Denitrification with methane as electron donor in oxygen-limited bioreactors

The microbial population from a reactor using methane as electron donor for denitrification under microaerophilic conditions was analyzed. High number s of aerobic methanotrophic bacteria (3 10(7) cells/ml) and high numbers of acetate-utilizing denitrifying bacteria (2 10(7) cells/ml) were detected, but only very low numbers of methanol-degrading denitrifying bacteria (4 10 (4) cells/ml) were counted. Two abundant acetate-degrading denitrifiers wer e isolated which, based on 16S rRNA analysis, were closely related to Mesor hizobium plurifarium (98.4% sequence similarity) and a Stenotrophomonas sp. (99.1% sequence similarity). A methanol-degrading denitrifying bacterium i solated from the bioreactor morphologically resembled Hyphomicrobium sp. an d was moderately related to H. vulgare (93.5% sequence similarity). The ini tial characterization of the most abundant methanotrophic bacterium indicat ed that it belongs to class II of the methanotrophs. "In vivo" C-13-NMR wit h concentrated cell suspensions showed that this methanotroph produced acet ate under oxygen limitation. The microbial composition of reactor material together with the NMR experiments suggest that in the reactor methanotrophs excrete acetate, which serves as the direct electron donor for denitrifica tion.