Intermediary metabolism in methanogenic paddy soil and the influence of temperature

Abstract

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 CHCl3, or inhibition of H2-producing syntrophic bacteria by increased partial pressures of H2 (0.2 bar) all resulted in the accumulation of metabolic intermediates. The temperature shift to 15°C resulted in a decrease of the CH4 production rate and of the H2 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, H2, propionate, caproate, lactate, and iso-propanol at both 15°C and 30°C. Addition of H2 resulted in the accumulation of propionate, caproate, lactate, formate and iso-propanol at both temperatures. The added H2 was consumed, together with CO2, 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 H2-producing reactions that was larger than that of the H2-consuming reactions. Addition of chloroform, and even more so of H2, also resulted in increased Gibbs free energies of H2-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 H2-producing reactions (presumably syntrophic bacteria) were more sensitive to low temperature than the H2-consuming reactions, and that H2 consumption by methanogenesis was more sensitive than H2 consumption by homoacetogenesis.