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Chapter 13 : Methanogenesis
Category: Microbial Genetics and Molecular Biology; Environmental Microbiology
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Recent genomic sequencing, proteomic analyses, and development of genetic systems continue to expand one's understanding of methanogenesis and the Archaea. The conversion of the methyl group of acetate to methane (acetate fermentation pathway) produces about two-thirds of the annual production, whereas one-third derives from the reduction of carbon dioxide with electrons supplied from the oxidation of formate or hydrogen (carbon dioxide reduction pathway). Thus, the methanogens rely on the first two groups to supply substrates for growth and methanogenesis. Methanogens are the main constituency of the Euryarchaeota and are subdivided into five orders, including such as Methanobacteriales, Methanococcales and Methanomicrobiales; each with distinctive characteristics. Methanofuran (MF) and tetrahydromethanopterin (THMPT) function as one-carbon carriers, the latter coenzyme also functioning in methylotrophic microbes from the Bacteria domain. A proteomic and transcriptional analysis of cold adaptation has revealed the thermal regulation of several genes essential for methanogenesis by dismutation of the methyl group of trimethylamine. The extent of regulation of genes essential for methanogenesis and other fundamental processes in response to temperature is consistent with a role in providing the cell with an ecological advantage in cold environments. The genome sequences of M. acetivorans, M. mazei, and M. thermophila harbor two homologs of mtaA, three homologs of mtaB, and three homologs of mtaC encoding enzymes specific for methanogenesis from methanol.
Reenactment of the Volta experiment illustrating methanogenesis in a freshwater pond.
The global carbon cycle. Steps: (1) Fixation of carbon dioxide into organic matter, (2) aerobic (oxygen-dependent) decomposition of organic matter to carbon dioxide, (3) deposition of organic matter into anaerobic (oxygen-free) environments and decomposition to metabolic end products by fermentative and obligate hydrogen-producing anaerobes, (4) conversion of the end products to methane by the methanoarchaea and escape of the methane to aerobic environments, (5) aerobic oxidation of methane to carbon dioxide by oxygen-requiring methylotrophs.
Cofactors and coenzymes utilized in methanogenic pathways.
Steps in the acetate fermentation pathway. Substrates and products are shown in bold.
Steps in the carbon dioxide reduction pathway. Substrates and products are shown in bold.
The H2:CoMS-SCoB oxidoreductase system in Methanosarcina species. MP, methanophenazine.
Predicted topology of the H2:CoMS-SCoB oxidoreductase system in obligate carbon dioxide-reducing species.
Predicted topology and function of the Fd:CoMS-SCoB oxidoreductase system in Methanosarcina barkeri. M P, methanophenazine.
Postulated topology and function of the Fd:CoMS-SCoB oxidoreductase system in Methanosarcina acetivorans. Please see reference 145 for a detailed understanding. The numbers correspond to open reading frames of the gene cluster encoding the Ma-Rnf complex. MP, methanophenazine.
Steps in the pathway for dismutation of methanol and monomethylamine to carbon dioxide and methane.
Postulated topology and function of the F420H2:CoMS-SCoB oxidoreductase system in Methanosarcina mazei. MP, methanophenazine.
Pyrrolysine ((4R,5R)-4-methyl-pyrroline-5-carboxylate) present in MtmB, MtbB, and MttB.