ELSEVIER FEMS Microbiology Ecology I8 (1995) 85- 102 Intermediary metabolism in methanogenic paddy soil and the influence of temperature

The intermediary metabolism in methanogenic rice paddy soil was studied by slurry incubation experiments at low (15°C) and high (30°C) temperatures. A shift to a low temperature, inhibition of methanogenesis by the addition of CHCI,, or inhibition of Hz-producing syntrophic bacteria by increased partial pressures of H, (0.2 bar) all resulted in the accumulation of metabolic intermediates. The temperature shift to 15°C resulted in a decrease of the CH, production rate and of the H, and CO, partial pressures, and resulted in the transient accumulation of acetate, propionate, caproate, lactate, and iso-propanol. Chloroform inhibited methanogenesis and resulted in the accumulation of acetate, H,, propionate, caproate, lactate, and iso-propanol at both 15°C and 30°C. Addition of H a resulted in the accumulation of propionate, caproate, lactate, formate and iso-propanol at both temperatures. The added Ha was consumed, together with CO>, mainly by methanogenesis at 30°C. but mainly by homoacetogenesis at 15°C. A decrease in temperature caused an increase (less exergonic) of the Gibbs free energy of the H,-producing reactions that was larger than that of the Ha-consuming reactions. Addition of chloroform, and even more so of H,, also resulted in increased Gibbs free energies of Hz-producing reactions, thus explaining why the intermediates detected did accumulate. The metabolites that accumulated when methanogenesis was inhibited by chloroform largely (766108%) accounted for the missing CH,. Carbon flow through acetate contributed 79983 % of the total carbon flow to CH,. Comparison of the relative amounts of accumulated intermediates indicates that the Hz-producing reactions (presumably syntrophic bacteria) were more sensitive to low temperature than the Hz-consuming reactions, and that H, consumption by methanogenesis was more sensitive than H2 consumption by homoacetogenesis