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 par tially depleted in endogenous electron donors by prolonged incubation under anaerobic conditions. Endogenous NO3- was reduced within 4 h, fo llowed by reduction of Fe3+ and SO42-, and later by production of CH4. Addition of NO3- slightly inhibited the production of Fe2+ in the dep leted 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 compo unds), indicating that these compounds served as electron donors for t he bacteria reducing NO3- and/or ferric iron. Addition on NO3- also in hibited the reduction of SO42- in the depleted paddy soil. This inhibi tion was only overcome by H-2, but not by acetate or a mixture of comp ounds, 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 reductio n of Fe3+ and/or SO42+ CH4 production was inhibited by NO3- both in de pleted and in fresh paddy soil. Fe3+ and SO42- also inhibited methanog enesis, but the inhibition was stronger in the depleted than in the fr esh paddy soil. Inhibition of CH4 production was paralleled by a decre ase 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 in dicate that in the presence of exogenous Fe3+ or SO42+, methanogenic b acteria were outcompeted for H-2 by bacteria reducing Fe3+ or SO42+.