Coupling methanogenesis with iron reduction by acetotrophic Methanosarcina mazei zm‐15

Abstract Application of ferric iron is conventionally considered to inhibit methanogenesis in anoxic environments. Here we show that Methanosarcina mazei zm‐15, a strain isolated from the natural wetland of Tibetan plateau, is capable of Fe(III) reduction, which significantly promotes its growth and methanogenesis. We grew Ms . mazei zm‐15 in a medium containing acetate supplemented with Fe(III) in ferric citrate or ferrihydrite and to some cultures anthraquinone‐2,6‐disulfonate (AQDS) was applied as an electron shuttle. The reduction of Fe(III) species occurred immediately. Ferric citrate was more readily reduced than ferrihydrite. The X‐ray diffraction spectra analysis showed the formation of magnetite from ferrihydrite and amorphous reduced products from ferrihydrite plus AQDS. The analysis of protein contents revealed that Fe(III) reduction contributed 36%–46% of the cell growth. The growth yield, estimated as protein increment per acetate consumed for Fe(III) reduction, increased by 20‐ to 30‐fold compared with methanogenesis, which is in consistence with the difference in free energy available by Fe(III) reduction relative to methanogenesis. We propose that the outer‐surface multiheme c‐type cytochrome predicted from Ms . mazei zm‐15 genome serves as the terminal reductase with the energy‐converting hydrogenase and F 420 H 2 dehydrogenase involved in electron transport chain for Fe(III) reduction. The findings shed a light to better understand the ecophysiology of Methanosarcina in anaerobic environments.