Physiology and biochemistry of the methane-producing archaea

Abstract

Methanoarchaea are an ancient monophyletic lineage within the Euryarchaeota, thriving by chemolithotrophic energy metabolism. The chapter discusses three major problems: (i) distinguishing taxa on the basis of their similar phenotype; (ii) the extreme genetic diversity of methanogens and, consequently, the high variability of their metabolism; and (iii) based upon analysis of environmental DNA, the notion that the cultured methanogens represent a very sparse sampling of the likely diversity in nature. Hence, knowledge of the species richness and metabolic diversity of this group is necessarily incomplete. The following will focus on the large differences in cellular structure, metabolic pathways, and regulation. Methanoarchaea derive their metabolic energy from the conversion of a restricted number of substrates to methane. Most methanoarchaea can reduce CO 2 to CH 4. The major electron donors for this reduction are H 2 and formate. In addition, some methanoarchaea can use alcohols like 2-propanol, 2-butanol, cyclopentanol, and ethanol as electron donors. The second type of substrate for methanogenesis includes C-1 compounds containing a methyl group carbon bonded to O, N, or S. Compounds of this type include methanol, monomethylamine, dimethylamine, trimethylamine, tetramethylammonium, dimethylsulfide, and methane thiol. The third type of substrate is acetate. In this reaction, the methyl (C-2) carbon of acetate is reduced to methane using electrons obtained from the oxidation of the carboxyl (C-1) carbon of acetate. This reaction is called the 'aceticlastic reaction' because it results in the splitting of acetate into methane and CO 2. Key reactions of the different methanogenic pathways including, electron chains and structures, as well as genomic information and methanogenic coenzymes and enzymes in nonmethanogenic Archaea and Bacteria are described.