In Clear Creek, which runs through the Iowa State University campus in Ames, Iowa, USA, several types of iron mineralisation occur within stagnant pools and slow-moving water. This includes rusty flocs, commonly observed in mineral springs, rust-stained sediments and iridescent films (‘schwimmeisen’) on the pool surfaces. Observations of iron mineralisation over the course of more than a year in a single reach indicated that mineralisation occurred after precipitation events once water levels in the stream had dropped. Iron extracted and quantified from Clear Creek sediments and pool waters indicated the stream and its sediments were unlikely to be supplying the iron for mineralisation. We hypothesise that the observed mineralisation could result from the discharge of shallow, reducing groundwater-bearing Fe(II) into stagnant pools that form in debris-dammed areas of the stream. Piezometers installed next to the creek documented that shallow groundwater contained dissolved Fe, with the source of Fe being the floodplain sediments and the hydraulic gradient promoted groundwater discharge into the stream. Microorganisms identified in mineralised pools using 16S rRNA amplicon sequencing revealed an elevated presence of putative iron-oxidizing and iron-reducing microorganisms in mineralised vs. non-mineralised pools. Further investigation of the iridescent films revealed them to be composed of amorphous Fe(III) minerals. We further hypothesise that microbial exudates reduce surface tension and potential micro-zones for subsequent microbial iron redox cycling with dissolved organic matter in the pools. Determining the processes controlling mineralisation can lead to a better understanding of the ecological role of iron mineralisation in agricultural watersheds and the importance of contaminant degradation and nutrient cycling.

Members of the Shewanella genus transfer electrons to metal and actinide electron acceptors such as hexavalent uranium, U(VI), via c-type cytochromes. The intracellular mechanism of electron transfer is well studied but the delivery of electrons to external electron acceptors less well so. MtrC, a decaheme c-type cytochrome located on the cell surface side of the outer membrane of many Shewanella species, and extending to the extracellular medium, transfers electrons to U(VI), both in vivo and in vitro when purified. However, it is unclear how the electron transfer between the terminal heme(s) of the protein and extracellular U(VI) occurs. In particular, the type of interaction between MtrC and U(VI), and the parameters controlling electron transfer remain to be elucidated. Here, we investigated the kinetics of U(VI) reduction by S. baltica MtrC in solution for U(VI) complexed with one of five ligands: carbonate, hydroxyl, citrate, nitrilotriacetic acid (NTA) or ethylenediaminetetraacetic acid (EDTA). We observed two initial reaction rates, one more rapid for U-citrate, U-NTA and U-EDTA, and another slower for U-carbonate and U-hydroxo. By combining Nuclear Magnetic Resonance spectroscopy and M4-edge High Resolution X-ray Absorption Near Edge Structure spectroscopy, we attributed these differences to the type of interaction between the U-ligand complex and MtrC, i.e., probably electrostatic interaction with the ligand of U-EDTA, hydrogen bonding to the ligand of U-citrate and U-NTA, and covalent bonding with U-carbonate and U-hydroxo. We also demonstrate the persistence of U(V) in the U-carbonate system when interacting with MtrC. Overall, we showed that the mechanism of electron transfer depended on the chemistry of the soluble U(VI) complex serving as the substrate.

Coastal wetland sediments are vital to the global carbon cycle as they represent large sinks of blue carbon – carbon from atmospheric and oceanic sources – which are threatened by ecosystem loss. The forms of sequestered carbon and the sequestration capability are affected by many bio- and geochemical factors that change unpredictably along coastal locales. In the present study, we investigated three unique coastal sites – a coastal mangrove and two sabkhas with contrasting geology and tidal influence in the Qatar peninsula – for their carbon capture ability to determine how biogeochemical indices affect their blue carbon sequestration potential. We applied a suite of biological and geochemical tools, collecting the sediment cores of approximately 40 cm depth; analysed sediment porewater; performed depth-profiling of the organic matter, sedimentary minerals, microbial community and analysis of sediment surface for pH, oxygen (O2); redox potential and hydrogen sulfide (H2S) by microsensors. High-resolution transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM-EDXS) and scanning transmission X-ray microscopy (STXM) revealed templating effects that promoted Mg-carbonate nucleation in coastal hypersaline environments. Microsensing reveals the intricacy of the oxic/anoxic transition at the sediment surface. Microbial DNA sequencing at various sediment depths shows the occurrence of microbial genera, whose functions explain the geochemical trends and carbon sequestration pathways observed at each site. Notably, we found that carbon sequestration in the mangrove and carbonate-sand sabkha was correlated with organic matter degradation and inorganic carbon content, while in the siliciclastic sabkha it was solely influenced by sediment density and depth.

