Based on their function as the major sink for methane in Arctic permafrost affected wetlands and tundra, methane-oxidizing Proteobacteria are also of importance for the greenhouse gas budget of these environments.
Methane-oxidizing Proteobacteria represent a subset of methylotrophic bacteria. Through the activity of their specific enzyme, methane monooxygenase, they are specialized to utilize methane as their single carbon and energy source (Hanson and Hanson 1996). The group of methane-oxidizing Proteobacteria comprises the three families Methylococcaceae, Methylocystaceae, and Beijerinckiaceae (Bowman 1999; Dedysh et al. 2000, 2001, 2002, 2004). The only exception is Crenothrix polyspora, a filamentous, sheathed microorganism recently discovered to be methanotrophic (Stoecker et al. 2006). Methylococcaceae include the genera Methylobacter, Methylomonas, Methylomicrobium, Methylosarcina, Methylosphaera, Methylohalobius, Methylosoma, Methylothermus, Methylococcus, and Methylocaldum (Hanson and Hanson 1996; Bowman et al. 1997; Wise et al. 2001; Heyer et al. 2005; Tsubota et al. 2005; Rahalkar et al. 2007). They belong to the gamma subdivision of the Proteobacteria phylum and are termed type I meth-anotrophs, except for the last two, which are also known as type X methanotrophs. The families Methylocystaceae, and Beijerinckiaceae include the genera Methylosinus, Methylocystis, Methylocella, and Methylocapsa (Hanson and Hanson 1996; Bowman 1999; Dedysh et al. 2000, 2001, 2002, 2004). Members of the Methylocystaceae and Beijerinckiaceae are termed type II methanotrophs, and belong to the alpha subdivision of the Proteobacteria phylum. Except for their phylogeny, type I and type II methanotrophs can also be distinguished by their carbon assimilation pathway, the structure of their intracytoplasmic membranes, their resting stages, G + C-content, the constitution of their methane monooxygenase, and by their major phospholipid fatty acids (PLFAs).
Several studies have revealed that methanotrophs are abundant and active also under very harsh environmental conditions of cold environments (review by Trotsenko and Khmelenina 2005). Viable methane oxidizers have even been detected in deep Siberian permafrost sediments with ages of 1,000-100,000 years (Khmelenina et al. 2001). Numerous psychrophilic and psychrotrophic methanotrophs, primarily affiliated to the type I group, are known, such as Methylobacter psychrophilus, isolated from Siberian tundra (Omelchenko et al. 1996), Methylobacter tundripaludum, isolated from Arctic wetland soils (Wartiainen et al. 2006a; Fig. 15.4), Methylosphaera hansonii, isolated from Antarctic, marine salinity, meromictic lakes (Bowman et al. 1997), and Methylomonas scandinavica, isolated from deep igneous rock ground water (Kaluzhnaya et al. 1999). Type I methanotrophs have also been discovered to dominate in Arctic permafrost-affected soils (Wartiainen et al. 2003; Wagner et al. 2005; Liebner and Wagner 2007). Within the type II group, Methylocystis rosea, isolated from an Arctic wetland soil (Wartiainen et al. 2006b; Fig. 15.4), and representatives of the acidophilic genera Methylocella and Methylocapsa were reported to be psychrotrophs (Dedysh et al. 2002, 2004).
Methane-oxidizing Proteobacteria have been shown to be highly abundant in permafrost soils of the Lena Delta, Siberia, with cell numbers ranging between 3 x 106 and 1 x 108 cells g-1 soil and contributing up to 10% to the total number of microbial cells (Liebner and Wagner 2007). In the same area, specific clusters of methane-oxidizing Proteobacteria closely related to Methylobacter psychrophilus and to Methylobacter tundripaludum were detected, indicating a micro-diverse community on the species level (Liebner et al. 2008). Also, highly divergent functional gene sequences of these methanotrophs were found in soils of the high Canadian Arctic (Pacheco-Oliver et al. 2002). In contrast, the diversity of methane-oxidizing Proteobacteria in an Arctic wetland on the island of Svalbard was observed to be restricted to only two genera (Wartiainen et al. 2003), whereas most methanotrophic Proteobacteria were detected in a Russian sub-Arctic tundra (Kaluzhnaya et al. 2002).
Still, diversity and composition of methane-oxidizing bacteria in permafrost soils are only poorly explored. Also, it remains unknown whether psychrophilic or cold-adapted mesophilic methantrophs are responsible for methane oxidation at low and subzero temperatures in permafrost sediments (Trotsenko and Khmelenina 2005). A recent study, though, observed a shift between a mesophilic methanotrophic community near the surface and a psychrophilic methanotrophic community near the permafrost table of Siberian permafrost soils (Liebner and Wagner 2007). This indicates that depending on the environmental conditions both mesophilic as well as psy-chrophilic methanotrophs are active in Siberian permafrost soils.
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