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.

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.

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.

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.