 36S/ 32S). When the 34S/ 32S fractionations are large, direct information about the fractionation mechanisms can be obtained, with the potential to distinguish the biological from abiological processes. For example, the reconstruction of the Roper Group suggests that seawater sulfate concentrations were high enough to buffer against spatial heterogeneities. Overall, our findings agree with previously proposed redox structure of the Proterozoic ocean, highlight contributions from the oxidative sulfur cycle, and outline a new tool for interpreting the state of the surface sulfur cycle.

Structure of the Dissimilatory Sulfite Reductase from the Hyperthermophilic Archaeon Archaeoglobus fulgidus

Conservation of energy based on the reduction of sulfate is of fundamental importance for the biogeochemical sulfur cycle. A key enzyme of this ancient anaerobic process is the dissimilatory sulfite reductase (dSir), which catalyzes the six-electron reduction of sulfite to hydrogen sulfide under participation of a unique magnetically coupled siroheme–[4Fe–4S] center. We determined the crystal structure of the enzyme from the sulfate-reducing archaeon Archaeoglobus fulgidus at 2-Å resolution and compared it with that of the phylogenetically related assimilatory Sir (aSir). dSir is organized as a heterotetrameric (αβ) 2 complex composed of two catalytically independent αβ heterodimers. In contrast, aSir is a monomeric protein built of two fused modules that are structurally related to subunits α and β except for a ferredoxin domain inserted only into the subunits of dSir. The [4Fe–4S] cluster of this ferredoxin domain is considered as the terminal redox site of the electron transfer pathway to the siroheme–[4Fe–4S] center in dSir. While aSir binds one siroheme–[4Fe–4S] center, dSir harbors two of them within each αβ heterodimer. Surprisingly, only one siroheme–[4Fe–4S] center in each αβ heterodimer is catalytically active, whereas access to the second one is blocked by a tryptophan residue. The spatial proximity of the functional and structural siroheme–[4Fe–4S] centers suggests that the catalytic activity at one active site was optimized during evolution at the expense of the enzymatic competence of the other. The sulfite binding mode and presumably the mechanism of sulfite reduction appear to be largely conserved between dSir and aSir. In addition, a scenario for the evolution of Sirs is proposed.

Changes in biomarker abundances and sulfur isotopes of pyrite across the Permian–Triassic (P/Tr) Schuchert Dal section (East Greenland)

In this study, we report on biomarker abundances through parts of the Permian/Triassic boundary (PTB) of Schuchert Dal (East Greenland) that contains rich marine faunal records and excellent terrestrial palynological records. Biomarker abundances and sulfur isotopes are used to correlate the series of events (including changes in element cycling and associated redox conditions of the ocean) surrounding the collapse of the marine and terrestrial ecosystems through this record of a major crisis of life on Earth during a mass extinction episode. The Upper Schuchert Dal Formation contains a low diversity palynological assemblage, ascribed to arborescent cordiaite–conifer–pteridosperm vegetation. Samples from this pre-collapse interval are characterised by high abundances of dibenzofuran (DBF), dibenzothiophene (DBT) and biphenyl. Since these compounds have similar base structures, and show comparable abundance curves, it is plausible that they probably derive from a common source. We propose that phenolic compounds of lignin of the woody plants present during this period could be the source for DBF, DBT and biphenyl. The redox conditions during this period of time also support the formation of DBF and DBT. Just above the extinction interval, there is a dramatic decrease in the abundances of DBF and DBT which occurs at the same time as a sudden change in the stable sulfur isotopic composition ( δ 34S) of pyrite, indicating a change in redox conditions from oxic to anoxic/euxinic conditions. δ 34S values leading up to the extinction are highly depleted in the heavy sulfur isotope (about − 40‰ vs. VCDT), whilst shortly after the extinction interval much more positive isotope values are observed (about − 25‰). An inferred change in the biogeochemical sulfur cycle is supported by facies evidence from similar neighbouring sections. It is suggested that two processes are operating closely here; 1) Changes in redox conditions and 2) extinction and/or transgression accounting for the absence of woody material.

Comparison of the diversity of sulfate-reducing bacterial communities in the water column and the surface sediments of a Japanese meromictic lake

