How specific is the inhibition by methyl fluoride of acetoclastic methanogenesis in anoxic rice field soil?

Acetoclastic methanogens have been described to be inhibited at much lower concentrations of methyl fluoride, CH3F, than H-2/CO2-utilizing methanogens . Therefore, we tested whether CH3F inhibition may be used to determine, in anoxic rice field soil, the contribution of H-2/CO2-dependent methanogenes is to the total CH4 production by comparing this technique with the incorpo ration of (CO2)-C-14 into CH4. In general, addition of 0.01-1% CH3F to the gas phase resulted in an immediate partial inhibition of the total CH4 prod uction which lasted for at least 200 h. Inhibition increased with the logar ithm of the initial CH3F concentration up to about 0.2-0.6%. The initial CH 3F concentration slowly decreased with time, probably due to decomposition. CH4 production sometimes completely recovered during the course of the exp eriment. The presence of CH3F resulted in the accumulation of acetate, the final concentration of which was usually stoichiometrically related to the deficit in CH4 production and increased with the initial CH3F concentration . In some experiments, acetate accumulation was larger than expected from t he CH4 deficit and a substantial incorporation of (CO2)-C-14 into acetate w as observed. Hydrogen, on the other hand, was only slightly elevated in the presence of CH3F. Addition of increasing CH3F resulted in an increase of t he percentage of H-2-dependent methanogenesis (measured by conversion of (C O2)-C-14 to (CH4)-C-14) demonstrating that acetoclastic methanogenesis was preferentially inhibited by CH3F. However, the conversion of (CO2)-C-14 to (CH4)-C-14 was also slightly inhibited by CH3F. Apparently, CH3F inhibited the Hz-dependent methanogenesis to some extent, depending on the concentrat ion of CH3F applied. Indeed, the ratio between the residual CH4 production rate and the fraction of CH4 produced from (CO2)-C-14 decreased with an inc reasing CH3F concentration. A ratio of unity was obtained at initial CH3F c oncentrations of 0.2-0.6% (58-174 mu M). Both methods, i.e. inhibition usin g 0.5% CH3F and conversion of (CO2)-C-14 to (CH4)-C-14, were applied to det ermine the temporal change of the contribution of H-2/CO2-dependent methano genesis to the total CH4 production in two different batches of Italian ric e field soil during a 120-days anoxic incubation period. The results of the two methods agreed well within the error of the methods and showed a relat ively constant contribution of H-2/CO2-dependent methanogenesis of about 25 -30% as soon as CH4 was produced at a steady rate and H-2 partial pressures had stabilized at about 1.5-2.5 Pa. (C) 1999 Federation of European Microb iological Societies. Published by Elsevier Science B.V. All rights reserved .