Microorganisms and their enzymes drive global biogeochemical cycles and impact ecosystem processes that directly influence plant, animal, and environmental health. Key reactions are catalyzed by metalloenzymes with active sites that contain iron (Fe), nickel (Ni), cobalt (Co), and molybdenum (Mo) sulfide cofactors. However, under anoxic conditions, such metals are bio-unavailable, raising questions of how anaerobes biosynthesize such metalloenzymes. Recent work indicates that methanogens can convert pyrite (FeS₂), the most abundant iron sulfide mineral in Earth’s crust, into metalloenzymes that interconvert CO₂ to CH₄, N₂ to NH₃, and H₂ to H⁺. Reduction of FeS₂ takes place via a yet to be defined mechanism of extracellular electron transfer that may involve extracellular conductive appendages. Reduction of FeS₂ yields the secondary mineral pyrrhotite (Fe_1-xS) that equilibrates to form aqueous iron sulfur clusters (FeS_aq) that are assimilated to meet Fe and S nutrient demands. Differential transcriptomics and proteomics experiments indicate up-expression of a homolog of the Fe(II) transporter, FeoB, and its putative transcriptional regulator, DtxR, in FeS₂-grown cells, suggesting that cells sense Fe(II) limitation. Two homologs of IssA, a protein putatively involved in coordinating thioferrate nanoparticles, were also up-expressed in FeS₂-grown cells. We interpret these data to indicate that, in FeS₂-grown cells, DtxR cannot sense Fe(II) and therefore cannot down-regulate FeoB. We suggest this is due to the transport of Fe(II) into the cells that is complexed with sulfur (FeS_(aq)) leading to excess Fe that is sequestered by IssA as a thioferrate-like species. New electron paramagnetic spectroscopy, Mössbauer spectroscopy, and cryo-electron microscopy data provide new insight into how FeS_aq is transported and stored intracellularly. These results provide a framework for the design of growth conditions and engineered proteins to bio-mine and concentrate other trace metals from pyrite, such as Co, Ni, and Mo.

Eric Boyd

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