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O. TRESSE, F. MOUNIEN, M.-J. LEVESQUE AND S. GUIOT. 2004. Aims: To understand the microbial ecology underlying trichloethene (TCE) degradation in a coupled anaerobic/ aerobic single stage (CANOXIS) reactor oxygenated with hydrogen peroxide (H2O2) and in an upflow anaerobic sludge bed (UASB) reactor. Methods and Results: The molecular study of the microbial population dynamics and a phylogenetic characterization were conducted using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR- DGGE). In both reactors, TCE had a toxic effect on two uncultured bacterial populations whereas oxygen favoured the growth of aerobic species belonging to Rhizobiaceae and Dechloromonas. No methanotrophic bacteria were detected when targeting 16S rRNA gene with universal primers. Alternatively, pmo gene encoding the particulate methane monooxygenase of Methylomonas sp. LW21 could be detected in the coupled reactor when H2O2 was supplied at 0AE 7gO 2 l � 1 R day )1 . Conclusions: Methylomonas sp. LW21 that could be responsible for the aerobic degradation of the TCE by- products is not among the predominant bacterial populations in the coupled reactor. It seems to have been outcompeted by heterotrophic bacteria (Rhizobiaceae and Dechloromonas sp.) for oxygen. Significance and Impact of the Study: The results obtained show the limitations of the coupled reactor examined in this study. Further investigations should focus on the operating conditions of this reactor in order to favour the growth of the methanotrophs.