ROLE OF INTERSPECIES H-2 TRANSFER TO SULFATE AND FERRIC IRON-REDUCINGBACTERIA IN ACETATE CONSUMPTION IN ANOXIC PADDY SOIL

Addition of sulfate resulted in complete inhibition of methanogenesis in anoxic paddy soil. About 20% of the CH4 was produced from H-2/(CO2) -C-14, the rest from-acetate. Inhibition of H-2-dependent methanogenes is was explained by successful competition by sulfate reducers for H-2 , as the H-2 partial pressures decreased upon addition of sulfate. How ever, acetate concentrations did not decrease. Sulfate reduction was s timulated by H-2, but not by acetate. Counts of acetate-utilizing sulf ate reducers were relatively low both in fresh and pasteurized soil in dicating that the bacteria were only present as spores. Inhibition of methanogenesis by chloroform resulted in accumulation of both H-2 and acetate. When sulfate was added in addition, H-2 accumulation stopped, but acetate still accumulated indicating that the activity of the met hanogens was necessary for acetate conversion and that acetate could n ot be utilized by the sulfate reducers directly. Conversion of [2-C-14 ]acetate resulted in formation of relatively more (CO2)-C-14, when sul fate was added, indicating that the methyl group of acetate was now be ing oxidized instead of reduced. Addition of chloroform strongly inhib ited the conversion of [2-C-14]acetate, even in the presence of sulfat e. A conceivable explanation is sulfate-dependent interspecies H-2 tra nsfer between acetate-utilizing methanogens and H-2-utilizing sulfate reducers, changing the electron now from CH4 production to sulfate red uction. Addition of ferrihydrite resulted only in incomplete inhibitio n of methanogenesis which could be explained by successful competition of ferric iron reducers for H-2. The ferric iron reducers were able t o use acetate directly, but did not outcompete the methanogens.