Bacterial Iron Reduction Research Articles

Biochar changes iron-reducing bacteria community in paddy soils and promotes the bacterial iron reduction

Biochar changes iron-reducing bacteria community in paddy soils and promotes the bacterial iron reduction

Elucidation of Iron(III) Bioleaching Properties of Gram-Positive Bacteria.

Microbial-based iron reduction is an emerging technology used as an alternative to conventional chemical-based iron reduction. The iron reduction in kaolin refinement is vital for enhancing its commercial value. Extensive studies on microbial-based iron reduction mainly focus on Gram-negative bacteria, whereas little is understood about Gram-positive bacteria’s mechanism and potential application. This study aims to investigate the iron-reducing mechanism of two Gram-positive bacterial isolates, Bacillus cereus (B. cereus) and Staphylococcus aureus (S. aureus). By varying the growth environment of bacteria and monitoring the biochemical changes during the process of iron reduction, the results show that Gram-positive bacterial iron reduction performance depends on the medium composition, differing from Gram-negative bacteria-based reduction processes. Nitrogen-rich medium facilitates the microbial basification of the medium, where the alkaline conditions impact the microbial iron reduction process by altering the gene expression involved in intracellular pH homeostasis and microbial growth. This discovery will contribute to the mineral refining processes and promote the development of microbial-based bioprocesses for ore purification, while also laying the foundation for investigating other Gram-positive bacterial iron-reducing ability.

Elucidation of Gram-Positive Bacterial Iron(III) Reduction for Kaolinite Clay Refinement.

Recently, microbial-based iron reduction has been considered as a viable alternative to typical chemical-based treatments. The iron reduction is an important process in kaolin refining, where iron-bearing impurities in kaolin clay affects the whiteness, refractory properties, and its commercial value. In recent years, Gram-negative bacteria has been in the center stage of iron reduction research, whereas little is known about the potential use of Gram-positive bacteria to refine kaolin clay. In this study, we investigated the ferric reducing capabilities of five microbes by manipulating the microbial growth conditions. Out of the five, we discovered that Bacillus cereus and Staphylococcus aureus outperformed the other microbes under nitrogen-rich media. Through the biochemical changes and the microbial behavior, we mapped the hypothetical pathway leading to the iron reduction cellular properties, and found that the iron reduction properties of these Gram-positive bacteria rely heavily on the media composition. The media composition results in increased basification of the media that is a prerequisite for the cellular reduction of ferric ions. Further, these changes impact the formation of biofilm, suggesting that the cellular interaction for the iron(III)oxide reduction is not solely reliant on the formation of biofilms. This article reveals the potential development of Gram-positive microbes in facilitating the microbial-based removal of metal contaminants from clays or ores. Further studies to elucidate the corresponding pathways would be crucial for the further development of the field.

Investigation of Biogenic Passivating Layers on Corroded Iron.

This study evaluates mechanisms of biogenic mineral formation induced by bacterial iron reduction for the stabilization of corroded iron. As an example, the Desulfitobacterium hafniense strain TCE1 was employed to treat corroded coupons presenting urban natural atmospheric corrosion, and spectroscopic investigations were performed on the samples’ cross-sections to evaluate the corrosion stratigraphy. The treated samples presented a protective continuous layer of iron phosphates (vivianite Fe2+3(PO4)2·8H2O and barbosalite Fe2+Fe3+2(PO4)2(OH)2), which covered 92% of the surface and was associated with a decrease in the thickness of the original corrosion layer. The results allow us to better understand the conversion of reactive corrosion products into stable biogenic minerals, as well as to identify important criteria for the design of a green alternative treatment for the stabilization of corroded iron.

