Oxidation and Reduction of Iron by Acidophilic Bacteria

Abstract

Redox reactions of iron in acidic environments are of economic and environmental significance, for example, for the leaching of metal ores and for the formation of acid mine drainage and acid sulfate soils. Until recently, research on microbial iron metabolism in acidic environments has mainly been focused on the role of aerobic, autotrophic ferrous iron‐oxidizing bacteria. In the present paper, recent new developments in the field of acidophilic iron metabolism are reviewed. In addition to the well‐known autotrophic ferrous iron‐oxidizing organisms, new heterotrophic isolates have been described that are capable of oxidizing ferrous iron. Microorganisms can also play an important role in the reductive part of the iron cycle. Both heterotrophic and autotrophic organisms may also be involved in this process. The contribution of heterotrophic organisms to acidophilic iron cycling can be twofold: In addition to their direct role as a catalyst, these organisms may scavenge organic compounds that inhibit their autotrophic counterparts. Detailed studies of acidophilic ecosystems are needed to assess the significance of the various types of microorganisms for the overall rate of iron cycling in these extreme environments.     

Promotion and nucleation of carbonate precipitation during microbial iron reduction

Iron‐bearing early diagenetic carbonate cements are common in sedimentary rocks, where they are thought to be associated with microbial iron reduction. However, little is yet known about how local environments around actively iron‐reducing cells affect carbonate mineral precipitation rates and compositions. Precipitation experiments with the iron‐reducing bacterium Shewanella oneidensis MR‐1 were conducted to examine the potential role of cells in promoting precipitation and to explore the possible range of precipitate compositions generated in varying fluid compositions. Actively iron‐reducing cells induced increased carbonate mineral saturation and nucleated precipitation on their poles. However, precipitation only occurred when calcium was present in solution, suggesting that cell surfaces lowered local ferrous iron concentrations by adsorption or intracellular iron oxide precipitation even as they locally raised pH. Resultant precipitates were a range of thermodynamically unstable calcium‐rich siderites that would likely act as precursors to siderite, calcite, or even dolomite in nature. By modifying local pH, providing nucleation sites, and altering metal ion concentrations around cell surfaces, iron‐reducing micro‐organisms could produce a wide range of carbonate cements in natural sediments.     

The role of microaerophilic Fe‐oxidizing micro‐organisms in producing banded iron formations

Despite the historical and economic significance of banded iron formations (BIFs), we have yet to resolve the formation mechanisms. On modern Earth, neutrophilic microaerophilic Fe‐oxidizing micro‐organisms (FeOM) produce copious amounts of Fe oxyhydroxides, leading us to wonder whether similar organisms played a role in producing BIFs. To evaluate this, we review the current knowledge of modern microaerophilic FeOM in the context of BIF paleoenvironmental studies. In modern environments wherever Fe(II) and O₂ co‐exist, microaerophilic FeOM proliferate. These organisms grow in a variety of environments, including the marine water column redoxcline, which is where BIF precursor minerals likely formed. FeOM can grow across a range of O₂ concentrations, measured as low as 2 μm to date, although lower concentrations have not been tested. While some extant FeOM can tolerate high O₂ concentrations, many FeOM appear to prefer and thrive at low O₂ concentrations (~3–25 μm ). These are similar to the estimated dissolved O₂ concentrations in the few hundred million years prior to the ‘Great Oxidation Event’ (GOE). We compare biotic and abiotic Fe oxidation kinetics in the presence of varying levels of O₂ and show that microaerophilic FeOM contribute substantially to Fe oxidation, at rates fast enough to account for BIF deposition. Based on this synthesis, we propose that microaerophilic FeOM were capable of playing a significant role in depositing the largest, most well‐known BIFs associated with the GOE, as well as afterward when global O₂ levels increased.     

