Tetrachloroethene is a prominent groundwater pollutant that can be reductively dechlorinated by mixed anaerobic microbial populations to the nontoxic product ethene. Strain 195, a coccoid bacterium that dechlorinates tetrachloroethene to ethene, was isolated and characterized. Growth of strain 195 with H2 and tetrachloroethene as the electron donor and acceptor pair required extracts from mixed microbial cultures. Growth of strain 195 was resistant to ampicillin and vancomycin; its cell wall did not react with a peptidoglycan-specific lectin and its ultrastructure resembled S-layers of Archaea. Analysis of the 16S ribosomal DNA sequence of strain 195 indicated that it is a eubacterium without close affiliation to any known groups.

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A biological process for remediation of groundwater contaminated with tetrachloroethylene (PCE) and trichloroethylene (TCE) can only be applied if the transformation products are environmentally acceptable. Studies with enrichment cultures of PCE-and TCE-degrading microorganisms provide evidence that, under methanogenic conditions, mixed cultures are able to completely dechlorinate PCE and TCE to ethylene, a product which is environmentally acceptable. Radiotracer studies with [14C]PCE indicated that ['4C]ethylene was the terminal product; significant conversion to 14Co2 or 14CH4 was not observed. The rate-limiting step in the pathway appeared to be conversion of vinyl chloride to ethylene. To sustain reductive dechlorination of PCE and TCE, it was necessary to supply an electron donor; methanol was the most effective, although hydrogen, formate, acetate, and glucose also served. Studies with the inhibitor 2-bromoethanesulfonate suggested that methanogens played a key role in the observed biotransformations of PCE and TCE.

Previous studies indicated that dechlorinators can utilize H 2 at lower concentrations than can methanogens. This suggests a strategy for selective enhancement of dechlorinationsmanaging H 2 delivery so as to impart a competitive advantage to dechlorinators. Four H 2 donorssbutyric and propionic acids, which can only be fermented when the H 2 partial pressure is lower than 10 -3.5 or 10 -4.4 atm, respectively, and ethanol and lactic acid, which are readily fermented at H 2 partial pressures 2-3 orders of magnitude higherswere administered to anaerobic mixed cultures. Comparison of the resulting enrichment cultures during time-intensive, short-term tests showed significant differences in patterns of donor degradation, H 2 production and use, and distribution of reduction equivalents between dechlorination and competing methanogenesis. Amendment with butyric and propionic acids resulted in less methanogenesis than did amendment with ethanol or lactic acid, which generated much higher H 2 levels. Ethanol did not support complete dechlorination during shortterm tests, but it was a viable donor over long-term testing because a portion was converted to a pool of slowly degraded propionic acid and because during long-term tests, cultures were routinely co-amended with pre-fermented yeast extract, a source of slowly fermented volatile fatty acids. Understanding the fate of electron donors and their fermentation products is an important component in understanding dechlorinating communities.