Methane production and archaeal community composition were studied in samples from an acidic peat bog incubated at different temperatures and pH values. H 2 -dependent methanogenesis increased strongly at the lowest pH, 3.8, and Methanobacteriaceae became important except for Methanomicrobiaceae and Methanosarcinaceae. An acidophilic and psychrotolerant Methanobacterium sp. was isolated using H 2 -plus-CO 2 -supplemented medium at pH 4.5.Wetlands are considered to be the largest natural sources of atmospheric CH 4 . Acidic peatlands are the most typical type of northern wetlands and are responsible for about 60% of total wetland emission (26). Peat bogs are characterized by low concentrations of mineral salts, low pH, and low temperature. Various factors have been identified as important controls of methanogenesis, with temperature, water table level, and content of organic matter being the most notable ones (4,9,12,27,32,35,38). However, there is little information on how pH influences the composition and functioning of the methanogenic community.In peatlands, hydrogentrophic methanogenesis is the predominant pathway of CH 4 formation, especially in deeper layers, accounting for 50 to 100% of total CH 4 production (12,18,28,40). However, acetoclastic methanogenesis has also been found to play an important role in acidic bogs (1, 2, 21). Relatively little is known about the archaeal communities inhabiting peatlands. Recent studies of different wetlands revealed the presence of methanogens belonging to the Methanomicrobiaceae, Methanobacteriaceae, Methanococcaceae, Methanosarcinaceae, and Methanosaetaceae as well as new archaeal lineages within the Euryarchaeota (3,7,8,13,17,34,36,37). However, the role of the methanogenic populations in CH 4 production under different in situ conditions is unknown.Attempts to isolate acidophilic or acidotolerant methanogens in pure culture have failed until very recently, although acid-tolerant enrichment cultures have been reported (6,11,15,34,41). It was only after we finished our study that Bräuer and coworkers reported the successful isolation of a moderately acidophilic methanogen belonging to the Methanomicrobiales order (5).The aim of the present study was to investigate how high acidity and low temperature can affect the functioning of the methanogenic community, its structure, and, hence, methane production in a peat bog, as well as to obtain a pure culture of an acidophilic methanogen. We used the same bog samples as in our previous study (21).We obtained peat samples from Bakchar Bog, which is located in West Siberia (57°N, 83°E). The main unforested part of the bog is covered with continuous Sphagnum moss and patches of vascular plants (Carex, Menyanthes, and Equisetum spp.). The detailed location of the bog and structure of the plant community have been described earlier (21, 29). The samples were taken in July 1999 at a depth of 30 to 50 cm below the water table from the site covered with Equisetum. The peat pH values were in a range of 3.5 to 5.5, with pH 4.8 at t...