Concentrations of sedimentary molybdenum (Mo) have been used as a proxy for palaeoceanographic redox conditions based on the distinctive behaviour of Mo under oxic versus euxinic (i.e., anoxic and sulfidic) conditions. However, the mechanisms that govern Mo sequestration in various euxinic settings are not fully resolved. It has previously been proposed that sulfate-reducing bacteria (SRB), the main drivers and regulators of euxinic conditions, can actively take up and reduce Mo intracellularly and passively induce Fe-independent Mo complexation and reduction at their cell surfaces. However, uncertainties remain regarding the underlying interactions and relative contributions of these proposed biotic Mo sequestration pathways. In this study, systematic experiments were carried out to examine the interactions among Mo(VI) species (MoO42- or MoS42-), ferrous iron (Fe2+) and SRB with a focus on combinations of conditions that lead to reductive Mo precipitation. The speciation of aqueous Mo and composition, structure, oxidation states and bonding environment of precipitated Mo-sulfides were analysed using UV-vis spectrophotometry (UV-vis), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and synchrotron-based X-ray absorption spectroscopy (XAS). Results indicate that SRB does not directly reduce Mo but, rather, plays a passive role in mediating Mo sequestration by providing sulfide and potential nucleation sites at their reactive cell surfaces for precipitation. However, even in the presence of SRB cells, Fe2+ was required for Mo precipitation in all conditions tested. By identifying the limiting (and non-limiting) factors in the Mo reduction and sequestration process, this study provides significant new insights for interpreting Mo palaeoredox proxies.

Hydraulic fracturing generates large volumes of flowback and produced water, composed of complex mixtures of organic and inorganic constituents. The solids associated with these fluids are Fe-rich and can contain toxic organics, heavy metals and naturally occurring radioactive materials (NORMs). Despite this, only a few studies have analysed their composition and there is a lack of understanding about their interactions with microbial communities and their long-term fate in the environment. In this study, we analysed the solids associated with flowback water derived from a hydraulically fractured well in the Bowland Shale, UK. We also investigated the microbial reduction of these Fe(III)-rich materials under anaerobic conditions using anthraquinone-2.6-disulfonate (AQDS) as an electron shuttle and identified the resulting bioreduced mineral phases. XRD characterization indicated that the solids contained akaganeite (β-FeOOH, Cl) and Ba-bearing celestine (SrSO4). These Fe(III)-containing solids served as an electron acceptor for Shewanella frigidimarina and a flowback-derived Fe(III)-reducing enrichment culture. The bioreduced Fe(II)-bearing mineral phase was identified as ankerite [Ca(Fe,Mg,Mn)(CO3)2]; however, the presence of amorphous mineral phases is not ruled out. Microbial community composition was analysed using 16S rRNA gene sequencing. Amplicon sequence variants (ASVs) most closely related to Chromohalobacter, Caminicella and putative Fe(III)-reducing genera were dominant across treatments. Our findings highlight the potential of these Fe(III)-bearing sludges to be harnessed for the development of wastewater treatment strategies; for example, coupling the oxidation of toxic organics with Fe(III) reduction through either the introduction of microbial inocula or biostimulation of the native microbial communities. Furthermore, microbial processing can also be optimized to transform the Fe(III) sludges into denser materials, which are easier to handle and can immobilize toxic metals, thereby reducing the toxicity of this waste.

Iron sulfide (Fe-S) minerals such as mackinawite (FeS), greigite (Fe3S4) and pyrite (FeS2) are widespread on Earth, where their formation and dissolution are strongly linked to the biogeochemical cycles of iron, sulfur, carbon, oxygen, nutrients and trace metals. Recent studies have shed light on how microorganisms mediate their formation, with breakthroughs linked to biogenic pyrite. In this review, we highlight the formation pathways of Fe-S minerals, starting with the increasingly recognized roles of Fe(III) and intermediate sulfur species (e.g. S0 and polysulfides) during the initial steps. The mechanisms by which microorganisms affect Fe-S mineral formation are compiled and discussed for low (25–35°C) and high (≥ 80°C) temperatures, with specific examples from experimental studies. The morphology and precipitation rates obtained from experiments are compared to natural environments, and their similarities and differences are critically discussed. We then review the current state of the art for Fe-S minerals in the context of the origin of life and as environmental proxies and biosignatures in the geological record using their texture and chemical and isotopic compositions. We end by highlighting the importance of Fe-S minerals for current societal issues, such as the sequestration of organic carbon, the formation of acid drainages, metal recovery and nitrate removal, and their potential use as technological bio-materials in the future.