Microbial driven iron reduction affects arsenic transformation and transportation in soil-rice system

The microbe-driven iron cycle plays an important role in speciation transformation and migration of arsenic (As) in soil-rice systems. In this study, pot experiments were used to investigate the effect of bacterial iron (Fe) reduction processes in soils on As speciation and migration, as well as on As uptake in soil-rice system. During the rice growth period, pH and electrical conductivity (EC) in soil solutions initially increased and then decreased, with the ranges of 7.4–8.8 and 116.3–820 mS cm−1, respectively. The concentrations of Fe, total As and As(III) showed an increasing trend in the rhizosphere and non-rhizosphere soil solutions with the increasing time. Fe concentrations were significantly positively correlated with total As and As(III) concentrations (***p < 0.001) in the soil solutions. The abundances of the arsenate reductase gene (arsC) and the As(III) S-adenosylmethionine methyltransferase gene (arsM) in rhizosphere soils were higher than those in non-rhizosphere soils, while the abundance of the Fe-reducing bacteria (Geo) showed an opposite trend. Moreover, it showed that the Geo abundance was significantly positively correlated with that of the arsC (***p < 0.001) and arsM (**p < 0.01) genes, respectively. The abundances of Geo, arsC and arsM genes were significantly positively correlated with the concentrations of Fe, total As and As(III) in the soil solutions (*p < 0.05). Moreover, the abundances of arsC and arsM genes were significantly negatively correlated with total As and As(III) in rice grains (*P < 0.05). These results showed that the interaction of bacterial Fe reduction process and radial oxygen loss from roots promoted the reduction and methylation of As, and then decreased As uptake by rice, which provided a theoretical basis for alleviating As pollution in paddy soils.

Biogeochemical cycling of iron: Implications for biocementation and slope stabilisation

Microbial biofilms growing in iron-rich seeps surrounding Lake Violão, Carajás, Brazil serve as a superb natural system to study the role of iron cycling in producing secondary iron cements. These seeps flow across iron duricrusts (referred to as canga in Brazil) into hydraulically restricted lakes in northern Brazil. Canga caps all of the iron ore deposits in Brazil, protecting them from being destroyed by erosion in this active weathering environment. Biofilm samples collected from these seeps demonstrated heightened biogeochemical iron cycling, contributing to the relatively rapid, seasonal formation of iron-rich cements. The seeps support iron-oxidising lineages including Sideroxydans, Gallionella, and an Azoarcus species revealed by 16S rRNA gene sequencing. In contrast, a low relative abundance of putative iron reducers; for example, Geobacter species (<5% of total sequences in any sample), corresponds to carbon limitation in this canga-associated ecosystem. This carbon limitation is likely to restrict anoxic niches to within biofilms. Examination of a canga rock sample collected from the edge of Lake Violão revealed an array of well- to poorly-preserved microbial fossils in secondary iron cements. These heterogeneous cements preserved bacterial cell envelopes and possibly extracellular polymeric substances within the microfossil iron-rich cements (termed biocements). Bacterial iron reduction initiates the sequence, and intuitively is the rate-limiting step in this broadly aerobic environment. The organic framework of the active- and paleo-biofilms appears to provide a scaffold for the formation of some cements within canga and likely expedites cement formation. The accelerated development of these resilient iron-rich biocements in the lake edge environment compared with surroundings duricrust-associated environments may provide insights into new approaches to remediate mined land, aiding to stabilise slopes, reduce erosion, restore functional hydrogeology and provide a substrate akin to natural canga for revegetation using endemic canga plant species, which have adapted to grow on iron-rich substrates.

Dissimilatory iron and sulfate reduction by native microbial communities using lactate and citrate as carbon sources and electron donors

The bacterial (dissimilatory) iron and sulfate reduction (BIR and BSR) are intimately linked to the biogeochemical cycling of C, Fe, and S in acid mine drainage (AMD) environments. This study examined the response of native microbial communities to the reduction of iron and sulfate in bench experimental systems. Results showed that the reduction of ferric iron and sulfate took place when the electron acceptors coexist. Existence of Fe(III) can postpone the reduction of sulfate, but can enhance the reduction rate. Cultures grown in the presence of 10 mM iron can reach the final level of sulfate bio-reduction rate (~100%) after 35 days incubation. 16 S rDNA -based microbial community analysis revealed that the three genera Anaeromusa, Acinetobacter and Bacteroides were dominated in the ferric-reducing conditions. SRB (Desulfobulbus, Desulfosporosinus and Desulfovibrio) were dominated in the sulfate reduction process. Results in this study highlighted the highly coupled nature of C, Fe, and S biogeochemical cycles in AMD and provided insights into the potential of environmental remediation by native microbial.

