Isotopic reconstruction of iron oxidation-reduction process based on an Archean Ocean analogue.

The chemical composition and redox conditions of the Precambrian ocean are key factors for reconstructing the temporal evolution of atmospheric oxygen through time. In particular, the isotopic composition of iron are useful proxies for reconstructing paleo-ocean environments. Yet, respective processes and related signatures are poorly constrained, hindering the reconstruction of iron redox mechanisms in the Archean ocean. This study centers on Sihailongwan Lake, a stratified water body with a euxinic lower water column considered as an Archean ocean analogue. Results show that the anaerobic oxidation layer is so different from other similar lakes in which dissolved Fe oxidation is present in redoxcline layer. And the fractionation factor between ferrous Fe and iron hydroxide observed in nature water body of Sihailongwan Lake reaches to 2.6‰, which would benefit the production of the oxidations of BIF in sediment. By the spatial distribution of Fe isotope, the benthic water in autumn and the hypolimnetic anoxic water in spring has been identified as iron sulfide zone, where iron isotopic fractionation factor during iron sulfide formation is 1.16‰, accounting for partial scavenging of dissolved Fe(II) with an associated isotopic fractionation. However, pyrite in the sediment records the iron isotopic signal from the redoxcline but not in the iron sulfide or oxide zones of the water column. Above findings indicate that neither the iron isotope fractionation during partial transfer of ferrous iron to iron sulfide nor the partial oxidation of ferrous iron are recorded as pyrite in sedimentary rock. Importantly, the signal of Fe isotopic fractionation in water was archived in the suspended particulate matter and transferred into the sediment, rather than via ferrous iron directly deposited in the sediment. This study reveals that Fe isotopes from modern natural environments are useful proxies for reconstructing iron oxidation-reduction process during Earth's early history.

Large sulfur isotope fractionation by bacterial sulfide oxidation.

A sulfide-oxidizing microorganism, Desulfurivibrio alkaliphilus (DA), generates a consistent enrichment of sulfur-34 (34 S) in the produced sulfate of +12.5 per mil or greater. This observation challenges the general consensus that the microbial oxidation of sulfide does not result in large 34 S enrichments and suggests that sedimentary sulfides and sulfates may be influenced by metabolic activity associated with sulfide oxidation. Since the DA-type sulfide oxidation pathway is ubiquitous in sediments, in the modern environment, and throughout Earth history, the enrichments and depletions in 34 S in sediments may be the combined result of three microbial metabolisms: microbial sulfate reduction, the disproportionation of external sulfur intermediates, and microbial sulfide oxidation.

The Biogeochemical Sulfur Cycle of Marine Sediments.

Microbial dissimilatory sulfate reduction to sulfide is a predominant terminal pathway of organic matter mineralization in the anoxic seabed. Chemical or microbial oxidation of the produced sulfide establishes a complex network of pathways in the sulfur cycle, leading to intermediate sulfur species and partly back to sulfate. The intermediates include elemental sulfur, polysulfides, thiosulfate, and sulfite, which are all substrates for further microbial oxidation, reduction or disproportionation. New microbiological discoveries, such as long-distance electron transfer through sulfide oxidizing cable bacteria, add to the complexity. Isotope exchange reactions play an important role for the stable isotope geochemistry and for the experimental study of sulfur transformations using radiotracers. Microbially catalyzed processes are partly reversible whereby the back-reaction affects our interpretation of radiotracer experiments and provides a mechanism for isotope fractionation. We here review the progress and current status in our understanding of the sulfur cycle in the seabed with respect to its microbial ecology, biogeochemistry, and isotope geochemistry.

Iron and sulfide nanoparticle formation and transport in nascent hydrothermal vent plumes.