Respiratory components in acidophilic bacteria that respire on iron

Abstract

Microorganisms capable of aerobic respiration on ferrous ions are spread throughout eubacterial and archaebacterial phyla. Phylogenetically distinct organisms were shown to express spectrally distinct redox‐active biomolecules during autotrophic growth on soluble iron. A new iron‐oxidizing eubacterium, designated as strain Funis, was investigated. Strain Funis was judged to be different from other known iron‐oxidizing bacteria on the bases of comparative lipid analyses, 16S rRNA sequence analyses, and cytochrome composition studies. When grown autotrophically on ferrous ions, Funis produced conspicuous levels of a novel acid‐stable, acid‐soluble yellow cytochrome with a distinctive absorbance peak at 579 nm in the reduced state.

Stopped‐flow spectrophotometric kinetic studies were conducted on respiratory chain components isolated from cell‐free extracts of Thiobacillus ferrooxidans. Experimental results were consistent with a model where the primary oxidant of ferrous ions is a highly aggregated c‐type cytochrome that then reduces the periplasmic rusticyanin. The Fe(II)‐dependent, cytochrome c‐catalyzed reduction of the rusticyanin possessed three kinetic properties in common with corresponding intact cells that respire on iron: the same anion specificity, a similar dependence of the rate on the concentration of ferrous ions, and similar rates at saturating concentrations of ferrous ions     

Phosphogenesis in the 2460 and 2728 million‐year‐old banded iron formations as evidence for biological cycling of phosphate in the early biosphere

The banded iron formation deposited during the first 2 billion years of Earth's history holds the key to understanding the interplay between the geosphere and the early biosphere at large geological timescales. The earliest ore‐scale phosphorite depositions formed almost at ~2.0–2.2 billion years ago bear evidence for the earliest bloom of aerobic life. The cycling of nutrient phosphorus and how it constrained primary productivity in the anaerobic world of Archean–Palaeoproterozoic eons are still open questions. The controversy centers about whether the precipitation of ultrafine ferric oxyhydroxide due to the microbial Fe(II) oxidation in oceans earlier than 1.9 billion years substantially sequestrated phosphate, and whether this process significantly limited the primary productivity of the early biosphere. In this study, we report apatite radial flowers of a few micrometers in the 2728 million‐year‐old Abitibi banded iron formation and the 2460 million‐year‐old Kuruman banded iron formation and their similarities to those in the 535 million‐year‐old Lower Cambrian phosphorite. The lithology of the 535 Million‐year‐old phosphorite as a biosignature bears abundant biomarkers that reveal the possible similar biogeochemical cycling of phosphorus in the Later Archean and Palaeoproterozoic oceans. These apatite radial flowers represent the primary precipitation of phosphate derived from the phytoplankton blooms in the euphotic zones of Neoarchean and Palaoeproterozoic oceans. The unbiased distributions of the apatite radial flowers within sub‐millimeter bands do not support the idea of an Archean Crisis of Phosphate. This is the first report of the microbial mediated mineralization of phosphorus before the Great Oxidation Event when the whole biosphere was still dominated by anaerobic microorganisms.     

Planktonic marine iron oxidizers drive iron mineralization under low‐oxygen conditions

Observations of modern microbes have led to several hypotheses on how microbes precipitated the extensive iron formations in the geologic record, but we have yet to resolve the exact microbial contributions. An initial hypothesis was that cyanobacteria produced oxygen which oxidized iron abiotically; however, in modern environments such as microbial mats, where Fe(II) and O₂ coexist, we commonly find microaerophilic chemolithotrophic iron‐oxidizing bacteria producing Fe(III) oxyhydroxides. This suggests that such iron oxidizers could have inhabited niches in ancient coastal oceans where Fe(II) and O₂ coexisted, and therefore contributed to banded iron formations (BIFs) and other ferruginous deposits. However, there is currently little evidence for planktonic marine iron oxidizers in modern analogs. Here, we demonstrate successful cultivation of planktonic microaerophilic iron‐oxidizing Zetaproteobacteria from the Chesapeake Bay during seasonal stratification. Iron oxidizers were associated with low oxygen concentrations and active iron redox cycling in the oxic–anoxic transition zone (<3 μm O₂, <0.2 μm H₂S). While cyanobacteria were also detected in this transition zone, oxygen concentrations were too low to support significant rates of abiotic iron oxidation. Cyanobacteria may be providing oxygen for microaerophilic iron oxidation through a symbiotic relationship; at high Fe(II) levels, cyanobacteria would gain protection against Fe(II) toxicity. A Zetaproteobacteria isolate from this site oxidized iron at rates sufficient to account for deposition of geologic iron formations. In sum, our results suggest that once oxygenic photosynthesis evolved, microaerophilic chemolithotrophic iron oxidizers were likely important drivers of iron mineralization in ancient oceans.     

