Acetate-Based Methane Production

This chapter explores the microbiology and biochemistry of acetate conversion to methane, a key component of biomethanation. It provides a fundamental background appropriate for stimulating advances to improve the process that will ensure biomethanation among the competitive alternatives to fossil fuels. Biomethanation of organic matter in nature occurs in diverse habitats such as freshwater sediments, rice paddies, sewage digesters, the rumen, the lower intestinal tract of monogastric animals, landfills, hydrothermal vents, coastal marine sediments, and the subsurface. Methanosarcina species synthesize tetrahydrosarcinapterin (H4SPT) which serves the same function as tetrahydromethanopterin (H4MPT). The aceticlastic and CO2 reduction pathways generate primary sodium and proton gradients that are the only possible driving forces for ATP synthesis. The production of acetate from complex biomass by fermentative and acetogenic anaerobes and the subsequent conversion of acetate to methane by aceticlastic methanogens are of primary importance in the biomethanation process. Aceticlastic methanogenesis is the major factor controlling the rate and reliability of the process; thus, a comprehensive understanding of these methanogens is paramount for developing an efficient process for biomethanation of renewable and waste biomass for use as a biofuel. Although the enzymology of reactions leading from acetate to methane by Methanosarcina species is fairly well understood, there have been only a few investigations reported on the mechanism of energy conservation and regulation of gene expression. Further, global proteomic and microarray analyses have identified a host of proteins and genes in Methanosarcina species, many with unknown functions, that may be important or essential.