Sites in the West Siberian peat bog 'Bakchar' were acidic (pH 4.2-4.8), low in nutrients, and emitted CH4 at rates of 0.2-1.5 mmol m(-2) h(-1). The vertical profile of delta13CH4 and delta13CO2 dissolved in the porewater indicated increasing isotope fractionation and thus increasing contribution of H2/CO2-dependent methanogenesis with depth. The anaerobic microbial community at 30-50 cm below the water table produced CH4 with optimum activity at 20-25 degrees C and pH 5.0-5.5 respectively. Inhibition of methanogenesis with 2-bromo-ethane sulphonate showed that acetate, phenyl acetate, phenyl propionate and caproate were important intermediates in the degradation pathway of organic matter to CH4. Further degradation of these intermediates indicated that 62-72% of the CH4 was ultimately derived from acetate, the remainder from H2/CO2. Turnover times of [2-14C]acetate were on the order of 2 days (15, 25 degrees C) and accounted for 60-65% of total CH4 production. Conversion of 14CO2 to 14CH4 accounted for 35-43% of total CH4 production. These results showed that acetoclastic and hydrogenotrophic methanogenesis operated closely at a ratio of approximately 2 : 1 irrespective of the incubation temperature (4, 15 and 25 degrees C). The composition of the archaeal community was determined in the peat samples by terminal restriction fragment length polymorphism (T-RFLP) analysis and sequencing of amplified SSU rRNA gene fragments, and showed that members of Methanomicrobiaceae, Methanosarcinaceae and Rice cluster II (RC-II) were present. Other, presumably non-methanogenic archaeal clusters (group III, RC-IV, RC-V, RC-VI) were also detected. Fluorescent in situ hybridization (FISH) showed that the number of Bacteria decreased (from 24 x 10(7) to 4 x 10(7) cells per gram peat) with depth (from 5 to 55 cm below the water table), whereas the numbers of Archaea slightly increased (from 1 x 10(7) to 2 x 10(7) cells per gram peat). Methanosarcina spp. accounted for about half of the archaeal cells. Our results show that both hydrogenotrophic and acetoclastic methanogenesis are an integral part of the CH4-producing pathway in acidic peat and were represented by appropriate methanogenic populations.

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...

Methanogenic degradation of organic matter occurs in a wide temperature range from psychrophilic to extreme thermophilic conditions. Mesophilic and thermophilic methanogenesis is relatively well investigated, but little is known about low temperature methanogenesis and psychrophilic methanogenic communities. The aim of the present work was to study methanogenesis in a wide range of temperatures with samples from sediments of deep lakes. These sediments may be considered deposits of different types of microorganisms, which are constantly exposed to low temperatures. The main question was how psychrophilic methanogenic microbial communities compare to mesophilic and thermophilic ones. Methanogenesis in a temperature range of 2–70°C was investigated using sediment samples from Baldegger lake (65 m) and Soppen lake (25 m), Switzerland. Methane production from organic matter of sediments occurred at all temperatures tested. An exponential dependence of methane production rate was found between 2 and 30°C. Methanogenesis occurred even at 70°C. At the same time stable methane production from organic matter of sediments was observed at temperatures below 10°C. Methanogenic microbial communities were enriched at different temperatures. The communities enriched at 4–8°C had the highest activity at low temperatures indicating that a specific psychrophilic community exists. Addition of substrates such as cellulose, volatile fatty acids (butyrate, propionate, acetate), methanol and H2/CO2 stimulated methane production at all temperatures. H2/CO2 as well as methanol were directly converted to methane under thermophilic conditions. At low temperatures these substrates were converted to methane by a two-step process. First acetate was formed, followed by methane production from acetate. When acetate concentrations were high, acetoclastic methanogenesis was inhibited at low temperatures. This reaction appears to be one of the “bottle neck” in psychrophilic methanogenesis.

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Methanogens and homoacetogens compete for available H2 in anoxic environments. The competitiveness of these microorganisms was studied by measuring H2 consumption kinetics (Vmax, Km, threshold) in different psychroactive strains as function of temperature. Methanogenic strains MSB and MSP and homoacetogenic Acetobacterium bakii, A. paludosum, A. fimetarium, A. tundrae, which were isolated from different low‐temperature environments, were all able to grow and consume H2 in a temperature range of 4–30°C. The H2 thresholds steadily decreased with decreasing temperature in cultures of A. bakii, A. tundrae and strain MSB. In A. paludosum, A. fimetarium and especially in strain MSP, however, H2 thresholds again increased below 10–15°C. With exception of strain MSP at ≤10°C, H2 thresholds were generally lower in the methanogens (<2 Pa H2) than homoacetogens (<200 Pa H2). The measured thresholds decreased in parallel to those predicted from thermodynamic theory, and thus allowed the calculation of a critical Gibbs’ free energy required for H2 consumption, i.e. approximately −5 to −8 kJ mol−1, being similar for methanogens and homoacetogens. Vmax increased with temperature. The increase was more pronounced in the methanogenic strains than in A. bakii, but the values of the latter were generally higher. Km also increased with temperature and was higher in A. bakii (about 190–520 Pa H2) than in the methanogens (about 50–190 Pa H2). The values of H2 threshold, Vmax and Km, were used to compare the relative competitiveness of the different microorganisms over the entire temperature range using a kinetic model. A. bakii revealed a generally strong competitiveness for H2 at high H2 concentrations because of higher Vmax. It also outcompeted the methanogenic strain MSP at low H2 and low temperature. However, the capacity of the methanogens to compete with A. bakii for H2 increased with decreasing H2 concentration because of more favorable Km and threshold. In the methanogenic strain MSB, the H2 thresholds were generally lower than those of the homoacetogens irrespective of the temperature, and thus it outcompeted A. bakii. Methanogenic strain MSP, on the other hand, was outcompeted by A. bakii at low temperature because of insufficient psychrotolerance.