Fig. 4

Metabolic reconstruction of methanogenesis reveals two mechanisms of energy conservation. Energy conservation through methanogenesis is detailed for electron bifurcation (a) and chemiosmotic coupling (b). Electron bifurcation conserves energy in methanogenesis by taking two pairs of electrons from two separate hydrogen molecules and splitting them into a high energy state (CO₂ or ferredoxin reduction) and a low energy state (CoB-S-S-CoM heterodisulfide reduction). Genes for this mechanism were found in Msph. cuniculi, Mbac. bryantii, and Mcor. parvum, although the coupled hydrogenase was not identified in Mcor. parvum. Msar. spelaei utilizes chemiosmotic coupling for energy conservation, where Na⁺ or H⁺ transport into the cell is linked to H₂ oxidation, and transport out of the cell is linked to methyltransferase and heterodisulfide reductase activity, establishing a net outward gradient for ATP production. The hydrogenase depicted in (a) represents the Mvh hydrogenase (Msph. cuniculi and Mbac. bryantii), and the hydrogenase in (b) represents the Eha (Mbac. bryantii and Mcor. parvum), Ehb (Mbac. bryantii and Msph. cuniculi), or Ech hydrogenase (Msar. spelaei and Mcor. parvum). Methylotrophic methanogenesis pathways are displayed in (c). Methanol utilization pathways are found in Msph. cuniculi, Msar. spelaei, and Mbac. bryantii. Acetate and methylamine utilization pathways are found only in Msar. spelaei. The acetyl-CoA synthase complex is found in Mbac. bryantii and Msar. spelaei, allowing them to fix CO₂