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

Abstract

Washed excised roots of rice (Oryza sativa) produced H(2), CH(4) 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 CH(4), a strong reduction of the production of fatty acids and a transient (75 h) but complete inhibition of the production of H(2). Radioactive bicarbonate was incorporated into CH(4), acetate, propionate and butyrate. About 20-40% of the fatty acid carbon originated from CO(2) reduction. In the presence of phosphate, CH(4) was exclusively produced from H(2)/CO(2), since phosphate selectively inhibited acetoclastic methanogenesis. Acetoclastic methanogenesis was also selectively inhibited by methyl fluoride, while chloroform or 2-bromoethane sulfonate inhibited CH(4) production completely. Production of CH(4), acetate, propionate and butyrate from H(2)/CO(2) was always exergonic with Gibbs free energies <-20 kJ mol(-1) product. Chloroform inhibited the production of acetate and the incorporation of radioactive CO(2) into acetate. Simultaneously, H(2) was no longer consumed and accumulated, indicating that acetate was produced from H(2)/CO(2). Chloroform also resulted in increased production of propionate and butyrate whose formation from CO(2) became more exergonic upon addition of chloroform. Nevertheless, the incorporation of radioactive CO(2) 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 CO(2). [U-(14)C]Glucose was indeed converted to acetate, propionate, butyrate, CO(2) and CH(4). Radioactive acetate, CO(2) and CH(4) were also products of the degradation of [U-(14)C]cellulose and [U-(14)C]xylose. Addition of chloroform and methyl fluoride did not affect the product spectrum of [U-(14)C]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.