Proposed pathways for methanogenesis and energy conservation in the Methanomassiliicoccales representatives. The protein names are in bold. The predicted pathways and enzymes present in the three Methanomassiliicoccales species are in blue, those absent from "Ca. M. intestinalis" are in green and those absent from "Ca. M. alvus" are in red. MtaA and MtbA are marked with an asterisk to signify that the homologs present in the Methanomassiliicoccales are not yet assigned to one or the other enzyme category. "X" refers to the uncharacterized lipid soluble electron transporter. The question mark points out that the enzymes involved in the reoxidation of the lipid soluble electron transporter remain to be uncovered. See Table 6 and Additional file 1: Table S10 for a description of the set of genes involved.