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Tetrachloroethylene (PCE) is a chlorinated aliphatic compound essentially used as a degreasing and drycleaning solvent. It is commonly found as a groundwater contaminant and because of its carcinogenic properties is considered a pollutant. Biotransformation of PCE in ethane or ethylene has been studied by many researchers at low concentrations under strict anaerobic conditions (De Bruin et al., 1992;Freedman and Gossett, 1989).The dechlorination process was observed in methanogenic, acetogenic, or sulfate-reducing habitats (Holliger and Schraa, 1994;Mohn and Tiedje, 1992). In anaerobic PCE-enrichment culture, methanogenesis and acetogenesis disappeared with high concentrations of PCE (Maymo-Gatell et al., 1995). Ecological understanding of such communities will probably be important in using dehalogenation in bioremediation processes (Mohn and Tiedje, 1992).A methanogenic and sulfate-reducing consortium degrading PCE was obtained in our laboratory from anaerobic-digested sludge from a wastewater treatment plant (Bourg-en-Bresse, France). Initially, high concentrations of PCE were dechlorinated in trichloroethylene (TCE) by methanogenic fermentation in batch conditions. The sulfate-reducing bacteria were not involved in PCE dehalogenation (Cabirol et al., 1996). This methanogenic and sulfate-reducing consortium was then cultivated continually in the presence of high concentrations of PCE. The methanogenic bacteria always had a deciding role in the dechlorination process after this enrichment period. The pollutant was completely degraded to carbon biomass and CO 2 . PCE dehalogenation has been studied in a semicontinuous fixed-bed reactor, revealing an attractive method for removing high concentrations of PCE in bioremediation processes (Cabirol et al., 1998 A methanogenic and sulfate-reducing consortium, which was enriched on medium containing tetrachloroethylene (PCE), had the ability to dechlorinate high concentrations of PCE. Dehalogenation was due to the direct activity of methanogens. However, interactions between methanogenic and sulfate-reducing bacteria involved modification of the dechlorination process according to culture conditions. In the absence of sulfate, the relative percentage of electrons used in PCE dehalogenation increased after an addition of lactate in batch conditions. The sulfate reducers would produce further reductant from lactate catabolism. This reductant might be used by methanogenic bacteria in PCE dechlorination. A mutualistic interaction was observed in the absence of sulfate. However in the presence of sulfate, methanogenesis and dechlorination decreased because of interspecific competition, probably between the H 2 -oxydizing methanogenic and sulfate-reducing bacteria in batch conditions. In the semicontinuous fixed-bed reactor, the presence of sulfate did not affect dechlorination and methanogenesis. The sulfate-reducing bacteria may not be competitors of H 2 -consuming methanogens in the reactor because of the existence of microbial biofilm. The presence of the fixed film may ...

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