Biogenicity of an Early Quaternary iron formation,Milos Island,Greece

A ~2.0‐million‐year‐old shallow‐submarine sedimentary deposit on Milos Island, Greece, harbours an unmetamorphosed fossiliferous iron formation (IF) comparable to Precambrian banded iron formations (BIFs). This Milos IF holds the potential to provide clues to the origin of Precambrian BIFs, relative to biotic and abiotic processes. Here, we combine field stratigraphic observations, stable isotopes of C, S and Si, rock petrography and microfossil evidence from a ~5‐m‐thick outcrop to track potential biogeochemical processes that may have contributed to the formation of the BIF‐type rocks and the abrupt transition to an overlying conglomerate‐hosted IF (CIF). Bulk δ¹³C isotopic compositions lower than ‐25‰ provide evidence for biological contribution by the Calvin and reductive acetyl–CoA carbon fixation cycles to the origin of both the BIF‐type and CIF strata. Low S levels of ~0.04 wt.% combined with δ³⁴S estimates of up to ~18‰ point to a non‐sulphidic depository. Positive δ³⁰Si records of up to +0.53‰ in the finely laminated BIF‐type rocks indicate chemical deposition on the seafloor during weak periods of arc magmatism. Negative δ³⁰Si data are consistent with geological observations suggesting a sudden change to intense arc volcanism potentially terminated the deposition of the BIF‐type layer. The typical Precambrian rhythmic rocks of alternating Fe‐ and Si‐rich bands are associated with abundant and spatially distinct microbial fossil assemblages. Together with previously proposed anoxygenic photoferrotrophic iron cycling and low sedimentary N and C potentially connected to diagenetic denitrification, the Milos IF is a biogenic submarine volcano‐sedimentary IF showing depositional conditions analogous to Archaean Algoma‐type BIFs.     

Dissolved silica affects the bulk iron redox state and recrystallization of minerals generated by photoferrotrophy in a simulated Archean ocean

Chemical sedimentary deposits called Banded Iron Formations (BIFs) are one of the best surviving records of ancient marine (bio)geochemistry. Many BIF precursor sediments precipitated from ferruginous, silica-rich waters prior to the Great Oxidation Event at ~2.43 Ga. Reconstructing the mineralogy of BIF precursor phases is key to understanding the coevolution of seawater chemistry and early life. Many models of BIF deposition invoke the activity of Fe(II)-oxidizing photoautotrophic bacteria as a mechanism for precipitating mixed-valence Fe(II,III) and/or fully oxidized Fe(III) minerals in the absence of molecular oxygen. Although the identity of phases produced by ancient photoferrotrophs remains debated, laboratory experiments provide a means to explore what their mineral byproducts might have been. Few studies have thoroughly characterized precipitates produced by photoferrotrophs in settings representative of Archean oceans, including investigating how residual Fe(II)_aq can affect the mineralogy of expected solid phases. The concentration of dissolved silica (Si) is also an important variable to consider, as silicate species may influence the identity and reactivity of Fe(III)-bearing phases. To address these uncertainties, we cultured Rhodopseudomonas palustris TIE-1 as a photoferrotroph in synthetic Archean seawater with an initial [Fe(II)_aq] of 1 mM and [Si] spanning 0–1.5 mM. Ferrihydrite was the dominant precipitate across all Si concentrations, even with substantial Fe(II) remaining in solution. Consistent with other studies of microbial iron oxidation, no Fe-silicates were observed across the silica gradient, although Si coprecipitated with ferrihydrite via surface adsorption. More crystalline phases such as lepidocrocite and goethite were only detected at low [Si] and are likely products of Fe(II)-catalyzed ferrihydrite transformation. Finally, we observed a substantial fraction of Fe(II) in precipitates, with the proportion of Fe(II) increasing as a function of [Si]. These experimental results suggest that photoferrotrophy in a Fe(II)-buffered ocean may have exported Fe(II,III)-oxide/silica admixtures to BIF sediments, providing a more chemically diverse substrate than previously hypothesized.     

