COMPETITION FOR ELECTRON-DONORS AMONG NITRATE REDUCERS, FERRIC IRON REDUCERS, SULFATE REDUCERS, AND METHANOGENS IN ANOXIC PADDY SOIL

Slurries of anoxic paddy soil were either freshly prepared or were partially depleted in endogenous electron donors by prolonged incubation under anaerobic conditions. Endogenous NO3- was reduced within 4 h, followed by reduction of Fe3+ and SO42-, and later by production of CH4. Addition of NO3- slightly inhibited the production of Fe2+ in the depleted but not in the fresh paddy soil. Inhibition was overcome by the addition of H-2, acetate, or a mixture of fatty acids (and other compounds), indicating that these compounds served as electron donors for the bacteria reducing NO3- and/or ferric iron. Addition on NO3- also inhibited the reduction of SO42- in the depleted paddy soil. This inhibition was only overcome by H-2, but not by acetate or a mixture of compounds, indicating that H-2 was the predominant electron donor for the bacteria involved in NO3- and/or SO42- reduction. SO42- reduction was also inhibited by exogenous Fe3+, but only in the depleted paddy soil. This inhibition was overcome by either H-2, acetate, or a mixture of compounds, suggesting that they served as electron donors for reduction of Fe3+ and/or SO42+ CH4 production was inhibited by NO3- both in depleted and in fresh paddy soil. Fe3+ and SO42- also inhibited methanogenesis, but the inhibition was stronger in the depleted than in the fresh paddy soil. Inhibition of CH4 production was paralleled by a decrease in the steady state concentration of H-2 to a level which provided a free enthalpy of less than Delta G = -17 kJ mol(-1) CH4 compared to more than Delta G = -32 kJ mol(-1) CH4 in the control. The results indicate that in the presence of exogenous Fe3+ or SO42+, methanogenic bacteria were outcompeted for H-2 by bacteria reducing Fe3+ or SO42+.