The Methanogenic Bacteria
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The Methanogenic Bacteria WILLIAM B. WHITMAN, TIMOTHY L. BOWEN AND DAVID R. BOONE
The methanogenic bacteria are a large and diverse group that is united by three features: 1) They form large quantities of methane as the major product of their energy metabolism. 2) They are strict anaerobes. 3) They are members of the domain Archaea, or archaebacteria, (see Chapter 1) and only distantly related to the more familiar classical bacteria or eubacteria. Like the photosynthetic eubacteria, the methanogenic bacteria are related to each other primarily by their mode of energy metabolism but are very diverse with respect to their other properties. Methanogenic bacteria obtain their energy for growth from the conversion of a limited number of substrates to methane gas. The major substrates are H2 + CO2, formate, and acetate. In addition, some other C-1 compounds such as methanol, trimethylamine, and dimethylsulfide and some alcohols such as isopropanol, isobutanol, cyclopentanol and ethanol are substrates for some methanogens. All of these substrates are converted stoichiometrically to methane. In this regard, the metabolism of the methanogens is strikingly different from that of the so-called “minimethane” producers, which are other anaerobic microorganisms that produce very small amounts of methane as a consequence of side reactions of their normal metabolism (for an overview of the minimethane producers, see Rimbault et al., 1988). Another feature that distinguishes the methanogens from the minimethane producers is that the methanogens are obligate methane-producers, that is, they only grow under conditions where methane is formed. The list of substrates for growth of methanogens may be divided into three groups (Table 1). In the first group, the energy substrate (electron donor) is H2, formate, or certain alcohols and the electron acceptor is CO2, which is reduced to methane. The ability to utilize H2 as an electron donor for CO2 reduction is almost universal among methanogens. Likewise, many methanogens also utilize formate, but the ability to utilize alcohols is less common (Bleicher et al., 1989; Zellner and Winter, 1987a). Some methanogens also utilize carbon monoxide as an electron donor, but growth is very slow (Daniels et al.,
1977). CO2 reduction is the major source of methane in certain habitats such as the rumen. In other environments, such as the sediments of freshwater lakes and certain bioreactors, only about one-third of the methane is formed from CO2 reduction. However, this reaction is still very important for maintaining the very low concentrations of H2 and formate typical of these anaerobic habitats and facilitating the process of interspecies electron transfer. In the second group, the energy substrate is one of a variety of methyl-containing C-1 compounds, which can serve as substrates for a few taxa of methanogens. Usually these compounds are disproportionated. Some molecules of the substrate are oxidized to CO2. The electron acceptors are the remaining met
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