Reductive biomining of pyrite by methanogens.

Pyrite (FeS2) is the most abundant iron sulfide mineral in Earth's crust. Until recently, FeS2 has been considered a sink for iron (Fe) and sulfur (S) at low temperature in the absence of oxygen or oxidative weathering, making these elements unavailable to biology. However, anaerobic methanogens can transfer electrons extracellularly to reduce FeS2 via direct contact with the mineral. Reduction of FeS2 occurs through a multistep process that generates aqueous sulfide (HS-) and FeS2-associated pyrrhotite (Fe1-xS). Subsequent dissolution of Fe1-xS provides Fe(II)(aq), but not HS-, that rapidly complexes with HS-(aq) generated from FeS2 reduction to form soluble iron sulfur clusters [nFeS(aq)]. Cells assimilate nFeS(aq) to meet Fe/S nutritional demands by mobilizing and hyperaccumulating Fe and S from FeS2. As such, reductive dissolution of FeS2 by methanogens has important implications for element cycling in anoxic habitats, both today and in the geologic past.