EVALUATION OF ALTERNATIVE SUBSTRATES FOR DETERMINING METHANE-OXIDIZING ACTIVITIES AND METHANOTROPHIC - POPULATIONS IN SOILS

The magnitude of methane emission is a net result of methane productio n and the oxidation rate. The possibility of measuring oxidized produc ts of alternative substrates of methane monooxygenase was examined to determine methane-oxidizing ability of soils, and to count methanotrop hic populations in soils. Wetland rice soils were incubated under meth ane containing air to enirch the methanotrophs. Methane loss and oxyge n uptake were inhibited by acetylene, dimethylether, and nitrapyrin (N -Serve). Acetylene was used routinely, because it inhibited methane ox idation even at a low concentration of 0.03 to 0.06 mu l ml(-1) in the incubation headspace. Propylene at 10 kPa was used as an alternative substrate of methane monooxygenase, and the formation of propylene oxi de was measured. When soils were incubated under methane, their methan e-oxidizing activity increased. Propylene oxide formation increased si multaneously. Acetylene also blocked propylene oxidation. The results of several experiments showed a significant correlation between methan e oxidation and propylene oxide formation (r = 0.87 after long-transfo rmation). These results indicate that propylene oxide formation can be used as a semiquantitative measure of the methane-oxidizing activity of soils. The colonies of soluble methane monooxygenase-forming methan otrophs were counted on Cu-deficient methanotroph agar medium by the f ormation of naphthol from naphthalene. The counts increased from 10(4) (0 days) to 10(7) (21 days) g(-1) soil during oxic incubation under m ethane.