Marine rust tubercles harbour iron corroding archaea and sulphate reducing bacteria

•31% of sequences from marine rust tubercles belonged to Methanococcus maripaludis.•This archaea was previously shown to extract electrons directly from steel.•Archaea may be significant contributors to marine corrosion.•Sulphate reducing bacteria were present in tubercles in relatively low numbers.

Marine corrosion has significant economic impacts globally. Marine rust on carbon steel in Western Australia was investigated to determine the importance of various microorganisms in corrosion. Microorganisms were imaged, identified and enumerated by pyrosequencing. The base of tubercles was anaerobic. Pyrosequencing demonstrated the presence of diverse bacteria and archaea. However, the dominant group were methanogenic archaea, representing 53.5% of all sequences. One methanogenic species, Methanococcus maripaludis, comprised 31% of sequences, and can significantly increase corrosion rates by extracting electrons directly from steel. Methanogenic archaea may be significant contributors to marine corrosion of carbon steel.