Acetoclastic and hydrogenotrophic methane production and methanogenic populations in an acidic West-Siberian peat bog

Summary 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 δ13CH4 and δ13CO2 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°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°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°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 × 107 to 4 × 107 cells per gram peat) with depth (from 5 to 55 cm below the water table), whereas the numbers of Archaea slightly increased (from 1 × 107 to 2 × 107 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.