Is seawater geochemical composition recorded in marine carbonate? Evidence from iron and manganese contents in Late Devonian carbonate rocks

Iron and manganese are the important redox-sensitive elements in the ocean. Previous studies have established a series of paleo-depositional redox proxies based on the form and content of iron in sedimentary rocks (e.g., degree of pyritization, FeHR/FeT, Fe/Al). These proxies were developed and applied on siliciclastic-rich marine sediments. Although marine carbonate rocks are generally considered to preserve the geochemical signals of ancient seawater, neither Fe nor Mn content in marine carbonate rocks (Fecarb, Mncarb) has been independently used as a proxy to quantify environmental cues in paleo-oceans. Both Fe and Mn are insoluble in oxic conditions (Fe2O3, Fe(OH)3, MnO2), while their reduced forms (Fe2+ and Mn2+) are soluble. Therefore, oxic seawater should have low concentrations of dissolved Fe2+ and Mn2+, and accordingly carbonate rocks precipitated from oxic seawater should have low Fecarb and Mncarb, and vice versa. To evaluate whether Fecarb and Mncarb can be used to quantify oxygen fugacity in seawater, we measured Fecarb and Mncarb of Upper Devonian marine carbonate rocks collected from nine sections in South China. Fecarb of intraplatform basin samples was significantly higher than that of shelf samples, while shelf and basin samples had comparable Mncarb. The modeling result indicates that the dramatic difference in Fecarb cannot be explained by variation in oxygen fugacity between the shelf and basin seawater. Instead, both Fecarb and Mncarb appear to be more sensitive to benthic flux from sediment porewater that is enriched in Fe2+ and Mn2+. Porewater Fe2+ and Mn2+ derive from bacterial iron and manganese reduction; flux was controlled by sedimentation rate and the depth of the Fe(Mn) reduction zone in sediments, the latter of which is determined by oxygen fugacity at the water–sediment interface. Thus, high Fecarb of the basin samples might be attributed to low sedimentation rate and/or low oxygen fugacity at the seafloor. However, invariant Mncarb of the shelf and basin samples might be the consequence of complete reduction of Mn in sediments. Our study indicates that marine carbonate rocks may not necessarily record seawater composition, particularly for benthic carbonate rocks. The influence of benthic flux might cause carbonate rocks’ geochemical signals to deviate significantly from seawater values. Our study suggests that interpretation of geochemical data from carbonate rocks, including carbonate carbon isotopes, should consider the process of carbonate formation.

Imaging redox activity and Fe(II) at the microbe-mineral interface during Fe(III) reduction

Dissimilatory iron-reducing bacteria (DIRB) play an important role in controlling the redox chemistry of Fe and other transition metals and radionuclides in the environment. During bacterial iron reduction, electrons are transferred from the outer membrane to poorly soluble Fe(III) minerals, although the precise physiological mechanisms and local impact on minerals of these redox processes remain unclear. The aim of this work was to use a range of microscopic techniques to examine the local environment of Geobacter sulfurreducens grown on thin films of Fe(III)-bearing minerals, to provide insight into spatial patterns of Fe(III) reduction and electron transfer. Confocal fluorescence microscopy showed that sparse biofilms formed on the mineral coatings, while the selective Fe(II) probe RhoNox-1 revealed Fe(II) patches on the minerals sometimes co-located with cells. Atomic force microscopy highlighted thin filamentous structures extending radially from the cell surface. Further analysis using fluorescent redox dyes showed redox-active, linear nanowires that formed cell to cell connections, although they were not implicated in playing a dominant role in direct electron transfer to the Fe(III) minerals. Overall this paper provides new methods and insights on studying Fe(III) reduction and other redox transformations in situ.

Bacterial iron reduction and biogenic mineral formation for the stabilisation of corroded iron objects

Exploiting bacterial metabolism for the stabilisation of corroded iron artefacts is a promising alternative to conventional conservation-restoration methods. Bacterial iron reduction coupled to biogenic mineral formation has been shown to promote the conversion of reactive into stable corrosion products that are integrated into the natural corrosion layer of the object. However, in order to stabilise iron corrosion, the formation of specific biogenic minerals is essential. In this study, we used the facultative anaerobe Shewanella loihica for the production of stable biogenic iron minerals under controlled chemical conditions. The biogenic formation of crystalline iron phosphates was observed after iron reduction in a solution containing Fe(III) citrate. When the same biological treatment was applied on corroded iron plates, a layer composed of iron phosphates and iron carbonates was formed. Surface and cross-section analyses demonstrated that these two stable corrosion products replaced 81% of the reactive corrosion layer after two weeks of treatment. Such results demonstrate the potential of a biological treatment in the development of a stabilisation method to preserve corroded iron objects.

