Iron Corrosion via Direct Metal-Microbe Electron Transfer

The concept that anaerobic microorganisms can directly accept electrons from Fe(0) has been controversial because direct metal-microbe electron transfer has previously only been indirectly inferred. Fe(0) oxidation was studied with Geobacter sulfurreducens strain ACL, an autotrophic strain that was previously shown to grow with electrons derived from a graphite cathode as the sole electron donor. Strain ACL grew with Fe(0) as the sole electron donor and fumarate as the electron acceptor. However, it appeared that at least a portion of the electron transfer was via H(2) produced nonenzymatically from the oxidation of Fe(0) to Fe(II). H(2), which accumulated in abiotic controls, was consumed during the growth of strain ACL, the cells were predominately planktonic, and genes for the uptake hydrogenase were highly expressed. Strain ACL(HF) was constructed to prevent growth on H(2) or formate by deleting the genes for the uptake of hydrogenase and formate dehydrogenases from strain ACL. Strain ACL(HF) also grew with Fe(0) as the sole electron donor, but H(2) accumulated in the culture, and cells heavily colonized Fe(0) surfaces with no visible planktonic growth. Transcriptomics suggested that the outer surface c-type cytochromes OmcS and OmcZ were important during growth of strain ACL(HF) on Fe(0). Strain ACL(HF) did not grow on Fe(0) if the gene for either of these cytochromes was deleted. The specific attachment of strain ACL(HF) to Fe(0), coupled with requirements for known extracellular electrical contacts, suggest that direct metal-microbe electron transfer is the most likely option for Fe(0) serving as an electron donor.