In methanogenic archaea growing on H
2
and CO
2
the first step in methanogenesis is the ferredoxin-dependent endergonic reduction of CO
2
with H
2
to formylmethanofuran and the last step is the exergonic reduction of the heterodisulfide CoM-S-S-CoB with H
2
to coenzyme M (CoM-SH) and coenzyme B (CoB-SH). We recently proposed that in hydrogenotrophic methanogens the two reactions are energetically coupled via the cytoplasmic MvhADG/HdrABC complex. It is reported here that the purified complex from
Methanothermobacter marburgensis
catalyzes the CoM-S-S-CoB-dependent reduction of ferredoxin with H
2
. Per mole CoM-S-S-CoB added, 1 mol of ferredoxin (Fd) was reduced, indicating an electron bifurcation coupling mechanism:
This stoichiometry of coupling is consistent with an ATP gain per mole methane from 4 H
2
and CO
2
of near 0.5 deduced from an H
2
-threshold concentration of 8 Pa and a growth yield of up to 3 g/mol methane.