Making magnetite late again: Evidence for widespread magnetite growth by thermal decomposition of siderite in Hamersley banded iron formations

Banded iron formations (BIFs) are marine chemical sedimentary rocks that were widely deposited before the start of the Great Oxidation Event (2.45–2.32 Ga). They represent an important archive into the chemistry of the Precambrian oceans and atmosphere, and the evolution of the marine biosphere. It is believed that BIFs were derived from ferric oxides/hydroxides that were deposited by iron-oxidizing bacteria or by reaction with oxygen released by Cyanobacteria. Hematite is traditionally interpreted to represent the dehydrated product of the ferric oxide/hydroxide precipitate, whereas magnetite is thought to have formed from ferric oxides/hydroxides via bacterial iron reduction and oxidation of microbial biomass. However, studies are equivocal about when and how magnetite formed in BIFs. We present the results of a detailed petrographic study of magnetite in BIFs of the Hamersley Group, Western Australia. We find no evidence that magnetite was precipitated from seawater or was a product of bacterial iron reduction. Instead, most of the magnetite formed by replacement of siderite and other ferrous-rich minerals after burial. Our observations suggest that magnetite growth was linked to the breakdown of siderite during metamorphism. Published experimental data indicate that in the presence of water and under the metamorphic conditions experienced in the southern Pilbara Craton (200–350 °C), the thermal decomposition of siderite could account for much of the magnetite in the Hamersley BIFs (3FeCO3 + H2O → Fe3O4 + 3CO2 + H2).The relationship between magnetite and the primary precipitates, which are preserved as greenalite nanoparticles and microgranules in chert bands, suggests that much of the magnetite formed in siderite-rich, iron-silicate bands that developed by compaction of non-silicified iron-silicate muds. These bands acted as reactive pathways for fluid flow and the formation of magnetite. If most of the magnetite formed after burial then it is necessary to reassess assumptions about the chemistry of the original precipitates and the role of microbial life in the redox cycling of iron. Our results are consistent with models for the deposition of BIFs that center around a precursor sediment comprising greenalite precipitates and diagenetic siderite that was partially replaced by magnetite and hematite after burial.

Electron shuttle-mediated microbial Fe(III) reduction under alkaline conditions

Extracellular Fe(III) reduction plays an important role in a variety of biogeochemical processes. Several mechanisms for microbial Fe(III) reduction in pH-neutral environments have been proposed, but pathways of microbial Fe(III) reduction within alkaline conditions have not been clearly identified. Alkaline soils are vastly distributed; thus, a better understanding of microbial Fe(III) reduction under alkaline conditions is of significance. The purpose of this study is to explore the dominant mechanism of bacterial iron reduction in alkaline environments. We used antraquinone-2,6-disulfonate (AQDS) as a representative of quinone moities of humic substances and elemental sulfur and sulfate as sulfur species to investigate the potential role of humic substances and sulfur species in mediating microbial Fe(III) reduction in alkaline environments. We carried out thermodynamic calculations to predict the ability of bacteria to reduce Fe(III) (oxyhydr)oxides under alkaline conditions and the ability of AQDS and sulfur species to serve as electron acceptors for microbial anaerobic respiration in an assumed alkaline soil environments. A series of incubation experiments with two model dissimilatory metal reducing bacteria, Shewanella oneidensis MR-1 and Geobacter sulfurreducens PCA as well as mixed bacteria enriched from a soil were performed to confirm the contribution of AQDS and sulfur species to Fe(III) reduction under alkaline conditions. Based on thermodynamic calculations, we predicted that, under alkaline conditions, the enzymatic reduction of Fe(III) (oxyhydr)oxides would be thermodynamically feasible but very weak. In our incubation experiments, the reduction of ferrihydrite by anaerobic cultures of Shewanella oneidensis MR-1, Geobacter sulfurreducens PCA or microbes enriched from a soil was significantly increased in the presence of S0 or AQDS. Notably, AQDS contributed more to promoting Fe(III) reduction as a soluble electron shuttle than S0 did under the alkaline conditions probably because of different mechanisms of microbial utilization of AQDS and S0. These results suggest that microbial reduction of Fe(III) (oxyhydr)oxides under alkaline conditions may proceed via a pathway mediated by electron shuttles such as AQDS and S0. Considering the high ability of electron shuttling and vast distribution of humic substances, we suggest that humic substance-mediated Fe(III) reduction may potentially be the dominant mechanism for Fe(III) reduction in alkaline environments.

