Turnover of propionate in methanogenic paddy soil

Article abstract of DOI:10.1016/S0168-6496(97)00017-2

Samples from planted Italian paddy soil exhibited most probable numbers (MPN) of about 10⁷ anaerobic propionate utilizers. In anoxic soil slurries that were either unamended or amended with rice straw production of CH₄ was measured together with concentrations of H₂, acetate and propionate. After a lag phase, during which ferric iron was depleted, CH₄ 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 H₂ partial pressures were elevated and the Gibbs free energy (ΔG) of propionate consumption to acetate, bicarbonate and H₂ 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 H₂/CO₂- dependent methanogenesis that were measured in experiments with [¹⁴C]bicarbonate. The only radioactive product of [1-¹⁴C]propionate was ¹⁴CO₂. However, [2-¹⁴C]propionate was converted to radioactive acetate. CO₂ and CH₄. This observation indicates that propionate was consumed via a randomizing pathway to CO₂ and acetate, the latter being then further degraded by acetotrophic methanogens to CO₂ and CH₄. Turnover of [1- ¹⁴C]propionate was almost completely inhibited by high H₂ 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 thai propionate was consumed by a syntrophic randomizing pathway, probably by bacteria that have also the capacity to reduce sulfate.