Anabolic Pathways in Methanogens

Even though the methanogens are morphologically and nutritionally diverse, every major phylogenetic group includes some autotrophic species. Even among methanogens that require an organic carbon source, acetate is usually the major carbon source, and acetyl-CoA is also the major product of autotrophic CO2 fixation. These observations are consistent with the hypothesis that all methanogens are either autotrophs or recently evolved from autotrophic ancestors. Because their biosynthetic pathways originate from simple carbon compounds, the role of the central metabolic pathways is the elaboration of precursors for macromolecular biosynthesis, and precursor biosynthesis is a major component of the cellular energy budget. The bioenergetic importance of precursor biosynthesis is further compounded by the low energy yields of methanogenesis and the low partial pressures of H2 that are typical of most methanogenic habitats. This situation is fundamentally different from that of many heterotrophs, where precursors are readily obtained from catabolic intermediates, and it is likely to have profound consequences. Even though these consequences are poorly understood, our examination of the anabolic pathways in methanogens will utilize this perspective to obtain insights into why and how methanogens use the pathways they do.