Use of Bacteria To Stabilize Archaeological Iron.

Iron artifacts are common among the findings of archaeological excavations. The corrosion layer formed on these objects requires stabilization after their recovery, without which the destruction of the item due to physicochemical damage is likely. Current technologies for stabilizing the corrosion layer are lengthy and generate hazardous waste products. Therefore, there is a pressing need for an alternative method for stabilizing the corrosion layer on iron objects. The aim of this study was to evaluate an alternative conservation-restoration method using bacteria. For this, anaerobic iron reduction leading to the formation of stable iron minerals in the presence of chlorine was investigated for two strains of Desulfitobacterium hafniense (strains TCE1 and LBE). Iron reduction was observed for soluble Fe(III) phases as well as for akaganeite, the most troublesome iron compound in the corrosion layer of archaeological iron objects. In terms of biogenic mineral production, differential efficiencies were observed in assays performed on corroded iron coupons. Strain TCE1 produced a homogeneous layer of vivianite covering 80% of the corroded surface, while on the coupons treated with strain LBE, only 10% of the surface was covered by the same mineral. Finally, an attempt to reduce iron on archaeological objects was performed with strain TCE1, which led to the formation of both biogenic vivianite and magnetite on the surface of the artifacts. These results demonstrate the potential of this biological treatment for stabilizing archaeological iron as a promising alternative to traditional conservation-restoration methods.IMPORTANCE Since the Iron Age, iron has been a fundamental material for the building of objects used in everyday life. However, due to its reactivity, iron can be easily corroded, and the physical stability of the object built is at risk. This is particularly true for archaeological objects on which a potentially unstable corrosion layer is formed during the time the object is buried. After excavation, changes in environmental conditions (e.g., higher oxygen concentration or lower humidity) alter the stability of the corrosion layer and can lead to the total destruction of the object. In this study, we demonstrate the feasibility of an innovative treatment based on bacterial iron reduction and biogenic mineral formation to stabilize the corrosion layer and protect these objects.

A comparison of Fe(III) reduction rates between fresh and aged biogenic iron oxides (BIOS) by Shewanella putrefaciens CN32

This study compared the chemical and physical properties of freshwater biogenic iron oxides (BIOS) aged at 4°C for ~5years in the dark to simulate early diagenesis to previously published findings from fresh samples. In addition, the reactivity of aged BIOS in the presence of a well-characterized iron reducing bacterium, Shewanella putrefaciens CN32, was investigated. The composition and proportion of the iron minerals were examined using XRD, Fe k-edge EXAFS, and chemical extractions, and results were similar in both fresh and aged BIOS. Aged BIOS were dominated by 2-line ferrihydrite indicating that the intermixed bacterial cells and their exudates inhibited the transformation of 2-line ferrihydrite into crystalline phases. However, despite the similarity of mineralogy between aged and fresh BIOS, the rates of bacterial iron reduction in aged BIOS were found to be significantly lower than that of their fresh counterparts. Further characterization of aged BIOS by TEM revealed unique morphologies which were not previously observed in fresh BIOS. TEM also showed thickly coated bacterial exudates and presumably those formed as a result of continued accretion or aggregation of iron oxide nanoparticles. Aggregation of the iron oxide particles and subsequent decrease in solubility are likely responsible for the low rates of bacterial iron reduction in aged BIOS. Phase stability and aggregation in BIOS have important implications for the long-term reactivity of BIOS and the preservation of organic matter in the environment.

