Evidence for anaerobic syntrophic acetate oxidation during methane production in the profundal sediment of subtropical Lake Kinneret (Israel)

Methane production was measured in samples of the profundal sediment from Lake Kinneret. Production rates of CH4 were higher at 30 degreesC than at the in situ temperature of 15 degreesC and were higher in the top 5 cm layer than below. Turnover of [2-C-14]-acetate resulted in the production of (CH4)-C-14 and (CO2)-C-14 with turnover times of <42 min. However, <30% of the added radioactivity was converted to gaseous products, indicating that only part of the acetate pool was microbially available. The calculated acetate turnover rates were sufficient to account for total CH4 production, indicating that CH4 was produced exclusively from acetate. This conclusion was confirmed by inhibition of methanogens with chloroform, which resulted in an almost stoichiometric accumulation of acetate. However, a large percentage (30-60%) of [2-C-14]-acetate was converted to (CO2)-C-14, despite lack of reducible sulphate or other oxidants in the sediment. Anoxic preincubation of the sediment did not result in reduced production of (CO2)-C-14. Therefore, part of the acetate must have been oxidized rather than methanogenically cleaved. Conversion of [C-14]-bicarbonate to (CH4)-C-14 indicated that 30-50% of total CH4 production originated from reduction Of CO2. To reconcile the relatively high contribution of H-2/CO2-dependent methanogenesis with the relatively high oxidative conversion of acetate, we assume that part of the acetate was used syntrophically by consortia of acetate-oxidizing bacteria and H-2/CO2-using methanogens. This conclusion is supported by favourable thermodynamic conditions for syntrophic acetate oxidation under in situ conditions and complete inhibition of [2-C-14]-acetate turnover at high H-2 partial pressures. Further evidence to support this conclusion comes from the analysis of the structure of the archaeal community. Terminal restriction fragment length polymorphism (T-RFLP) and partial sequence analysis of the SSU rRNA genes amplified from DNA extracts of the sediment showed Methanomicrobiaceae as the dominant methanogenic group, whereas acetoclastic methanogens could not be detected.