The diversity and abundance of sulfate-reducing bacteria (SRB) were investigated in Lake Suigetsu, a meromictic lake in Japan characterized by a permanent oxycline at a depth between 3 and 8 m separating the aerobic freshwater epilimnion from the anaerobic, saline, sulfidogenic hypolimnion. A quantitative competitive PCR targeting the gene coding for a portion of the α-subunit of dissimilatory sulfite reductase (dsrA) was used to assess the distribution of the SRB in the stratified water column and the surface sediments. The diversity of the SRB communities was assessed using a sequence analysis of the differing dsrA isomers. The dsrA gene copy numbers of SRB in the hypolimnic waters were from 9.6 × 103 to 7.5 × 105 copies ml−1, whereas higher dsrA copy numbers of SRB were observed in surface sediments, ranging from 1.8–8.1 × 107 copies ml−1 as estimated by competitive PCR. Phylogenetic analysis of the dsrA sequences retrieved from the surface sediments shows most belong to a deeply branching lineage of diverse dsrA sequences not related to any cultured SRB group. In contrast, dsrA sequences found in the oxycline waters were related to sequences of members of the genera Desulfonema, Desulfosarcina, and Dusulfococcus and to sequences of the incomplete oxidizers from the Desulfobulbaceae family. Diversity and abundance of dsrA genes significantly differed between the samples from the oxycline waters and the surface sediments of Lake Suigetsu, indicating habitat-specific SRB communities may contribute to the biogeochemical cycling of carbon and sulfur.

Parallel, high-resolution carbon and sulfur isotope records of the evolving Paleozoic marine sulfur reservoir

Links between the biogeochemical cycles of carbon and sulfur are expressed in the evolving stable isotope composition of the ocean. Carbonate rocks record the inorganic carbon isotope composition of the oceanic reservoir through geological time, along with the sulfate sulfur isotope composition preserved as carbonate-associated sulfate (CAS). An inverse relationship exists between the first-order carbon and sulfur records of the Paleozoic; however, the isotope curves also reveal higher-frequency variations superimposed on the first-order (10 7–10 8 yr) trend. Because of links between carbon and sulfur cycling, high-resolution, CAS-based S isotope data have the potential to shed essential light on the mechanisms behind carbon isotope excursions observed in the geologic record. Results from the Late Cambrian SPICE event at Shingle Pass, Nevada, show parallel positive C (5‰) and S (25‰) isotope excursions, likely recording a large-scale marine organic carbon burial event with sympathetic pyrite burial. Carbon and sulfur records from other events throughout the Paleozoic reveal different and evolving relationships. A Silurian–Devonian boundary section at Strait Creek, West Virginia, reveals a positive C excursion of 6‰ but with an enigmatic, apparently negative S excursion (15‰) antithetic to the C shift. By contrast, a Kinderhookian–Osagean (Early Mississippian) section in the Confusion Range, Utah, shows invariant S isotope ratios across a C isotope excursion of 6‰. The coupling of the high-resolution C–S isotope records may have weakened in the mid-to-late Paleozoic with decreasing sensitivity of the seawater S isotope reservoir to flux changes as the marine sulfate concentrations increased through the Paleozoic. The progressively decreasing isotopic variability of CAS across three positive carbon excursions suggests that the Paleozoic was transitional between a Proterozoic ocean with rapid isotopic variability and thus low sulfate concentration and the comparatively more stable isotopic properties of a relatively more sulfate-rich Mesozoic and Cenozoic ocean. Furthermore, the emergence of the terrestrial ecosystems during the late middle and late Paleozoic yielded new loci of organic burial that also likely played a central role in the decoupling carbon and sulfur isotope records. Organic carbon burial in these sulfate-limited environments occurred in the absence of significant pyrite burial.

Microbial communities and geochemical dynamics in an extremely acidic, metal‐rich stream at an abandoned sulfide mine (Huelva, Spain) underpinned by two functional primary production systems

An extremely acidic (pH 2.5-2.75) metal-rich stream draining an abandoned mine in the Iberian Pyrite Belt, Spain, was ramified with stratified macroscopic gelatinous microbial growths ('acid streamers' or 'mats'). Microbial communities of streamer/mat growths sampled at different depths, as well as those present in the stream water itself, were analysed using a combined biomolecular and cultivation-based approach. The oxygen-depleted mine water was dominated by the chemolithotrophic facultative anaerobe Acidithiobacillus ferrooxidans, while the streamer communities were found to be highly heterogeneous and very different to superficially similar growths reported in other extremely acidic environments. Microalgae accounted for a significant proportion of surface streamer biomass, while subsurface layers were dominated by heterotrophic acidophilic bacteria (Acidobacteriacae and Acidiphilium spp.). Sulfidogenic bacteria were isolated from the lowest depth streamer growths, where there was also evidence for selective biomineralization of copper sulfide. Archaeal clones (exclusively Euryarchaeota) were recovered from streamer samples, as well as the mine stream water. Both sunlight and reduced inorganic chemicals (predominantly ferrous iron) served as energy sources for primary producers in this ecosystem, promoting complex microbial interactions involving transfer of electron donors and acceptors and of organic carbon, between microorganisms in the stream water and the gelatinous streamer growths. Microbial transformations were shown to impact the biogeochemical cycling of iron and sulfur in the acidic stream, severely restricting the net oxidation of ferrous iron even when the initially anoxic waters were oxygenated by indigenous acidophilic algae. A model accounting for the biogeochemistry of iron and sulfur in the mine waters is described, and the significance of the acidophilic communities in regulating the geochemistry of acidic, metal-rich waters is described.