Effect of temperature on carbon and electron flow and on the archaeal community in methanogenic rice field soil

Temperature is an important factor controlling CH4 production in anoxic rice soils. Soil slurries, prepared from Italian rice field soil, were incubated anaerobically in the dark at six temperatures of between 10 to 37 degreesC or in a temperature gradient block covering the same temperature range at intervals of 1 degreesC. Methane production reached quasi-steady state after 60 to 90 days. Steady-state CH4 production rates increased with temperature, with an apparent activation energy of 61 kJ mol(-1). Steady-state partial pressures of the methanogenic precursor H-2 also increased with increasing temperature from <0.5 to 3.5 Pa, so that the Gibbs free energy change of H-2 plus CO2-dependent methanogenesis was kept at -20 to -25 kJ mol of CH4-1 over the whole temperature range. Steady-state concentrations of the methanogenic precursor acetate, on the other hand, increased with decreasing temperature from <5 to 50 <mu>M. Simultaneously, the relative contribution of H-2 as methanogenic precursor decreased, as determined by the conversion of radioactive bicarbonate to (CH4)-C-14, so that the carbon and electron flow to CH4 was increasingly dominated by acetate, indicating that psychro-tolerant homoacetogenesis was important. The relative composition of the archaeal community was determined by terminal restriction fragment length polymorphism (T-RFLP) analysis of the 16S rRNA genes (16S rDNA), T-RFLP analysis differentiated the archaeal Methanobacteriaceae, Methanomicrobiaceae, Methanosaetaceae, Methanosarcinaceae, and Rice clusters I, III, IV, V, and VI, which were all present in the rice field soil incubated at different temperatures. The 16S rRNA genes of Rice cluster I and Methanosaetaceae were the most frequent methanogenic groups. The relative abundance of Rice cluster I decreased with temperature. The substrates used by this microbial cluster, and thus its function in the microbial community, are unknown. The relative abundance of acetoclastic methanogens, on the other hand, was consistent with their physiology and the acetate concentrations observed at the different temperatures, i.e., the high-acetate-requiring Methanosarcinaceae decreased and the more modest Methanosaetaceae increased with increasing temperature. Our results demonstrate that temperature not only affected the activity but also changed the structure and the function (carbon and electron how) of a complex methanogenic system.