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Abstract:
The intermediary metabolism in methanogenic rice paddy soil was studied by slurry incubation experiments at low (15 degrees C) and high (30 degrees C) temperatures. A shift to a low temperature, inhibition of methanogenesis by the addition of CHCl3, or inhibition of H-2-producing syntrophic bacteria by increased partial pressures of H-2 (0.2 bar) all resulted in the accumulation of metabolic intermediates. The temperature shift to 15 degrees C resulted in a decrease of the CH4 production rate and of the H-2 and CO2 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-2, propionate, caproate, lactate, and iso-propanol at both 15 degrees C and 30 degrees C. Addition of H-2 resulted in the accumulation of propionate, caproate, lactate, formate and iso-propanol at both temperatures. The added H-2 was consumed, together with CO2, mainly by methanogenesis at 30 degrees C, but mainly by homoacetogenesis at 15 degrees C. A decrease in temperature caused an increase (less exergonic) of the Gibbs free energy of the H-2-producing reactions that was larger than that of the H-2-consuming reactions. Addition of chloroform, and even more so of H-2, also resulted in increased Gibbs free energies of H-2-producing reactions, thus explaining why the intermediates detected did accumulate. The metabolites that accumulated when methanogenesis was inhibited by chloroform largely (76-108%) accounted for the missing CH4. Carbon flow through acetate contributed 79-83% of the total carbon flow to CH4. Comparison of the relative amounts of accumulated intermediates indicates that the H-2-producing reactions (presumably syntrophic bacteria) were more sensitive to low temperature than the H-2-consuming reactions, and that H-2 consumption by methanogenesis was more sensitive than H-2 consumption by homoacetogenesis.