The coupling of biological iron cycling and mineral weathering during saprolite formation, Luquillo Mountains, Puerto Rico

Corestones of quartz diorite bedrock in the Rio Icacos watershed in Puerto Rico weather spheroidally to form concentric sets of partially weathered rock layers (referred to here as rindlets) that slowly transform to saprolite. The rindlet zone (0.2–2 m thick) is overlain by saprolite (2–8 m) topped by soil (0.5–1 m). With the objective of understanding interactions between weathering, substrate availability, and resident micro‐organisms, we made geochemical and microbiological measurements as a function of depth in 5 m of regolith (soil + saprolite). We employed direct microscopic counting of total cell densities; enumeration of culturable aerobic heterotrophs; extraction of microbial DNA for yield calculations; and biochemical tests for iron‐oxidizing bacteria. Total cell densities, which ranged from 2.5 × 10⁶ to 1.6 × 10¹⁰ g^?1 regolith, were higher than 10⁸ g^?1 at three depths: in the upper 1 m, at 2.1 m, and between 3.7 and 4.9 m, just above the rindlet zone. High proportions of inactive or unculturable cells were indicated throughout the profile by very low percentages of culturable heterotrophs (0.0004% to 0.02% of total cell densities). The observed increases in total and culturable cells and DNA yields at lower depths were not correlated with organic carbon or total iron but were correlated with moisture and HCl‐extractable iron. Biochemical tests for aerobic iron‐oxidizers were also positive at 0.15–0.6 m, at 2.1–2.4 m, and at 4.9 m depths. To interpret microbial populations within the context of weathering reactions, we developed a model for estimating growth rates of lithoautotrophs and heterotrophs based on measured substrate fluxes. The calculations and observations are consistent with a model wherein electron donor flux driving bacterial growth at the saprolite–bedrock interface is dominated by Fe(II) and where autotrophic iron‐oxidizing bacteria support the heterotrophic population and contribute to bedrock disaggregation and saprolite formation.     

Effects of ferrous iron on the performance and microbial community in aerobic granular sludge in relation to nutrient removal

Lab‐scale experiments were conducted to investigate the effects of ferrous iron on nutrient removal performance and variations in the microbial community inside aerobic granular sludge for 408 days. Two reactors were simultaneously operated, one without added ferrous iron (SBR1), and one with 10 mg Fe²⁺ L^?1 of added ferrous iron (SBR2). A total of 1 mg Fe²⁺ L^?1 of added ferrous iron was applied to SBR1 starting from the 191st day to observe the resulting variations in the nutrient removal performance and the microbial community. The results show that ammonia‐oxidizing bacteria (AOB) could not oxidize ammonia due to a lack of iron compounds, but they could survive in the aerobic granular sludge. Limited ferrous iron addition encouraged nitrification. Enhanced biological phosphorus removal (EBPR) from both reactors could not be maintained regardless of the amount of ferrous iron that was applied. EBPR was established in both reactors when the concentration of mixed liquor suspended solid (MLSS) and the percentage of Accumulibacteria increased. A total of 10 mg Fe²⁺ L^?1 of added ferrous iron had a relatively adverse effect on the growth of AOB species compared to 1 mg Fe²⁺ L^?1 of added ferrous iron, but it encouraged the growth of Nitrospira sp. and Accumulibacteria, which requires further study. It could be said that the compact and stable structure of aerobic granular sludge preserved AOB and NOB from Fe‐deficient conditions, and wash‐out during the disintegration period. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:716–725, 2017     

