The Order Methanosarcinales
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The Order Methanosarcinales MELISSA M. KENDALL AND DAVID R. BOONE
Introduction Methanogens (methane-producing microbes) belong to the domain Archaea, and 16S rRNA gene sequences indicate that they form a phylogenetically distinct clade within the kingdom Euryarchaeota. The order Methanosarcinales is one of the five orders of methanogens. These microbes catalyze the terminal step in the degradation of organic matter in anoxic environments where light and terminal electron-acceptors other than CO2 are limiting. Representatives of the Methanosarcinales are widespread and cosmopolitan in anaerobic environments. They are found in freshwater, marine environments, and extremely halophilic sediments as well as in anaerobic sludge digestors and the gastrointestinal tracts of animals. Use of molecular techniques has also identified Methanosarcinales in the deep terrestrial subsurface environments such as in the gold mines in South Africa. Members of the order Methanosarcinales are coccoids, pseudosarcinae, or sheathed rods. Most cells have a protein cell wall, and some are surrounded by a sheath or acidic heteropolysaccharide; no peptidoglycan or pseudomurein is present. Hydroxyarchaeol is usually present in the lipids, which contain myo-inositol, ethanolamine and glycerol as polar headgroups. Cells are very strictly anaerobic and obtain energy with the concomitant production of methane. Methanosarcinales have the widest substrate range of methanogens: many can grow by reducing CO2 with H2, by dismutating methyl compounds, or by the splitting of acetate. Some species can use only one of those catabolic schemes, but others can use all three. The order Methanosarcinales comprises two families, Methanosarcinaceae and Methanosaetaceae. Some of their distinctive characteristics are listed in Table 1.
Habitats and Metabolism of Methanosarcinales Similarly to other methanogens, Methanosarcinales are widespread in diverse anaerobic habi-
tats, including freshwater and marine mud and sediments, rumens of ungulates, animal waste lagoons, sludge from anaerobic sewage sludge digestors, and animal feces. Genetic sequences of 16S rRNA that are apparently from organisms that belong to Methanosarcinales have been detected in DNA from fissure water of deep South African gold mines (Takai et al., 2001), gas industry pipelines (Zhu et al., 2003), and a human periodontal pocket (Robichauz et al., 2003). In most natural environments where Methanosarcinales occur, they grow by splitting acetate to methane (CH4) and carbon dioxide (CO2), or they catabolize methyl compounds (methanol, methyl amines, methyl sulfides, etc.). In anoxic environments that lack light and alternate electron acceptors other than CO2, methanogenesis is the major fate of organic matter that is degraded, and acetate splitting is the major source of methane. There are no other organisms than Methanosarcinales that are capable of catabolizing acetate in this manner. It is accomplished by members of one genus of the family Methanosarc
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