Certain methanogens deteriorate steel surfaces through a process called microbiologically influenced corrosion (MIC). However, the mechanisms of MIC, whereby methanogens oxidize zerovalent iron (Fe ), are largely unknown. In this study, Fe -corroding Methanococcus maripaludis strain OS7 and its derivative (strain OS7mut1) defective in Fe -corroding activity were isolated. Genomic analysis of these strains demonstrated that the strain OS7mut1 contained a 12-kb chromosomal deletion. The deleted region, termed "MIC island", encoded the genes for the large and small subunits of a NiFe hydrogenase, the TatA/TatC genes necessary for the secretion of the NiFe hydrogenase, and a gene for the hydrogenase maturation protease. Thus, the NiFe hydrogenase may be secreted outside the cytoplasmic membrane, where the NiFe hydrogenase can make direct contact with Fe , and oxidize it, generating hydrogen gas: Fe  + 2 H  → Fe  + H . Comparative analysis of extracellular and intracellular proteomes of strain OS7 supported this hypothesis. The identification of the MIC genes enables the development of molecular tools to monitor epidemiology, and to perform surveillance and risk assessment of MIC-inducing M. maripaludis.