Organic matter mineralization with the reduction of ferric iron: A review

A review of the literature indicates that numerous microorganisms can reduce ferric iron during the metabolism of organic matter. In most cases, the reduction of ferric iron appears to be enzymatically catalyzed and, in some instances, may be coupled to an electron transport chain that could generate ATP. However, the physiology and biochemistry of ferric iron reduction are poorly understood. In pure culture, ferric iron‐reducing organisms metabolize fermentable substrates, such as glucose, primarily to typical fermentation products, and transfer only a minor portion of the electron equivalents in the fermentable substrates to ferric iron. However, fermentation products, especially hydrogen and acetate, may be important electron donors for ferric iron reduction in natural environments. The ability of some organisms to couple the oxidation of fermentation products to the reduction of ferric iron means that it is possible for a food chain of iron‐reducing organisms to completely mineralize nonrecalcitrant organic matter with ferric iron as the sole electron acceptor. The rate and extent of ferric iron reduction depend on the forms of ferric iron that are available. Most of the ferric iron in sediments is resistant to microbial reduction. Ferric iron‐reducing organisms can exclude sulfate reduction and methane production from the zone of ferric iron reduction in sediments by outcompeting sulfate‐reducing and methanogenic food chains for organic matter when ferric iron is available as amorphic ferric oxyhydroxide. There are few quantitative estimates of the rates of ferric iron reduction in natural environments, but there is evidence that ferric iron reduction can be an important pathway for organic matter decomposition in some environments. There is a strong need for further study on all aspects of microbial reduction of ferric iron.     

Oncoidal granular iron formation in the Mesoarchaean Pongola Supergroup,southern Africa: Textural and geochemical evidence for biological activity during iron deposition

We document the discovery of the first granular iron formation (GIF) of Archaean age and present textural and geochemical results that suggest these formed through microbial iron oxidation. The GIF occurs in the Nconga Formation of the ca. 3.0–2.8 Ga Pongola Supergroup in South Africa and Swaziland. It is interbedded with oxide and silicate facies micritic iron formation (MIF). There is a strong textural control on iron mineralization in the GIF not observed in the associated MIF. The GIF is marked by oncoids with chert cores surrounded by magnetite and calcite rims. These rims show laminated domal textures, similar in appearance to microstromatolites. The GIF is enriched in silica and depleted in Fe relative to the interbedded MIF. Very low Al and trace element contents in the GIF indicate that chemically precipitated chert was reworked above wave base into granules in an environment devoid of siliciclastic input. Microbially mediated iron precipitation resulted in the formation of irregular, domal rims around the chert granules. During storm surges, oncoids were transported and deposited in deeper water environments. Textural features, along with positive δ⁵⁶Fe values in magnetite, suggest that iron precipitation occurred through incomplete oxidation of hydrothermal Fe²⁺ by iron‐oxidizing bacteria. The initial Fe³⁺‐oxyhydroxide precipitates were then post‐depositionally transformed to magnetite. Comparison of the Fe isotope compositions of the oncoidal GIF with those reported for the interbedded deeper water iron formation (IF) illustrates that the Fe²⁺ pathways and sources for these units were distinct. It is suggested that the deeper water IF was deposited from the evolved margin of a buoyant Fe²⁺_aq‐rich hydrothermal plume distal to its source. In contrast, oncolitic magnetite rims of chert granules were sourced from ambient Fe²⁺_aq‐depleted shallow ocean water beyond the plume.     

