The Reductive Dehalogenation of Chloroethenes, Dichlorophenol, and Chlorobenzenes
The improper disposal of chlorinated compounds has become an environmental concern and its remediation is a priority. Dehalococcoides ethenogenes strain 195 is able to reductively dehalogenate tetrachloroethene (PCE), trichloroethene (TCE) and 2,3-dichlorophenol (2,3-DCP) to less toxic products. Its genome contains 19 putative reductive dehalogenase (RD) genes including pceA (PCE RD) and tceA (TCE RD). Differential transcription was examined with strain 195 grown on PCE, TCE, or 2,3-DCP. Gene product and function was confirmed using proteomic analysis and resting cell assays. When grown on PCE or TCE, transcript levels of tceA, pceA and DET0162 were several folds higher than housekeeping gene, rpoB. DET1559 was only expressed in PCE-grown cells. In 2,3-DCP-grown cells, pceA and DET0162 were the only RD genes expressed and tceA transcript level was 300-fold lower than in PCE-grown cells. DET0162 is presumed nonfunctional as it contains a translational stop codon and was not detected in proteomic analysis. PceA was the only RD detected in 2,3-DCP-grown cells. PCE-grown resting cells were able to reductively dehalogenated 2,3-DCP and 2,3-DCP-grown resting cells 2,3-DCP without lag. PceA has been identified as the 2,3-DCP reductive dehalogenase. In the case of dichlorobenzene (DCB) and monochlorobenzene (MCB), no microorganism has been identified capable of their reductive dehalogenation. From a historically chlorobenzene-contaminated site, sediment microcosms were used to enrich for DCB and MCB dehalogenators. Microcosms were able to reductively dehalogenate all three DCB isomers to MCB and benzene. If only given MCB, reductive dehalogenation to benzene only occurred in specific sediment samples and was considered unreliable. Inoculating microcosms with sediment slurries from dehalogenating-microcosms reduced the lag time of MCB reductive dehalogenation and allowed for reliable growth on MCB in the absence of DCBs. A mixed culture was established by transferring sediment slurry from a DCB-dehalogenating microcosm into mineral salts medium supplemented with yeast extract and vitamins. A 16S rRNA gene clone library of the mixed culture contained sequences >99% identical to Dehalobacter restrictus, a known dehalogenator. The Dehalobacter population, monitored by real time PCR, was linked to reductive dehalogenation of DCBs and 16S rRNA gene sequences account for 53% of the total bacterial sequences in the mixed culture.