Turnover of propionate in methanogenic paddy soil

Abstract

Samples from planted Italian paddy soil exhibited most probable numbers (MPN) of about 10 7 anaerobic propionate utilizers. In anoxic soil slurries that were either unamended or amended with rice straw production of CH 4 was measured together with concentrations of H 2, acetate and propionate. After a lag phase, during which ferric iron was depleted, CH 4 was produced at a constant rate which was slightly higher in the straw-amended than in the unamended soil. Propionate concentrations were relatively low at about 5–15 μM. However, in the straw-amended soil propionate transiently accumulated to about 35 μM just after onset of methanogenesis. During the period of propionate accumulation H 2 partial pressures were elevated and the Gibbs free energy (ΔG) of propionate consumption to acetate, bicarbonate and H 2 was endergonic or higher than −3 kJ mol −1 propionate. Propionate concentrations decreased again when the ΔG decreased to more negative values. In unamended paddy soil, propionate did not accumulate transiently and ΔG was always <−6 kJ mol −1 propionate. Propionate radiolabelled in the C-1 or C-2 position was utilized with turnover times of 30–60 min. Propionate turnover rates approximately accounted for the rates of H 2/CO 2-dependent methanogenesis that were measured in experiments with [ 14C]bicarbonate. The only radioactive product of [1- 14C]propionate was 14CO 2. However, [2- 14C]propionate was converted to radioactive acetate, CO 2 and CH 4. This observation indicates that propionate was consumed via a randomizing pathway to CO 2 and acetate, the latter being then further degraded by acetotrophic methanogens to CO 2 and CH 4. Turnover of [1- 14C]propionate was almost completely inhibited by high H 2 concentrations, chloroform or molybdate. The MPN of bacteria that utilized propionate either in syntrophy with methanogens or by reduction of sulfate was identical. All these observations suggest that propionate was consumed by a syntrophic randomizing pathway, probably by bacteria that have also the capacity to reduce sulfate.