High-resolution 2D and 3D cryo-TEM reveals structural adaptations of two stalk-forming bacteria to an Fe-oxidizing lifestyle

Aerobic neutrophilic Fe‐oxidizing bacteria (FeOB) thrive where oxic and iron‐rich anoxic waters meet. Here, iron microbial mats are commonly developed by stalk‐forming Fe‐oxidizers adapted to these iron‐rich gradient environments, somehow avoiding iron encrustation. Few details are known about FeOB physiology; thus, the bases of these adaptations, notably the mechanisms of interactions with iron, are poorly understood. We examined two stalked FeOB: the marine Zetaproteobacterium Mariprofundus ferrooxydans and a terrestrial Betaproteobacterium Gallionella‐like organism. We used cryo‐transmission electron microscopy and cryo‐electron tomography to provide unprecedented ultrastructural data on intact cell‐mineral systems. Both FeOB localize iron mineral formation at stalk extrusion sites, while avoiding surface and periplasmic mineralization. The M. ferrooxydans cell surface is densely covered in fibrils while the terrestrial FeOB surface is smooth, suggesting a difference in surface chemistry. Only the terrestrial FeOB exhibited a putative chemotaxis apparatus, which may be due to differences in chemotaxis mechanisms. Both FeOB have a single flagellum, which alone is insufficient to account for cell motion during iron oxidation, suggesting that stalk extrusion is a mechanism for motility. Our results delineate the physical framework of iron transformations and characterize possible structural adaptations to the iron‐oxidizing lifestyle. This study shows ultrastructural similarities and differences between two distinct FeOB, setting the stage for further (e.g. genomic) comparisons that will help us understand functional differences and evolutionary history.     

Atmospheric hydrogen peroxide and Eoarchean iron formations

It is widely accepted that photosynthetic bacteria played a crucial role in Fe(II) oxidation and the precipitation of iron formations (IF) during the Late Archean–Early Paleoproterozoic (2.7–2.4 Ga). It is less clear whether microbes similarly caused the deposition of the oldest IF at ca. 3.8 Ga, which would imply photosynthesis having already evolved by that time. Abiological alternatives, such as the direct oxidation of dissolved Fe(II) by ultraviolet radiation may have occurred, but its importance has been discounted in environments where the injection of high concentrations of dissolved iron directly into the photic zone led to chemical precipitation reactions that overwhelmed photooxidation rates. However, an outstanding possibility remains with respect to photochemical reactions occurring in the atmosphere that might generate hydrogen peroxide (H₂O₂), a recognized strong oxidant for ferrous iron. Here, we modeled the amount of H₂O₂ that could be produced in an Eoarchean atmosphere using updated solar fluxes and plausible CO₂, O₂, and CH₄ mixing ratios. Irrespective of the atmospheric simulations, the upper limit of H₂O₂ rainout was calculated to be <10⁶ molecules cm^?2 s^?1. Using conservative Fe(III) sedimentation rates predicted for submarine hydrothermal settings in the Eoarchean, we demonstrate that the flux of H₂O₂ was insufficient by several orders of magnitude to account for IF deposition (requiring ~10¹¹ H₂O₂ molecules cm^?2 s^?1). This finding further constrains the plausible Fe(II) oxidation mechanisms in Eoarchean seawater, leaving, in our opinion, anoxygenic phototrophic Fe(II)‐oxidizing micro‐organisms the most likely mechanism responsible for Earth's oldest IF.     

The potential significance of microbial Fe(III) reduction during deposition of Precambrian banded iron formations

