Biogenic FeS nanoparticles modulate the extracellular electron transfer and schwertmannite transformation

Abstract

Iron sulfide nanoparticles (e.g., FeS NPs), which are ubiquitous in sulfate (SO₄²⁻)-rich anaerobic environments, can act as an electrical wire for long-distance extracellular electron transfer (EET) and bridge spatially discrete redox environments. However, whether FeS NPs can effectively promote EET may depend on the FeS NPs dosage, while how the FeS NPs affect mineral transformation is also unclear. It has been demonstrated that Shewanella oneidensis MR-1 possesses the ability to self-regulate and synthesize biogenic FeS NPs. Subsequently, the effect of FeS NPs on schwertmannite (Sch) reduction driven by S. oneidensis MR-1 and its mutants (ΔomcA, ΔmtrC, and ΔcymA) was investigated. It was found that the FeS NPs dosage strongly affected the Fe(III) reduction rate, thus altering the intermediate formed by Sch transformation. A high dosage of FeS resulted in a lower Fe(III) reduction rate, leading to the transformation of Sch into lepidocrocite, while goethite was formed with a low dosage of FeS. Moreover, the FeS NPs lowered the microbial abundance and gene expression involved in mineral transformation. The effect of the FeS NPs dosage on their mediated EET was shown to be hormetic, that is, low-dosage FeS (≤2 mM) indeed promoted the EET-mediated microbial reduction of Sch, while high-dosage FeS (≥4 mM) attached to the cell surface and formed a thick encrustation, which hindered EET and matter exchange, thus slowing down the microbial reduction of minerals. This study provides new insights into the potential role of FeS NPs in mediating the EET process and the microbe–mineral interactions in SO₄²⁻-rich anaerobic environments.