Deep-sea hydrothermal vents are a significant source of dissolved metals to the global oceans, producing midwater plumes enriched in metals that are transported thousands of kilometers from the vent source. Particle precipitation upon emission of hydrothermal fluids controls metal speciation and the magnitude of metal export. Here, we document metal sulfide particles, including pyrite nanoparticles, within the first meter of buoyant plumes from three high-temperature vents at the East Pacific Rise. We observe a zone of particle settling 10-20 cm from the orifice, indicated by stable sulfur isotopes; however, we also demonstrate that nanoparticulate pyrite (FeS2) is not removed from the plume and can account for over half of the filtered Fe (≤0.2 µm) up to one meter from the vent orifice. The persistence of nanoparticulate pyrite demonstrates that it is an important mechanism for near-vent Fe stabilisation and highlights the potential role of nanoparticles in element transport.

Disguised as a Sulfate Reducer: Growth of the Deltaproteobacterium Desulfurivibrio alkaliphilus by Sulfide Oxidation with Nitrate.

This study demonstrates that the deltaproteobacterium Desulfurivibrio alkaliphilus can grow chemolithotrophically by coupling sulfide oxidation to the dissimilatory reduction of nitrate and nitrite to ammonium. Key genes of known sulfide oxidation pathways are absent from the genome of D. alkaliphilus Instead, the genome contains all of the genes necessary for sulfate reduction, including a gene for a reductive-type dissimilatory bisulfite reductase (DSR). Despite this, growth by sulfate reduction was not observed. Transcriptomic analysis revealed a very high expression level of sulfate-reduction genes during growth by sulfide oxidation, while inhibition experiments with molybdate pointed to elemental sulfur/polysulfides as intermediates. Consequently, we propose that D. alkaliphilus initially oxidizes sulfide to elemental sulfur, which is then either disproportionated, or oxidized by a reversal of the sulfate reduction pathway. This is the first study providing evidence that a reductive-type DSR is involved in a sulfide oxidation pathway. Transcriptome sequencing further suggests that nitrate reduction to ammonium is performed by a novel type of periplasmic nitrate reductase and an unusual membrane-anchored nitrite reductase.IMPORTANCE Sulfide oxidation and sulfate reduction, the two major branches of the sulfur cycle, are usually ascribed to distinct sets of microbes with distinct diagnostic genes. Here we show a more complex picture, as D. alkaliphilus, with the genomic setup of a sulfate reducer, grows by sulfide oxidation. The high expression of genes typically involved in the sulfate reduction pathway suggests that these genes, including the reductive-type dissimilatory bisulfite reductases, are also involved in as-yet-unresolved sulfide oxidation pathways. Finally, D. alkaliphilus is closely related to cable bacteria, which grow by electrogenic sulfide oxidation. Since there are no pure cultures of cable bacteria, D. alkaliphilus may represent an exciting model organism in which to study the physiology of this process.

Turnover Rates of Intermediate Sulfur Species ([Formula: see text], S0, S2[Formula: see text], S4[Formula: see text], [Formula: see text]) in Anoxic Freshwater and Sediments.

The microbial reduction of sulfate to sulfide coupled to organic matter oxidation followed by the transformation of sulfide back to sulfate drives a dynamic sulfur cycle in a variety of environments. The oxidative part of the sulfur cycle in particular is difficult to constrain because the eight electron oxidation of sulfide to sulfate occurs stepwise via a suite of biological and chemical pathways and produces a wide variety of intermediates ([Formula: see text], S0, S2[Formula: see text], S4[Formula: see text], and [Formula: see text]), which may in turn be oxidized, reduced or disproportionated. Although the potential processes affecting these intermediates are well-known from microbial culture and geochemical studies, their significance and rates in the environment are not well constrained. In the study presented here, time-course concentration measurements of intermediate sulfur species were made in amended freshwater water column and sediment incubation experiments in order to constrain consumption rates and processes. In sediment incubations, consumption rates were [Formula: see text] [Formula: see text] [Formula: see text] S4[Formula: see text] S2[Formula: see text], which is consistent with previous measurements of [Formula: see text], S4[Formula: see text], and S2[Formula: see text] consumption rates in marine sediments. In water column incubations, however, the relative reactivity was [Formula: see text] [Formula: see text] [Formula: see text] S2[Formula: see text] S4[Formula: see text]. Consumption of thiosulfate, tetrathionate and sulfite was primarily biological, whereas it was not possible to distinguish between abiotic and biological polysulfide consumption in either aqueous or sediment incubations. [Formula: see text] consumption in water column experiments was biologically mediated, however, rapid sedimentary consumption was likely due to reactions with iron minerals. These experiments provide important constraints on the biogeochemical reactivity of intermediate sulfur species and give further insight into the diversity of biological and geochemical processes that comprise (cryptic) environmental sulfur cycling.

