Turnover of glucose and acetate coupled to reduction of nitrate, ferric iron and sulfate and to methanogenesis in anoxic rice field soil

Turnover of glucose and acetate in the presence of active reduction of nitr ate, ferric iron and sulfate was investigated in anoxic rice field soil by using [U-C-14]glucose and [2-C-14]acetate. The turnover of glucose was not much affected by addition of ferrihydrite or sulfate, but was partially inh ibited (60%) by addition of nitrate. Nitrate addition also strongly reduced acetate production from glucose while ferrihydrite and sulfate addition di d not. These results demonstrate that ferric iron and sulfate reducers did not outcompete fermenting bacteria for glucose at endogenous concentrations . Nitrate reducers may have done so, but glucose fermentation may also have been inhibited by accumulation of toxic denitrification intermediates (nit rite, NO, N2O). Addition of nitrate resulted in complete inhibition of CH4 production from [U-C-14]glucose and [2-C-14]acetate. However, addition of f errihydrite or sulfate decreased the production of (CH4)-C-14 from [U-C-14] glucose by only 70 and 65%, respectively. None of the electron accepters si gnificantly increased the production of (CO2)-C-14 from [U-C-14]glucose, bu t all increased the production of (CO2)-C-14 from [2-C-14]acetate. Uptake o f acetate was faster in the presence of either nitrate, ferrihydrite or sul fate than in the unamended control. Addition of ferrihydrite and sulfate re duced (CH4)-C-14 production from [2-C-14]acetate by 83 and 92%, respectivel y. Chloroform completely inhibited the methanogenic consumption of acetate. It also inhibited the oxidation of acetate, completely in the presence of sulfate, but not in the presence of nitrate or ferrihydrite. Our results sh ow that, besides the possible toxic effect of products of nitrate reduction (NO, NO2- and N2O) on methanogens, nitrate reducers, ferric iron reducers and sulfate reducers were active enough to outcompete methanogens for aceta te and channeling the flow of electrons away from CH4 towards CO2 productio n. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.