Methanogenesis from main methane precursors H(2)/CO(2) and acetate was investigated in a temperature range of 2-70 degrees C using sediments from Lake Baldegg, Switzerland. Psychrophilic, psychrotrophic, mesophilic, and thermophilic methanogenic microbial communities were enriched by incubations for 1-3 months of nonamended sediment slurries at 5, 15, 30, and 50 degrees C. Isotope experiments with slurries amended with (14)C-labeled bicarbonate and (14)C-2-acetate showed that in the psychrophilic community (enriched at 5 degrees C), about 95% of methane originated from acetate, in contrast to the thermophilic community (50 degrees C) where up to 98% of methane was formed from bicarbonate. In the mesophilic community (30 degrees C), acetate was the precursor of about 80% of the methane produced. When the hydrogen-carbon dioxide mixture (H(2)/CO(2)) was used as a substrate, it was directly converted to methane under thermophilic conditions (70 and 50 degrees C). Under mesophilic conditions (30 degrees C), both pathways, hydrogenotrophic and acetoclastic, were observed. At low temperatures (5 and 15 degrees C), H(2)/CO(2) was converted into methane by a two-step process; first acetate was formed, followed by methane production from acetate. When slurries were incubated at high partial pressures of H(2)/CO(2), the high concentrations of acetate produced of more than 20 mM inhibited acetoclastic methanogenesis at a temperature below 15 degrees C. However, slow adaptation of the psychrophilic microbial community to high acetate concentrations was observed.