Microbial impact on polysulfide dynamics in the environment.

Polysulfides (Sx (2-)) are sulfide oxidation intermediates that are important for a variety of environmentally relevant processes including pyrite formation, organic matter sulfidization, isotope exchange among reduced sulfur species, and metal chelation. In addition to their chemical reactivity, laboratory experiments with microbial cultures and enzymes indicate both indirect and direct roles for microorganisms in affecting polysulfide chemistry in natural environments through production and consumption. As polysulfides have been detected in a wide array of natural systems ranging from microbial mats to hydrothermal vents, constraining their biogeochemical cycling has broad impacts. However, many questions remain regarding the processes responsible for polysulfide dynamics in these environments and the precise role that microorganisms play in these processes. This review provides a summary of laboratory experiments investigating the role of polysulfides in microbial metabolism, and observations of polysulfides in the environment in order to provide further insight into and highlight open questions about this significant component of the sulfur cycle.

Light-dependent sulfide oxidation in the anoxic zone of the Chesapeake Bay can be explained by small populations of phototrophic bacteria.

Microbial sulfide oxidation in aquatic environments is an important ecosystem process, as sulfide is potently toxic to aerobic organisms. Sulfide oxidation in anoxic waters can prevent the efflux of sulfide to aerobic water masses, thus mitigating toxicity. The contribution of phototrophic sulfide-oxidizing bacteria to anaerobic sulfide oxidation in the Chesapeake Bay and the redox chemistry of the stratified water column were investigated in the summers of 2011 to 2014. In 2011 and 2013, phototrophic sulfide-oxidizing bacteria closely related to Prosthecochloris species of the phylum Chlorobi were cultivated from waters sampled at and below the oxic-anoxic interface, where measured light penetration was sufficient to support populations of low-light-adapted photosynthetic bacteria. In 2012, 2013, and 2014, light-dependent sulfide loss was observed in freshly collected water column samples. In these samples, extremely low light levels caused 2- to 10-fold increases in the sulfide uptake rate over the sulfide uptake rate under dark conditions. An enrichment, CB11, dominated by Prosthecochloris species, oxidized sulfide with a Ks value of 11 µM and a Vmax value of 51 µM min(-1) (mg protein(-1)). Using these kinetic values with in situ sulfide concentrations and light fluxes, we calculated that a small population of Chlorobi similar to those in enrichment CB11 can account for the observed anaerobic light-dependent sulfide consumption activity in natural water samples. We conclude that Chlorobi play a far larger role in the Chesapeake Bay than currently appreciated. This result has potential implications for coastal anoxic waters and expanding oxygen-minimum zones as they begin to impinge on the photic zone.

Using in situ voltammetry as a tool to identify and characterize habitats of iron-oxidizing bacteria: from fresh water wetlands to hydrothermal vent sites.