Microbial sulfur metabolism evidenced from pore fluid isotope geochemistry at Site U1385

At Site U1385, drilled during IODP Expedition 339 off the coast of Portugal on the continental slope, high-resolution sulfate concentration measurements in the pore fluids display non-steady-state behavior. At this site there is a zone of sulfate reduction in the uppermost seven meters of sediment, followed by a 38-meter interval where sulfate concentrations do not change, and finally sulfate concentrations are depleted to zero between 45 and 55meters below seafloor. Below the sulfate minimum zone, there is abundant methane, suggesting that the lower sulfate consumption zone is coupled to anaerobic methane oxidation. We analyze pore water samples from IODP Site U1385 for sulfur and oxygen isotope ratios of dissolved sulfate, as well as the sulfur isotope composition of sedimentary pyrite. The sulfur isotopes in pore fluid sulfate display similar non-steady-state behavior similar to that of the sulfate concentrations, increasing over the uppermost zone of sulfate reduction and again over the lower zone of sulfate-driven anaerobic methane oxidation. The oxygen isotopes in sulfate increase to the ‘apparent equilibrium’ value in the uppermost zone of sulfate reduction and do not change further. Our calculations support the idea that sulfite to sulfide reduction is the limiting step in microbial sulfate reduction, and that the isotope fractionation expressed in the residual pore water sulfate pool is inversely proportional to the net sulfate reduction rate. The sulfur isotope composition of pyrite acquires one value in the uppermost sediments, which may be overprinted by a second value in the deeper sediments, possibly due to iron release during anaerobic methane oxidation or iron diffusion from a higher zone of bacterial iron reduction. Our results have implications for modeling the sulfur isotope composition of the pyrite burial flux in the global biogeochemical sulfur cycle.

The geochemistry of the 2.75 Ga-old Helen Iron Formation, Wawa, Ontario – Insights into iron formation deposition from carbon isotopes and rare earth elements

A high resolution stratigraphically controlled rare earth element and carbon isotope study of the 2.75 Ga old Algoma-type Helen Iron Formation has provided new insights into the chemistry of late Archean seawater. The Helen Iron Formation passes from a sediment dominated by rhyolitic detrital material in the first 20m of the section into a purely chemical sediment reflecting the chemistry of the seawater from which it precipitated. An overprint of a hydrothermal signature, seen in the development of a positive europium anomaly, increasing in magnitude upsection is in good accordance with field observations indicating a deepening of the Helen basin through time. The lack of a cerium anomaly along with the predominance of Fe2+ over Fe3+ indicates that oxygen played a minor role in the deposition of the Helen Iron Formation.Carbon isotope data are unusual for iron formation as they cluster around 0‰ (VPDB) and indicate direct precipitation from seawater. However, highly negative carbon isotope excursions suggest two episodes of dissimilatory iron reduction in sediments rich in organic matter, the negative carbon isotope signatures being inherited from the oxidation of this organic matter. These excursions have high magnitudes of 3 and 10.5‰, respectively.These results indicate that the Helen Iron Formation was deposited in a restricted basin with high input of Fe2+ from distal hydrothermal sources and high levels of CO2, explained by submarine volcanic activity in the Helen Basin. We suggest that CO2 and Fe oversaturation led to the direct precipitation of siderite and minor ankerite and that the negative carbon isotope excursions observed are the result of the oxidation of organic matter previously produced by bacterial dissimilatory iron reduction. The Helen Iron Formation thus is a direct precipitate from seawater in a basin increasing in water depth from moderately deep to deep water and presents a significant local hiatus in submarine volcanism.

Mercury mobilization and speciation linked to bacterial iron oxide and sulfate reduction: A column study to mimic reactive transfer in an anoxic aquifer

Mercury (Hg) mobility and speciation in subsurface aquifers is directly linked to its surrounding geochemical and microbial environment. The role of bacteria on Hg speciation (i.e., methylation, demethylation and reduction) is well documented, however little data is available on their impact on Hg mobility.The aim of this study was to test if (i) Hg mobility is due to either direct iron oxide reduction by iron reducing bacteria (IRB) or indirect iron reduction by sulfide produced by sulfate reducing bacteria (SRB), and (ii) to investigate its subsequent fate and speciation.Experiments were carried out in an original column setup combining geochemical and microbiological approaches that mimic an aquifer including an interface of iron-rich and iron depleted zones. Two identical glass columns containing iron oxides spiked with Hg(II) were submitted to (i) direct iron reduction by IRB and (ii) to indirect iron reduction by sulfides produced by SRB. Results show that in both columns Hg was leached and methylated during the height of bacterial activity. In the column where IRB are dominant, Hg methylation and leaching from the column was directly correlated to bacterial iron reduction (i.e., FeII release). In opposition, when SRB are dominant, produced sulfide induced indirect iron oxide reduction and rapid adsorption of leached Hg (or produced methylmercury) on neoformed iron sulfides (e.g., Mackinawite) or its precipitation as HgS. At the end of the SRB column experiment, when iron-oxide reduction was complete, filtered Hg and Fe concentrations increased at the outlet suggesting a leaching of Hg bound to FeS colloids that may be a dominant mechanism of Hg transport in aquifer environments.These experimental results highlight different biogeochemical mechanisms that can occur in stratified sub-surface aquifers where bacterial activities play a major role on Hg mobility and changes in speciation.

