Electron flow in hydrogenotrophic methanogens under nickel limitation.

Methanogenic archaea are the main producers of the potent greenhouse gas methane. In the methanogenic pathway from CO and H studied under laboratory conditions, low-potential electrons for CO reduction are generated by a flavin-based electron-bifurcation reaction catalysed by heterodisulfide reductase (Hdr) complexed with the associated [NiFe]-hydrogenase (Mvh). F-reducing [NiFe]-hydrogenase (Frh) provides electrons to the methanogenic pathway through the electron carrier F (ref. ). Here we report that under strictly nickel-limited conditions, in which the nickel concentration is similar to those often observed in natural habitats, the production of both [NiFe]-hydrogenases in Methanothermobacter marburgensis is strongly downregulated. The Frh reaction is substituted by a coupled reaction with [Fe]-hydrogenase (Hmd), and the role of Mvh is taken over by F-dependent electron-donating proteins (Elp). Thus, Hmd provides all electrons for the reducing metabolism under these nickel-limited conditions. Biochemical and structural characterization of Elp-Hdr complexes confirms the electronic interaction between Elp and Hdr. The conservation of the genes encoding Elp and Hmd in CO-reducing hydrogenotrophic methanogens suggests that the Hmd system is an alternative pathway for electron flow in CO-reducing hydrogenotrophic methanogens under nickel-limited conditions.