. Meanwhile the autotrophic methanogenesis via using
H2 as energy source could not only form extra CH4 but also consume
part of CO2 (Eq. (6)) and, as a consequence, the percentage
of methane increased with the applied ZVSI dosages (Fig. 1b). Likewise
homoacetogenic bacteria, as indicated by Thauer et al. [49]
and Rajagopal and LeGall [50], could also use CO2/H2 as sole energy
source for the formation of methanogenic precursor acetate (Eq.
(7)), which further reduced CO2/H2 contents and raised methane
percentage (Eq. (8)) [44]. Other authors similarly noted that iron
could be involved in energy metabolism as a cytochrome and ferredoxin
in methylotrophic methanogens [51].