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

Abstract

Acetoclastic methanogens have been described to be inhibited at much lower concentrations of methyl fluoride, CH 3F, than H 2/CO 2-utilizing methanogens. Therefore, we tested whether CH 3F inhibition may be used to determine, in anoxic rice field soil, the contribution of H 2/CO 2-dependent methanogenesis to the total CH 4 production by comparing this technique with the incorporation of 14CO 2 into CH 4. In general, addition of 0.01–1% CH 3F to the gas phase resulted in an immediate partial inhibition of the total CH 4 production which lasted for at least 200 h. Inhibition increased with the logarithm of the initial CH 3F concentration up to about 0.2–0.6%. The initial CH 3F concentration slowly decreased with time, probably due to decomposition. CH 4 production sometimes completely recovered during the course of the experiment. The presence of CH 3F resulted in the accumulation of acetate, the final concentration of which was usually stoichiometrically related to the deficit in CH 4 production and increased with the initial CH 3F concentration. In some experiments, acetate accumulation was larger than expected from the CH 4 deficit and a substantial incorporation of 14CO 2 into acetate was observed. Hydrogen, on the other hand, was only slightly elevated in the presence of CH 3F. Addition of increasing CH 3F resulted in an increase of the percentage of H 2-dependent methanogenesis (measured by conversion of 14CO 2 to 14CH 4) demonstrating that acetoclastic methanogenesis was preferentially inhibited by CH 3F. However, the conversion of 14CO 2 to 14CH 4 was also slightly inhibited by CH 3F. Apparently, CH 3F inhibited the H 2-dependent methanogenesis to some extent, depending on the concentration of CH 3F applied. Indeed, the ratio between the residual CH 4 production rate and the fraction of CH 4 produced from 14CO 2 decreased with an increasing CH 3F concentration. A ratio of unity was obtained at initial CH 3F concentrations of 0.2–0.6% (58–174 μM). Both methods, i.e. inhibition using 0.5% CH 3F and conversion of 14CO 2 to 14CH 4, were applied to determine the temporal change of the contribution of H 2/CO 2-dependent methanogenesis to the total CH 4 production in two different batches of Italian rice 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 relatively constant contribution of H 2/CO 2-dependent methanogenesis of about 25–30% as soon as CH 4 was produced at a steady rate and H 2 partial pressures had stabilized at about 1.5–2.5 Pa.