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Category: Applied and Industrial Microbiology
Acetate-Based Methane Production, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555815547/9781555819057_Chap13-1.gif /docserver/preview/fulltext/10.1128/9781555815547/9781555819057_Chap13-2.gifAbstract:
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.
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The global carbon cycle in nature. (A) Fixation of CO2 into organic matter; (B) aerobic decomposition of organic matter to CO2; (C) anaerobic decomposition of organic matter to fermentative end products; (D) anaerobic conversion of fermentative end products to methane and escape into aerobic environments; (E) aerobic oxidation of methane to CO2 by O2-requiring methylotrophs.
Methane-producing freshwater consortia.
Phase-contrast micrographs of Methanosarcina mazei showing cells in pseudosarcinal aggregates (left) and single cells (right). Bars, 10 μm. Reprinted from Applied and Environmental Microbiology ( Sowers et al., 1993 ) with permission of the publisher.
Thin-section electron micrograph of Methanosarcina thermophila showing a cell aggregate enclosed by an outer membrane. Bar, 1 μm. Reprinted from International Journal of Systematic Bacteriology ( Zinder et al., 1985 ) with permission of the publisher.
Scanning electron micrograph of Methanosaeta concilii strain T-3 filaments. Reprinted from Bioscience, Biotechnology, and Biochemistry ( Mizukami et al., 2006 ) with permission of the publisher.
Thin section of “Methanothrix” (Methanosaeta) concilii. Spacer plugs are indicated by arrows, and “M” indicates the amorphous granular layer. Bar, 1 μm. Reprinted from Canadian Journal of Microbiology ( Beveridge et al., 1986 ) with permission of the publisher.
Cofactors and coenzymes utilized in the pathway for aceticlastic methanogenesis.
Overview of the aceticlastic and CO2 reduction pathways. H4M(S)PT, tetrahydromethanopterin (H4MPT) or tetrahydrosarcinapterin (H4SPT); CoM, coenzyme M; CoB, coenzyme B.
Proposed mechanism of sodium translocation by the methyl-H4M(S)PT:coenzyme M methyltransferase. Reprinted from Biochimica et Biophysica Acta ( Gottschalk and Thauer, 2001 ) with permission of the publisher.
Mechanism proposed for methyl-coenzyme M methylreductase. Adapted from Ermler et al., 1997.
Pathways for conversion of acetate to methane by M. mazei (A) and M. acetivorans (B). Ack, acetate kinase; Pta, phosphotransacetylase; CoA-SH, coenzyme A; H4SPT, tetrahydrosarcinapterin; Fdr, reduced ferredoxin; Fdo, oxidized ferredoxin; Cdh, CO dehydrogenase/acetyl-CoA synthase; CoM-SH, coenzyme M; Mtr, methyl-H4SPT:CoM-SH methyltransferase; CoB-SH, coenzyme B; Cam, carbonic anhydrase; Ech, H2-evolving hydrogenase; Vho, H2-consuming hydrogenase; Ma-Rnf, M. acetivorans Rnf; MP, methanophenazine; Hdr-DE, heterodisulfide reductase; Mrp, multiple resistance/pH regulation Na+ /H antiporter; Atp, H -translocating A1A0 ATP synthase. Adapted from Li et al., 2006.
The A cluster of Acs from Moorella thermoacetica. Reprinted from Critical Reviews in Biochemistry and Molecular Biology ( Ragsdale, 2004 ) with permission of the publisher.