During deposition of late Archean–early Palaeoproterozoic Precambrian banded iron formations (BIFs) the downward flux of ferric hydroxide (Fe(OH)₃) and phytoplankton biomass should have facilitated microbial Fe(III) reduction. However, quantifying the significance of such a metabolic pathway in the Precambrian is extremely difficult, considering the post‐depositional alteration of the rocks and the lack of ideal modern analogues. Consequently, we have very few constraints on the Fe cycle at that time, namely (i) the concentration of dissolved Fe(II) in the ocean waters; (ii) by what mechanisms Fe(II) was oxidized (chemical, photochemical or biological, the latter using either O₂ or light); (iii) where the ferric hydroxide was precipitated (over the shelf vs. open ocean); (iv) the amount of phytoplankton biomass, which relates to the nutrient status of the surface waters; (v) the relative importance of Fe(III) reduction vs. the other types of metabolic pathways utilized by sea floor microbial communities; and (vi) the proportion of primary vs. diagenetic Fe(II) in BIF. Furthermore, although estimates can be made regarding the quantity of reducing equivalents necessary to account for the diagenetic Fe(II) component in Fe‐rich BIF layers, those same estimates do not offer any insights into the magnitude of Fe(III) actually generated within the water column, and hence, the efficiency of Fe and C recycling prior to burial. Accordingly, in this study, we have attempted to model the ancient Fe cycle, based simply on conservative experimental rates of photosynthetic Fe(II) oxidation in the euphotic zone. We estimate here that under ideal growth conditions, as much as 70% of the biologically formed Fe(III) could have been recycled back into the water column via fermentation and organic carbon oxidation coupled to microbial Fe(III) reduction. By comparing the potential amount of biomass generated phototrophically with the reducing equivalents required for Fe(III) reduction and magnetite formation, we also hypothesize that another anaerobic metabolic pathway might have been utilized in the surface sediment to oxidize the fermentation by‐products. Based on the premise that the deep ocean waters were anoxic, this role could have been fulfilled by methanogens, and maybe even methanotrophs that employed Fe(III) reduction.     

The microbially driven formation of siderite in salt marsh sediments

We employ complementary field and laboratory‐based incubation techniques to explore the geochemical environment where siderite concretions are actively forming and growing, including solid‐phase analysis of the sediment, concretion, and associated pore fluid chemistry. These recently formed siderite concretions allow us to explore the geochemical processes that lead to the formation of this less common carbonate mineral. We conclude that there are two phases of siderite concretion growth within the sediment, as there are distinct changes in the carbon isotopic composition and mineralogy across the concretions. Incubated sediment samples allow us to explore the stability of siderite over a range of geochemical conditions. Our incubation results suggest that the formation of siderite can be very rapid (about two weeks or within 400 hr) when there is a substantial source of iron, either from microbial iron reduction or from steel material; however, a source of dissolved iron is not enough to induce siderite precipitation. We suggest that sufficient alkalinity is the limiting factor for siderite precipitation during microbial iron reduction while the lack of dissolved iron is the limiting factor for siderite formation if microbial sulfate reduction is the dominant microbial metabolism. We show that siderite can form via heated transformation (at temperature 100°C for 48 hr) of calcite and monohydrocalcite seeds in the presence of dissolved iron. Our transformation experiments suggest that the formation of siderite is promoted when carbonate seeds are present.     

Morphology of biogenic iron oxides records microbial physiology and environmental conditions: toward interpreting iron microfossils

Despite the abundance of Fe and its significance in Earth history, there are no established robust biosignatures for Fe(II)‐oxidizing micro‐organisms. This limits our ability to piece together the history of Fe biogeochemical cycling and, in particular, to determine whether Fe(II)‐oxidizers played a role in depositing ancient iron formations. A promising candidate for Fe(II)‐oxidizer biosignatures is the distinctive morphology and texture of extracellular Fe(III)‐oxyhydroxide stalks produced by mat‐forming microaerophilic Fe(II)‐oxidizing micro‐organisms. To establish the stalk morphology as a biosignature, morphologic parameters must be quantified and linked to the microaerophilic Fe(II)‐oxidizing metabolism and environmental conditions. Toward this end, we studied an extant model organism, the marine stalk‐forming Fe(II)‐oxidizing bacterium, Mariprofundus ferrooxydans PV‐1. We grew cultures in flat glass microslide chambers, with FeS substrate, creating opposing oxygen/Fe(II) concentration gradients. We used solid‐state voltammetric microelectrodes to measure chemical gradients in situ while using light microscopy to image microbial growth, motility, and mineral formation. In low‐oxygen (2.7–28 μm ) zones of redox gradients, the bacteria converge into a narrow (100 μm–1 mm) growth band. As cells oxidize Fe(II), they deposit Fe(III)‐oxyhydroxide stalks in this band; the stalks orient directionally, elongating toward higher oxygen concentrations. M. ferrooxydans stalks display a narrow range of widths and uniquely biogenic branching patterns, which result from cell division. Together with filament composition, these features (width, branching, and directional orientation) form a physical record unique to microaerophilic Fe(II)‐oxidizer physiology; therefore, stalk morphology is a biosignature, as well as an indicator of local oxygen concentration at the time of formation. Observations of filamentous Fe(III)‐oxyhydroxide microfossils from a ~170 Ma marine Fe‐Si hydrothermal deposit show that these morphological characteristics can be preserved in the microfossil record. This study demonstrates the potential of morphological biosignatures to reveal microbiology and environmental chemistry associated with geologic iron formation depositional processes.     

