Selective inhibition of reactions involved in methanogenesis and fatty acid production on rice roots

Washed excised roots of rice (Oryza sativa) produced H-2, CH4 and fatty acids (millimolar concentrations of acetate: propionate, butyrate, micromolar concentrations of isovalerate, valerate) when incubated under anoxic conditions. Surface sterilization of the root material resulted in the inactivation of the production of CH4, a strong reduction of the production of fatty acids and a transient (75 h) but complete inhibition of the production of Hz. Radioactive bicarbonate was incorporated into CH4, acetate, propionate and butyrate. About 30-40% of the fatty acid carbon originated from CO2 reduction. In the presence of phosphate, CH4 was exclusively produced from H-2/CO2, since phosphate selectively inhibited acetoclastic methanogenesis. Acetoclastic methanogenesis was also selectively inhibited by methyl fluoride, while chloroform or 2-bromoethane sulfonate inhibited CH4 production completely. Production of CH4, acetate, propionate and butyrate from H-2/CO2 was always exergonic with Gibbs free energies < -20 kJ mol(-1) product. Chloroform inhibited the production of acetate and the incorporation of radioactive CO2 into acetate. Simultaneously, Hz was no longer consumed and accumulated, indicating that acetate was produced from H-2/CO2 Chloroform also resulted in increased production of propionate and butyrate whose formation from CO2 became more exergonic upon addition of chloroform. Nevertheless. the incorporation of radioactive CO into propionate and butyrate was inhibited by chloroform. The accumulation of propionate and butyrate in the presence of chloroform probably occurred by fermentation of organic matter, rather than by reduction of acetate and CO2. [U-C-14]Glucose was indeed converted to acetate, propionate, butyrate, CO2 and CH4. Radioactive acetate, CO2 and CH4 were also products of the degradation of [U-C-14]cellulose and [U-C-14]xylose. Addition of chloroform and methyl fluoride did not affect the product spectrum of [U-C-14]glucose degradation. The application of combinations of selective inhibitors may be useful to elucidate anaerobic metabolic pathways in mixed microbial cultures and natural microbial communities. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.