Microbial Insights

Known for the ability to convert 1,1,1 TCA to chloroethane

Dehalobacter is another genus of anaerobic bacteria capable of reductive dechlorination of chlorinated compounds including PCE and TCE.  Unlike most Dehalococcoides isolates that have been characterized, Dehalobacter pure cultures studied to date can utilize PCE and TCE but cannot metabolize or cometabolize the daughter products cis-DCE and VC.  While the range of chlorinated ethenes utilized appears relatively limited, members of the genus Dehalobacter have the somewhat unique ability to utilize chlorinated ethanes including 1,1,1-trichloroethane (1,1,1-TCA) and 1,1,2-trichloroethane (1,1,2-TCA).  TCAs were extensively used in industrial applications as degreasers (ToxFAQs) and are therefore common co-contaminants at PCE/TCE impacted sites.  The presence of 1,1,1-TCA is especially problematic at PCE impacted sites due to inhibition of reductive dechlorination of chlorinated ethenes particularly VC (Duhamel).  Under aerobic conditions, TCA can be co-oxidized by methane-, ammonia-, and butane-oxidizing bacteria (Field review).  Due to the presence of chlorinated ethanes as co-contaminants at PCE sites and the fact that impacted groundwater is often anoxic or anaerobic, we will focus on anaerobic mechanisms for biodegradation of chlorinated ethanes.

The range of chlorinated ethanes biodegraded by Dehalobacter varies by species and strain.  Dehalobacter restrictus strain TCA1 converts 1,1,1-TCA to chloroethane with the transient accumulation of 1,1-dichlorethane as an intermediate (Sun et al., 2002).  1,1,2-TCA did not serve as a growth supporting electron acceptor for strain TCA1, however, Grostern and Edwards attributed the dichloroelimination of 1,1,2-TCA to vinyl chloride to a Dehalobacter species.  The Grostern and Edwards study is particularly interesting because it was performed with mixed culture (Dehalococcoides sp. KB-1 and Dehalobacter sp. MS), chlorinated ethenes (TCE and daughter products), and 1,1,1-TCA.  In cultures containing both organisms, degradation of cis-DCE and VC to ethene by Dehalococcoides proceeded only after dechlorination of 1,1,1-TCA by Dehalobacter thus demonstrating important roles for both genera at PCE/TCE sites co-contaminated by TCA.