Bacterial diversity in Fe‐rich hydrothermal sediments at two South Tonga Arc submarine volcanoes

Seafloor iron oxide deposits are a common feature of submarine hydrothermal systems. Morphological study of these deposits has led investigators to suggest a microbiological role in their formation, through the oxidation of reduced Fe in hydrothermal fluids. Fe‐oxidizing bacteria, including the recently described Zetaproteobacteria, have been isolated from a few of these deposits but generally little is known about the microbial diversity associated with this habitat. In this study, we characterized bacterial diversity in two Fe oxide samples collected on the seafloor of Volcanoes 1 and 19 on the South Tonga Arc. We were particularly interested in confirming the presence of Zetaproteobacteria at these two sites and in documenting the diversity of groups other than Fe oxidizers. Our results (small subunit rRNA gene sequence data) showed a surprisingly high bacterial diversity, with 150 operational taxonomic units belonging to 19 distinct taxonomic groups. Both samples were dominated by Zetaproteobacteria Fe oxidizers. This group was most abundant at Volcano 1, where sediments were richer in Fe and contained more crystalline forms of Fe oxides. Other groups of bacteria found at these two sites include known S‐ and a few N‐metabolizing bacteria, all ubiquitous in marine environments. The low similarity of our clones with the GenBank database suggests that new species and perhaps new families were recovered. The results of this study suggest that Fe‐rich hydrothermal sediments, while dominated by Fe oxidizers, can be exploited by a variety of autotrophic and heterotrophic micro‐organisms.     

Microbial acceleration of aerobic pyrite oxidation at circumneutral pH

Pyrite (FeS₂) is the most abundant sulfide mineral on Earth and represents a significant reservoir of reduced iron and sulfur both today and in the geologic past. In modern environments, oxidative transformations of pyrite and other metal sulfides play a key role in terrestrial element partitioning with broad impacts to contaminant mobility and the formation of acid mine drainage systems. Although the role of aerobic micro‐organisms in pyrite oxidation under acidic‐pH conditions is well known, to date there is very little known about the capacity for aerobic micro‐organisms to oxidize pyrite at circumneutral pH. Here, we describe two enrichment cultures, obtained from pyrite‐bearing subsurface sediments, that were capable of sustained cell growth linked to pyrite oxidation and sulfate generation at neutral pH. The cultures were dominated by two Rhizobiales species (Bradyrhizobium sp. and Mesorhizobium sp.) and a Ralstonia species. Shotgun metagenomic sequencing and genome reconstruction indicated the presence of Fe and S oxidation pathways in these organisms, and the presence of a complete Calvin–Benson–Bassham CO₂ fixation system in the Bradyrhizobium sp. Oxidation of pyrite resulted in thin (30–50 nm) coatings of amorphous Fe(III) oxide on the pyrite surface, with no other secondary Fe or S phases detected by electron microscopy or X‐ray absorption spectroscopy. Rates of microbial pyrite oxidation were approximately one order of magnitude higher than abiotic rates. These results demonstrate the ability of aerobic microbial activity to accelerate pyrite oxidation and expand the potential contribution of micro‐organisms to continental sulfide mineral weathering around the time of the Great Oxidation Event to include neutral‐pH environments. In addition, our findings have direct implications for the geochemistry of modern sedimentary environments, including stimulation of the early stages of acid mine drainage formation and mobilization of pyrite‐associated metals.