Thermodynamics of propionate degradation in anoxic paddy soil from different rice-growing regions

Abstract

Rice-field soils from China, the Philippines and Italy were incubated under anoxic conditions at 30°C for 110 days. Propionate, acetate, methane, carbon dioxide and hydrogen were analyzed during the course of the experiment and used to calculate the actual Gibbs free energies (Δ G) of propionate degradation to acetate, bicarbonate and H 2. Hydrogen partial pressures and concentrations of propionate and acetate were high at the beginning of incubation so that propionate degradation was endergonic. After a few days, however, H 2 partial pressures decreased, presumably being consumed in the reduction of iron and sulfate, so that Δ G values of propionate degradation became rather negative (−23±5 kJ mol −1 propionate ). After 10–20 days of incubation, when Fe(III) and sulfate had been depleted and CH 4 production started, Δ G values of propionate degradation had increased to −15±4 kJ mol −1 propionate. Δ G values still further increased and finally reached −7±2 kJ mol −1 propionate. At this time, propionate concentrations were below the detection limit (<5 μM), acetate concentrations were constant at 30–120 μM, H 2 partial pressures had stabilized at about 2–5 Pa, and CH 4 was produced at a relatively constant rate. Microbial propionate degradation at Δ G values of −7 to −15 kJ mol −1 propionate requires a highly efficient system for ATP synthesis at minimum quantum yields of 1/4 ATP, demonstrating that propionate turnover in anoxic rice-field soil is thermodynamically tightly constrained.