Investigating Abiotic and Biotic Mechanisms of Pyrite Reduction

Pyrite (FeS(2)) has a very low solubility and therefore has historically been considered a sink for iron (Fe) and sulfur (S) and unavailable to biology in the absence of oxygen and oxidative weathering. Anaerobic methanogens were recently shown to reduce FeS(2) and assimilate Fe and S reduction products to meet nutrient demands. However, the mechanism of FeS(2) mineral reduction and the forms of Fe and S assimilated by methanogens remained unclear. Thermodynamic calculations described herein indicate that H(2) at aqueous concentrations as low as 10(–10) M favors the reduction of FeS(2), with sulfide (HS(–)) and pyrrhotite (Fe(1–)(x)S) as products; abiotic laboratory experiments confirmed the reduction of FeS(2) with dissolved H(2) concentrations greater than 1.98 × 10(–4) M H(2). Growth studies of Methanosarcina barkeri provided with FeS(2) as the sole source of Fe and S resulted in H(2) production but at concentrations too low to drive abiotic FeS(2) reduction, based on abiotic laboratory experimental data. A strain of M. barkeri with deletions in all [NiFe]-hydrogenases maintained the ability to reduce FeS(2) during growth, providing further evidence that extracellular electron transport (EET) to FeS(2) does not involve H(2) or [NiFe]-hydrogenases. Physical contact between cells and FeS(2) was required for mineral reduction but was not required to obtain Fe and S from dissolution products. The addition of a synthetic electron shuttle, anthraquinone-2,6-disulfonate, allowed for biological reduction of FeS(2) when physical contact between cells and FeS(2) was prohibited, indicating that exogenous electron shuttles can mediate FeS(2) reduction. Transcriptomics experiments revealed upregulation of several cytoplasmic oxidoreductases during growth of M. barkeri on FeS(2), which may indicate involvement in provisioning low potential electrons for EET to FeS(2). Collectively, the data presented herein indicate that reduction of insoluble FeS(2) by M. barkeri occurred via electron transfer from the cell surface to the mineral surface resulting in the generation of soluble HS(–) and mineral-associated Fe(1–)(x)S. Solubilized Fe(II), but not HS(–), from mineral-associated Fe(1–)(x)S reacts with aqueous HS(–) yielding aqueous iron sulfur clusters (FeS(aq)) that likely serve as the Fe and S source for methanogen growth and activity. FeS(aq) nucleation and subsequent precipitation on the surface of cells may result in accelerated EET to FeS(2), resulting in positive feedback between cell activity and FeS(2) reduction.