Fundamentals of methanogenic pathways that are key to the biomethanation of complex biomass

The conversion of biomass to CH4 (biomethanation) involves an anaerobic microbial food chain composed of at least three metabolic groups of which the first two decompose the complex biomass primarily to acetate, formate, and H2. The thermodynamics of these conversions are unfavorable requiring a symbiosis with the CH4-producing group (methanogens) that metabolize the decomposition products to favorable concentrations. The methanogens produce CH4 by two major pathways, conversion of the methyl group of acetate and reduction of CO2 coupled to the oxidation of formate or H2. This review covers recent advances in the fundamental understanding of both methanogenic pathways with the view of stimulating research towards improving the rate and reliability of the overall biomethanation process.

► The biomethanation of complex biomass requires microbial food chains comprised of at least three interacting metabolic groups.
► Methane-producing species are terminal organisms of the food chain that function in symbiosis with other metabolic groups.
► The two major pathways for methanogenesis are conversion of the methyl group of acetate and reduction of carbon dioxide to methane.
► Although one-carbon reactions of the pathways leading to methane are well understood, there are gaps in the understanding of energy conversion for growth.