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.