Iron-oxidizing bacteria (FeOB) likely play a large role in the biogeochemistry of iron, making the detection and understanding of the biogeochemical processes FeOB are involved in of critical importance. By deploying our in situ voltammetry system, we are able to measure a variety of redox species, specifically Fe(ii) and O2, simultaneously. This technique provides significant advantages in both characterizing the environments in which microaerophilic FeOB are found, and finding diverse conditions in which FeOB could potentially thrive. Described here are four environments with different salinities [one fresh groundwater seep site, one beach-groundwater mixing site, one hydrothermal vent site (Mid-Atlantic Ridge), and one estuary (Chesapeake Bay)] where in situ voltammetry was deployed, and where the presence of FeOB were confirmed by either culturing methods or molecular data. The sites varied in both O2 and Fe(ii) content with O2 ranging from below the 3 µM detection limit of the electrodes at the Chesapeake Bay suboxic zone, to as high 150 µM O2 at the vent site. In addition, a range of Fe(ii) concentrations supported FeOB communities, from 3 µM Fe(ii) in the Chesapeake Bay to 300 µM in the beach aquifer. In situ electrochemistry provides the means to quickly measure these redox gradients at appropriate resolution, making it possible in real time to detect niches likely inhabited by microaerophilic FeOB, then accurately sample for proof of FeOB presence and activity. This study demonstrates the utility of this approach while also greatly expanding our knowledge of FeOB habitats.

Phototrophic sulfide oxidation: environmental insights and a method for kinetic analysis.

Previously, we presented data that indicated microbial sulfide oxidation would out-compete strictly chemical, abiotic sulfide oxidation reactions under nearly all conditions relevant to extant ecosystems (Luther et al., 2011). In particular, we showed how anaerobic microbial sulfide oxidation rates were several orders of magnitude higher than even metal catalyzed aerobic sulfide oxidation processes. The fact that biotic anaerobic sulfide oxidation is kinetically superior to abiotic reactions implies that nearly all anaerobic and sulfidic environments should host microbial populations that oxidize sulfide at appreciable rates. This was likely an important biogeochemical process during long stretches of euxinia in the oceans suggested by the geologic record. In particular, phototrophic sulfide oxidation allows the utilization of carbon dioxide as the electron acceptor suggesting that this process should be particularly widespread rather than relying on the presence of other chemical oxidants. Using the Chesapeake Bay as an example, we argue that phototrophic sulfide oxidation may be more important in many environments than is currently appreciated. Finally, we present methodological considerations to assist other groups that wish to study this process.

Thermodynamics and kinetics of sulfide oxidation by oxygen: a look at inorganically controlled reactions and biologically mediated processes in the environment.

The thermodynamics for the first electron transfer step for sulfide and oxygen indicates that the reaction is unfavorable as unstable superoxide and bisulfide radical ions would need to be produced. However, a two-electron transfer is favorable as stable S(0) and peroxide would be formed, but the partially filled orbitals in oxygen that accept electrons prevent rapid kinetics. Abiotic sulfide oxidation kinetics improve when reduced iron and/or manganese are oxidized by oxygen to form oxidized metals which in turn oxidize sulfide. Biological sulfur oxidation relies on enzymes that have evolved to overcome these kinetic constraints to affect rapid sulfide oxidation. Here we review the available thermodynamic and kinetic data for H(2)S and HS• as well as O(2), reactive oxygen species, nitrate, nitrite, and NO(x) species. We also present new kinetic data for abiotic sulfide oxidation with oxygen in trace metal clean solutions that constrain abiotic rates of sulfide oxidation in metal free solution and agree with the kinetic and thermodynamic calculations. Moreover, we present experimental data that give insight on rates of chemolithotrophic and photolithotrophic sulfide oxidation in the environment. We demonstrate that both anaerobic photolithotrophic and aerobic chemolithotrophic sulfide oxidation rates are three or more orders of magnitude higher than abiotic rates suggesting that in most environments biotic sulfide oxidation rates will far exceed abiotic rates due to the thermodynamic and kinetic constraints discussed in the first section of the paper. Such data reshape our thinking about the biotic and abiotic contributions to sulfide oxidation in the environment.