Effects of O-2 and CH4 on presence and activity of the indigenous methanotrophic community in rice field soil

The activity and distribution of methanotrophs in soil depend on the availa bility of CH4 and O-2. Therefore, we investigated the activity and structur e of the methanotrophic community in rice field soil under four factorial c ombinations of high and low CH4 and O-2 concentrations. The methanotrophic population structure was resolved by denaturant gradient gel electrophoresi s (DGGE) with different PCR primer sets targeting the 16S rRNA gene, and tw o functional genes coding for key enzymes in methanotrophs, i.e. the partic ulate methane monooxygenase (pmoA) and the methanol dehydrogenase (mxaF), C hanges in the biomass of type I and II methanotrophic bacteria in the rice soil were determined by analysis of phospholipid-ester-linked fatty acid (P LFA) biomarkers. The relative contribution of type I and II methanotrophs t o the measured methane oxidation activity was determined by labelling of so il samples with (CH4)-C-14 followed by analysis of [C-14]-PLFAs. CH4 oxidat ion was repressed by high O-2 (20.5%), and enhanced by low O-2 (1%). Depend ing on the CH4 and O-2 mixing ratios, different methanotrophic communities developed with a higher diversity at low than at high CH4 concentration as revealed by PCR-DGGE, However, a prevalence of type I or II populations was not detected, The [C-14]-PLFA fingerprints, on the other hand, revealed th at CH4 oxidation activity was dominated by type I methanotrophs in incubati ons with low CH4 mixing ratios (1000 p.p.m.v.) and during initiation of CH4 consumption regardless of O-2 or CH4 mixing ratio. At high methane mixing ratios (10 000 p.p.m.v.), type I and II methanotrophs contributed equally t o the measured CH4 metabolism. Collectively, type I methanotrophs responded fast and with pronounced shifts in population structure and dominated the activity under all four gas mixtures. Type II methanotrophs, on the other h and, although apparently more abundant, always present and showing a largel y stable population structure, became active later and contributed to CH4 o xidation activity mainly under high CH4 mixing ratios.