Selenium is a widespread contaminant released by industrial activities such as coal combustion. In selenium-contaminated groundwater, native microbial communities commonly have the capability of reducing the toxic oxyanions selenate and selenite to insoluble elemental selenium. The impact of local hydrogeography on microbial selenium reduction was tested by constructing laboratory microcosms using biofilm and groundwater collected from four monitoring wells screened in three distinct stratigraphic units near fly ash disposal ponds in southeastern Montana. Glycerol, methanol and molasses were tested as carbon amendments. Nitrate and selenium concentrations were monitored, and microbial communities were sequenced to examine differences among sites and carbon amendments. All site and carbon combinations resulted in nitrate removal, though molasses had the highest removal rate. Selenium removal was significantly impacted by stratigraphic unit, with microcosms from alluvial wells removing more total selenium than those from coal and interburden wells. Microbial community composition was correlated with site, carbon amendment, and nitrate and selenium removal. Furthermore, two genera from the order Clostridiales, Desulfosporosinus and Gracilibacter, emerged as potential indicator organisms for selenium reduction in this environment. The site, carbon amendment, and microbial community were all found to potentially impact remediation efficacy.

Microbial mineral weathering has been predominantly investigated at shallow depths in humid and tropical environments. Much less is understood about its role in the deeper subsurface of arid and semi-arid environments where microbial weathering is limited by the availability of water and energy sources for microbial metabolism. However, the deep subsurface in these climate zones may host a microbial community that thrives on weathering of iron (Fe)-bearing minerals that serve as electron donors or acceptors.

To investigate the role of microorganisms in weathering of Fe-bearing minerals in a dry climate, we recovered a >80 m deep weathering profile in a semi-arid region of the Chilean Coastal Cordillera. The bedrock is rich in Fe-bearing minerals (hornblende, biotite, chlorite, magnetite and hematite) but lacks detectable organic carbon. We evaluated the bioavailability of Fe(III)-bearing minerals that may serve as an electron acceptor for Fe(III)-reducing microorganisms. Using geochemical, mineralogical and cultivation-based methods, we found enhanced Fe bioavailability and more in vitro microbial Fe(III) reduction at increased depth. We obtained an Fe(III)-reducing enrichment culture from the deepest weathered rock found at 77 m depth. This enrichment culture is capable of reducing ferrihydrite (up to 0.6 mM d–1) using lactate or dihydrogen as an electron donor and grows at circumneutral pH. The main organism in the enrichment culture is the spore-forming Desulfotomaculum ruminis (abundance of 98.5%) as revealed by 16S rRNA gene amplicon sequencing.

Our findings provide evidence for a microbial contribution to the weathering of Fe-bearing minerals in semi-arid environments. While microorganisms are probably not contributing to the weathering of Fe(II)-bearing silicate minerals, they are most likely of importance regarding reductive dissolution of secondary weathering products. The Fe(III) reduction quantified in this weathering profile by the in situ microbial community suggests that microorganisms are active weathering agents in semi-arid climates.

Microbial O2 production via oxygenic photosynthesis was vital in oxygenating the Earth’s surface environment during the Great Oxygenation Event (GOE) ca. 2.5 to 2.3 billion years ago. However, geochemical, paleontological and genomic data suggest the emergence of oxygenic photosynthesis precedes the GOE by at least 500 million years. This demonstrates that the first appearance of microbial O2 in the environment cannot explain the timing of atmospheric oxygenation. Instead, the GOE was facilitated by Earth’s geodynamic evolution, expanding cyanobacterial habitats and the changing redox state of the mantle, decreasing the abundance of reduced surface rocks, volcanic gases and aqueous solutes. These trends ultimately resulted in magnified O2 production rates and diminished O2 consumption rates. Thus, the GOE can be understood as a misbalance between O2 sources and sinks. One of the most critical O2 sinks on modern Earth is microbial O2 consumption via aerobic respiration, and accumulating evidence suggests its emergence well before the GOE. However, the role of aerobic microorganisms as an O2 sink delaying the GOE remains poorly explored. Here, we review the redox evolution of Earth’s mantle and surface environments, as well as the Archean evolution of aerobic microbial metabolisms. Oxygenic photosynthesis released O2 to the environment, but the secular oxidation of the solid Earth was critical in allowing O2 accumulation. Aerobic respiration expanded in response to the GOE, but our survey suggests it could have been a critical O2 sink even earlier. Hence, aerobic respiration can be seen as geobiological feedback to changes in the Earth system from deep in the mantle up to the surface. However, the timing and rate of O2 consumption by aerobic respiration before the GOE remain poorly constrained. We conclude by highlighting open questions and future research directions to understand the role of the aerobic O2 sink in delaying the GOE.