Importance of hydrogenotrophic, aceticlastic and methylotrophic methanogenesis for methane production in terrestrial, aquatic and other anoxic environments: A mini review

Abstract Microbial methanogenesis is a major source of the greenhouse gas methane (CH4). It is the final step in the anaerobic degradation of organic matter when inorganic electron acceptors such as nitrate, ferric iron, or sulfate have been depleted. Knowledge of this degradation pathway is important for the creation of mechanistic models, prediction of future CH4 emission scenarios, and development of mitigation strategies. In most anoxic environments, CH4 is produced from either acetate (aceticlastic methanogenesis) or hydrogen (H2) plus carbon dioxide (CO2) (hydrogenotrophic methanogenesis). Hydrogen can be replaced by other CO2-type methanogenesis, using formate, carbon monoxide (CO), or alcohols as substrates. The ratio of these two pathways is tightly constrained by the stoichiometry of conversion processes. If the degradation of organic matter is complete (e.g., degradation of straw in rice paddies), then fermentation eventually results in production of acetate and H2 at a ratio of > 67% aceticlastic and