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

Rice-field soils from China, the Philippines and Italy were incubated under anoxic conditions at 30 degreesC for 110 days. Propionate, acetate, methan e, carbon dioxide and hydrogen were analyzed during the course of the exper iment and used to calculate the actual Gibbs free energies (DeltaG) of prop ionate degradation to acetate, bicarbonate and H-2. Hydrogen partial pressu res and concentrations of propionate and acetate were high at the beginning of incubation so that propionate degradation was endergonic. After a few d ays, however, H-2 partial pressures decreased, presumably being consumed in the reduction of iron and sulfate, so that DeltaG values of propionate deg radation became rather negative (-23 +/- 5 kJ mol(-1) propionate). After 10 -20 days of incubation, when Fe(III) and sulfate had been depleted and CH4 production started, DeltaG values of propionate degradation had increased t o -15 +/- 4 kJ mol(-1) propionate, DeltaG values still further increased an d finally reached -7 +/- 2 kJ mol(-1) propionate. At this time, propionate concentrations were below the detection limit (<5 <mu>M), acetate concentra tions were constant at 30-120 muM, H-2 partial pressures had stabilized at about 2-5 Pa, and CH4 was produced at a relatively constant rate. Microbial propionate degradation at DeltaG values of -7 to -15 kJ mol(-1) propionate requires a highly efficient system for ATP synthesis at minimum quantum yi elds of 1/4 ATP, demonstrating that propionate turnover in anoxic rice-fiel d soil is thermodynamically tightly constrained. (C) 2001 Elsevier Science Ltd. All rights reserved.