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 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.