Responses of methanogenic archaeal community to oxygen exposure in rice field soil.

Methanogens are regarded as strict anaerobes and hence sensitive to O2 exposure. It has been demonstrated that CH4 production and emission from rice field soil are substantially reduced when soil is drained or aerated even shortly. However, the response of different methanogenic populations to O2 stress remains unclear. Therefore, we determined CH4 production and structure of the methanogenic community in a Chinese rice field soil after short-term (24 h) and long-term (72 h) exposure to O2 under laboratory conditions. O2 stress strongly inhibited CH4 production, and the inhibitory effect increased with the duration of O2 exposure. O2 exposure also resulted in dramatic increase of ferric iron and sulfate concentrations. H2 partial pressures were significantly reduced, most probably due to the competitive consumption by iron and sulfate-reducing bacteria. However, substrate competition could not explain the inhibition of acetoclastic methanogenesis, since acetate accumulated after O2 exposure compared with the control. Quantitative (real-time) PCR analyses of both archaeal 16S rRNA and mcrA genes (coding for alpha subunit of the methyl coenzyme M reductase) revealed that growth of the methanogenic populations was suppressed after O2 exposure. However, terminal restriction fragment length polymorphism (T-RFLP) analyses of both 16S rDNA and 16S rRNA showed that the structure of the methanogenic archaeal community remained remarkably stable, and that acetoclastic Methanosarcinaceae were always dominant whether with or without O2 exposure. Thus, O2 stress apparently did not differentially affect the various methanogenic populations, but instead inhibited CH4 production by enabling competition, generally suppressing growth and differentially affecting existing enzyme activity.