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A methanogenic consortium able to use 3-chlorobenzoic acid as its sole energy and carbon source was enriched from anaerobic sewage sludge. Seven bacteria were isolated from the consortium in monoor coculture. They included: one dechlorinating bacterium (strain DCB-1), one benzoate-oxidizing bacterium (strain BZ-2), two butyrate-oxidizing bacteria (strains SF-1 and NSF-2), two H2-consuming methanogens (Methanospirillum hungatei PM-1 and Methanobacterium sp. strain PM-2), and a sulfate-reducing bacterium (Desulfovibrio sp. strain PS-1). The dechlorinating bacterium (DCB-I) was a gram-negative, obligate anaerobe with a unique "collar" surrounding the cell. A medium containing rumen fluid supported minimal growth; pyruvate was the only substrate found to increase growth. The bacterium had a generation time of 4 to 5 days. 3-Chlorobenzoate was dechlorinated stoichiometrically to benzoate, which accumulated in the medium; the rate of dechlorination was ca. 0.1 pmol bacterium-' day-'. The benzoate-oxidizing bacterium (BZ-2) was a gram-negative, obligate anaerobe and could only be grown as a syntroph. Benzoate was the only substrate observed to support growth, and, when grown in coculture with M. hungatei, it was fermented to acetate and CH4. One butyrate-oxidizing bacterium (NSF-2) was a gram-negative, non-sporeforming, obligate anaerobe; the other (SF-1) was a gram-positive, sporeforming, obligate anaerobe. Both could only be grown as syntrophs. The substrates observed to support growth of both bacteria were butyrate, 2-DL-methylbutyrate, valerate, and caproate; isobutyrate supported growth of only the sporeforming bacterium (SF-1). Fermentation products were acetate and CH4 (from butyrate, isobutyrate, or caproate) or acetate, propionate, and CH4 (from 2-DLmethylbutyrate or valerate) when grown in coculture with M. hungatei. A mutualism among at least the dechlorinating, benzoate-oxidizing, and methane-forming members was apparently required for utilization of the 3-chlorobenzoate substrate. Many chlorinated organic compounds are not biodegraded under aerobic conditions, often because the chlorine substitutions prevent ring cleavage and thus subsequent dechlorination. Recently, however, certain chlorinated aromatic compounds have been shown to be dechlorinated in anaerobic habitats such as sediment, flooded soil, and digested sludge. These chemicals include chlorinated benzoates (5, 15, 16), chlorinated phenols (2, 3, 6, 12), and the pesticides diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] (1), Techlofthalam [N-(2,3-dichlorophenyl)-3,4,5,6-tetrachlorophthalmic acid] (7), chloronitrofen (4-nitrophenyl-2,4,6-trichlorophenyl ether) (19), and 2,4,5-trichlorophenoxyacetic acid (17). In all of these cases, chlorine is removed from the aromatic ring before ring cleavage, which is in contrast to aerobic metabolism of chloroaromatic compounds. Thus, this reaction-a reductive dechlorination-is of particular interest because it has the potential for making some of the highly chlorinated, serious pollutants less persistent and...

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Microorganisms of lake sediment and sewage sludge anaerobically metabolized halobenzoates by a novel pathway. The primary degradative event was loss of the aryl halide without the alteration of the aromatic ring. Dehalogenation required strict anaerobic conditions and depended on the halogen and position, but not the number of halogen substituents. A stable methanogenic bacterial consortium was enriched from sludge and found capable of dehalogenating and often mineralizing a variety of halobenzoates to CH(4) and CO(2). The results suggest that reductive dehalogenation of aromatics could be important in removal of some chlorinated xenobiotics from the environment.