Sulfur-mediated electron shuttling during bacterial iron reduction.

Microbial reduction of ferric iron [Fe(III)] is an important biogeochemical process in anoxic aquifers. Depending on groundwater pH, dissimilatory metal-reducing bacteria can also respire alternative electron acceptors to survive, including elemental sulfur (S(0)). To understand the interplay of Fe/S cycling under alkaline conditions, we combined thermodynamic geochemical modeling with bioreactor experiments using Shewanella oneidensis MR-1. Under these conditions, S. oneidensis can enzymatically reduce S(0) but not goethite (α-FeOOH). The HS(-) produced subsequently reduces goethite abiotically. Because of the prevalence of alkaline conditions in many aquifers, Fe(III) reduction may thus proceed via S(0)-mediated electron-shuttling pathways.

Phosphorus sources for phosphatic Cambrian carbonates

The fossilization of organic remains and shell material by calcium phosphate minerals provides an illuminating, but time-bounded, window into Ediacaran–Cambrian animal evolution. For reasons that remain unknown, phosphatic fossil preservation declined significantly through Cambrian Series 2. Here, we investigate the phosphorus (P) sources for phosphatic Cambrian carbonates, presenting sedimentological, petrographic, and geochemical data from the Cambrian Series 2–3 Thorntonia Limestone, Australia, some of the youngest Cambrian strata to display exceptional phosphatic preservation of small shelly fossils. We find that within Thorntonia sediments, phosphate was remobilized by organic decay and bacterial iron reduction, with subsequent reprecipitation largely as apatite within the interiors of small shelly fossils. We discuss the merits of bioclastic-derived, organic matter–bound, or iron-bound P as potential sources to these strata. Petrographic observations suggest that the dissolution of phosphatic skeletal material did not provide the P for fossil preservation. In contrast, high organic carbon contents imply significant organic fluxes of P to Thorntonia sediments. Sedimentology and iron-speciation data indicate that phosphorus enrichment occurred during times of expanded anoxic, ferruginous conditions in subsurface water masses, suggesting that phosphorus adsorption to iron minerals precipitating from the water column provided a second significant P source to Thorntonia sediments. Simple stoichiometric models suggest that, by themselves, neither organic carbon burial nor an iron shuttle can account for the observed phosphorus enrichment. Thus, we infer that both processes were necessary for the observed phosphorus enrichment and subsequent fossil preservation in the Thorntonia Limestone.

Iron reduction and mineralization of deep‐sea iron reducing bacterium Shewanella piezotolerans WP3 at elevated hydrostatic pressures

In this study, iron reduction and concomitant biomineralization of a deep-sea iron reducing bacterium (IRB), Shewanella piezotolerans WP3, were systematically examined at different hydrostatic pressures (0.1, 5, 20, and 50MPa). Our results indicate that bacterial iron reduction and induced biomineralization are influenced by hydrostatic pressure. Specifically, the iron reduction rate and extent consistently decreases with the increase in hydrostatic pressure. By extrapolation, the iron reduction rate should drop to zero by ~68MPa, which suggests a possible shut-off of enzymatic iron reduction of WP3 at this pressure. Nano-sized superparamagnetic magnetite minerals are formed under all the experimental pressures; nevertheless, even as magnetite production decreases, the crystallinity and grain size of magnetite minerals increase at higher pressure. These results imply that IRB may play an important role in iron reduction, biomineralization, and biogeochemical cycling in deep-sea environments.