The Order Methanomicrobiales
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The Order Methanomicrobiales JEAN-LOUIS GARCIA, BERNARD OLLIVIER AND WILLIAM B. WHITMAN
Characteristics of Methanomicrobiales Although their morphology is diverse, Methanomicrobiales can be distinguished from other methanogens by growth properties, cell wall and lipid composition, and rDNA sequence. All Methanomicrobiales can use H2 + CO2 as a substrate for methanogenesis, most species can utilize formate, and many species utilize alcohols (Table 1). They cannot use acetate and methylated C-1 compounds such as methanol, methylamines and methyl sulfides as substrates for methanogenesis, and this property distinguishes them from the Methanosarcinales. Notably, even though the Methanomicrobiales cannot use acetate as a substrate for methanogenesis, many species require acetate as a carbon source. The absence of hydroxyarchaeol in their lipids further distinguishes them from the Methanosarcinales (Koga et al., 1998). In addition, all of the Methanomicrobiales so far examined contain aminopentanetetrols, galactose and glycerol in their lipids, and aminopentanetetrols are unique to this taxon. The absence of pseudomurein further distinguishes the Methanomicrobiales from both the Methanobacteriales and Methanopyrales. Additional distinctive features are listed in Table 1. Based upon the phylogeny of the 16S rRNA genes as well as phenotypic and genotypic characteristics, the order Methanomicrobiales has been divided into three families and nine genera of hydrogenotrophic methanogens (Boone et al., 1993; Rouvière et al., 1992; Garcia et al., 2000; Tables 2 and 3). Twenty-four species have been described so far within this order (Tables 3 and 4). Because some species epithets are not in agreement with the International Nomenclature Code for Taxonomy, proposed changes for the original spelling are shown in brackets after the current name (Euzéby, 1997; Chong and Boone, 2004; Ferry and Boone, 2004).
Habitats of Methanomicrobiales Like methanogens belonging to other orders, Methanomicrobiales inhabit diverse anaerobic habitats comprising marine and fresh water sediments, anaerobic digesters and the rumen. Although based upon classical microbiological characterizations, this conclusion has been well supported recently by a wide variety of molecular and immunological studies (Kudo et al., 1997; Munson et al., 1997; Grossköpf et al., 1998; Sekiguchi et al., 1999). Moreover, a hybridization probe specific to the three families belonging to this order has been designed and characterized for environmental and determinative microbiological studies (Raskin et al., 1994). Eight of the 24 validly published species within the order Methanomicrobiales including species of the genera Methanoculleus, Methanofollis, and Methanocorpusculum have been isolated from anaerobic digestors or sewage sludge (Whitehead and Cotta, 1999). Methanoculleus bourgense was isolated from a tannerybyproducts enrichment-culture inoculated with sewage sludge (Ollivier et al., 1986), whereas Methanoculleus palmolei wa
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