Dichloromethane (DCM) as the sole substrate supported growth of a Dehalobacter sp. in an enrichment culture derived from noncontaminated river sediment. DCM was not reductively dechlorinated, and acetate was produced, indicating DCM fermentation and further suggesting Dehalobacter growth is not limited to organohalide respiration.T he chlorinated methanes carbon tetrachloride (CT), chloroform (CF), and dichloromethane (DCM) are common groundwater contaminants (23). No microbes capable of metabolizing CT have been reported, but the interaction of CT with electron transfer-active biomolecules can yield the trichloromethyl radical, which leads to unspecific transformation reactions and, consequently, CF formation (22). CF can be cometabolically transformed under sulfate-reducing and methanogenic conditions (11,30), and a recent report demonstrated growth of a Dehalobacter sp. linked to reductive dechlorination of CF to DCM and inorganic chloride as end products (8). This finding suggested that CF organohalide respiration provides an additional source of DCM in anoxic environments. Under oxic conditions, DCM is readily degraded by methylotrophic bacteria harboring glutathione-dependent DCM dehalogenases (3,15,29); however, the fate of DCM in anoxic habitats is poorly understood. A few studies have demonstrated DCM degradation under denitrifying (7), acetogenic (5, 19), and methanogenic (6) conditions, and DCM fermentation was observed in a packed-bed digester sludge reactor (4). The latter study suggested that methanogens, sulfate reducers, and nitrate reducers were not directly responsible for DCM degradation. To date, Dehalobacterium formicoaceticum is the only organism described to anaerobically metabolize DCM via a fermentative pathway to acetate, formate, and inorganic chloride (18,20).Dehalobacter spp. are characterized as strictly organohaliderespiring organisms, and no other growth-supporting substrates or substrate combinations could be identified (21, 26). Dehalobacter spp. respire chlorinated ethenes (14), chlorinated ethanes (9, 27), 4,5,6,7-tetrachlorophthalide (31), -hexachlorocyclohexane (28), and CF (8) and recently have been implicated in reductive dechlorination of dichlorobenzenes and monochlorobenzene (24).Pristine freshwater sediment was collected from Rio Mameyes in Luquillo, Puerto Rico, in October 2009 (latitude 18°21=43.9Љ, longitude Ϫ65°46=10Љ). Microcosms were established as described previously (13) with the following modifications. A pipettable slurry (0.2 g [dry weight] ml Ϫ1 ) was prepared by mixing the sediment with anoxic, bicarbonate-buffered (30 mM, pH 7.2) and HEPES-buffered (10 mM, pH 7.2) mineral salts medium (17). Inside an anoxic chamber (Coy, Ann Arbor, MI) filled with 3% H 2 -97% N 2 (vol/vol), 12-ml aliquots were dispensed into sterile 24-ml (nominal capacity) glass vials. Each microcosm received 20 mg liter Ϫ1 DCM (Ϸ128 M aqueous phase concentration), and triplicate microcosms were incubated statically at room temperature in the dark. DCM and chloromethane (CM) were monit...