Methanogenic archaea represent a small group of strictly anaerobic microorganisms (Hedderich and Whitman 2006). They can be found either in temperate habitats like paddy fields (Grosskopf et al. 1998), lakes (Jurgens et al. 2000; Keough et al. 2003), freshwater sediments (Chan et al. 2005), in the gastrointestinal tract of animals (Lin et al. 1997), or in extreme habitats such as hydrothermal vents (Jeanthon et al. 1999), hypersaline habitats (Mathrani and Boone 1985) or permafrost soils and sediments (Rivkina et al. 1998; Kobabe et al. 2004). In cold environments, two main pathways of energy-metabolism dominate: (i) the reduction of CO2 to CH4 using H2 as a reductant, and (ii) the fermentation of acetate to CH4 and CO2 (Conrad 2005). However, only a few psychrophilic (cold-adapted) strains of methanogenic archaea have been described so far (Simankova et al. 2003; Cavicchioli 2006).
Although permafrost environments are characterized by extreme climate conditions, it was recently shown that the abundance and composition of the methano-genic population is similar to that of communities of comparable temperate soil ecosystems (Wagner et al. 2005). The highest cell counts of methanogenic archaea were detected in the active layer of permafrost, with numbers of up to 3 x 108 cells g-1 soil (Kobabe et al. 2004). Methanogenic archaea represented between 0.5 and 22.4% of the total cell counts. Phylogenetic analyses revealed a great diversity of methanogens in the active layer, with species belonging to the families Methanobacteriaceae, Methanomicrobiaceae, Methanosarcinaceae, and Methano-saetaceae (H0j et al. 2005; Metje and Frenzel 2007; Ganzert et al. 2007; Fig. 15.3). Other sequences detected were affiliated to the euryarchaeotal Rice Clusters II and V (Hales et al. 1996; Grosskopf et al. 1998; Ramakrishnan et al. 2001) as well as to the Group I.3b of the uncultured Crenarchaeota (non-methanogenic archaea; Ochsenreiter et al. 2003). Environmental sequences from the Laptev Sea coast form four specific permafrost clusters (Ganzert et al. 2007). Permafrost Cluster I was recovered mainly from cold horizons (with temperatures of less than 40C) of the active layer, and was related to Methanosarcinacea. Permafrost Clusters II and III were related to Methanomicrobiales, and Permafrost Cluster IV was related to Rice Cluster II. It was hypothesized that these clusters comprise methanogenic archaea with a specific physiological potential to survive under harsh environmental conditions. The phylogenetic affiliation of the sequences recovered in this study indicated that both hydrogenotrophic and acetoclastic methanogenesis exist in
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