Cells of magnetotactic bacteria are used as model systems for studying the magnetic properties of ferrimagnetic nanocrystals. Each individual bacterial strain produces magnetosomes (membrane-bounded magnetic crystals) that have distinct sizes, shapes, crystallographic orientations and spatial arrangements, thereby providing nanoparticle systems whose unique magnetic properties are unmatched by synthetic chemically-produced crystals. Here, we use off-axis electron holography in the transmission electron microscope to study the magnetic properties of isolated and closely-spaced bullet-shaped magnetite (Fe3O4) magnetosomes biomineralized by the following magnetotactic bacterial strains: the cultured Desulfovibrio magneticus RS-1 and the uncultured strains LO-1 and HSMV-1. These bacteria biomineralize magnetite crystals whose crystallographic axes of elongation are parallel to <100> (RS-1 and LO-1) or <110> (HSMV-1). We show that the individual magnetosome crystals are single magnetic domains and measure their projected in-plane magnetization distributions and magnetic dipole moments. We use analytical modelling to assess the interplay between shape anisotropy and the magnetically preferred <111> magneto-crystalline easy axis of magnetite.

Sulfidated nanoscale zerovalent iron (S-nZVI) materials show enhanced reactivity and selectivity towards chlorinated solvents compared to non-sulfidated nZVI, thus they have a high potential for subsurface chlorinated solvent remediation. However, little is known about the possible toxic effects of S-nZVI towards microbial communities, which is of particular concern with regard to combined abiotic–biotic chlorinated solvent treatment scenarios. In this study, the toxicity of two different S-nZVI materials towards Shewanella oneidensis MR-1 (S. MR-1) was examined under anaerobic and aerobic conditions using colony forming units (CFU) and adenosine triphosphate (ATP) measurements, and the results were then compared to identical exposures performed with non-sulfidated nZVI. In a second step, the toxicity of S-nZVI and nZVI materials was tested on the commercial bioremediation culture KB-1® and on an in-house trichloroethylene enrichment culture. Under aerobic conditions, S. MR-1 viability was less affected by S-nZVI materials compared to non-sulfidated nZVI materials (up to three times higher viability) and it was generally lower compared to anaerobic conditions where little difference in S. MR-1 viability was observed between the tested materials. In terms of the two dechlorinating cultures, they exhibited significantly higher ATP viability during anaerobic exposures to S-nZVI and nZVI materials. Particularly for KB-1®, which retained comparable ATP-viability after ~60 hours exposure as S. MR-1 after two hours. Moreover, the ATP viability of the mixed cultures was generally higher in S-nZVI exposures compared to nZVI exposures (up to three times higher viability). The observed viability patterns are explained by differences in the shell structure, chemistry and stability of the tested S-nZVI and nZVI materials towards corrosion, while the substantially enhanced resilience of KB-1® is argued to stem from its year-long cultivation in the presence of reduced FeS particulates.

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.