| Dehalococcoides |
Only "Known" Organism to completely dechlorinate PCE to etheneDehalococcoides spp. are hydrogen consuming, obligate halorespiring bacteria, who have received much attention due to their ability to utilize chlorinated ethenes as electron acceptors. Chlorinated ethenes including tetrachloroethene (PCE) and trichloroethene (TCE) were widely used as dry cleaning fluids and industrial solvents and are now prominent groundwater contaminants. Under anaerobic conditions, PCE and TCE can undergo sequential reductive dechlorination through the daughter products cis-dichloroethene (cis-DCE) and vinyl chloride (VC) to ethene (DiStefano et al., 1991; Freedman and Gossett, 1989). While a number of bacterial cultures including Dehalococcoides, Dehalobacter, Desulfitobacterium, and Desulfuromonas species capable of utilizing PCE and TCE as growth supporting electron acceptors have been isolated (Hollinger et al., 1993; Gerritse et al., 1996, 1999; Loffler et al., 1999; Krumholz et al., 1996; Krumholz 1997), the Dehalococcoides spp. may be the most important because they are the only bacterial group that has been isolated to date which is capable of complete reductive dechlorination of PCE to ethene (Maymo-Gatell, 1997). In fact, the presence of Dehalococcoides spp. has been associated with the full dechlorination to ethene at sites across North America and Europe (Hendrickson et al., 2002). Thus, qPCR monitoring of the abundance of Dehalococcoides spp. allows site managers to evaluate the feasibility of complete reductive dechlorination under MNA conditions and the effectiveness of biostimulation (electron donor addition) to promote growth of key reductive dechlorinating bacteria and enhance bioremediation.
The accumulation of the daughter products cis-DCE and VC termed “DCE stall” is relatively common at PCE/TCE sites especially under monitored natural attenuation (MNA) conditions. DCE stall is particularly problematic because VC is generally considered more carcinogenic than the parent compounds. Within the Dehalococcoides genus, the range of chlorinated ethenes metabolized and cometabolized varies by species and strain. For example, Dehalococcoides ethenogenes str. 195 metabolizes PCE, TCE, and cis-DCE and cometabolizes VC (Maymo-Gatell, 1997) to produce ethene. Conversely, Dehalococcoides sp. CBDB1 utilizes PCE and TCE but does not cometabolize additional chloroethenes (Adrian 2000). Therefore, additional qPCR assays targeting vinyl chloride reductase genes (bvcA and vcrA) were developed to more definitively confirm the potential for biodegradation of VC.
Although most research has focused on chlorinated ethenes, some Dehalococcoides species are also capable of reductive dechlorination of chlorinated aromatic hydrocarbons. Again, biodegradation of individual compounds and isomers varies by isolate, however, qPCR enumeration of Dehalococcoides may also provide valuable insight into the feasibility of bioremediation of chlorobenzenes, chlorophenols, and polychlorinated biphenyls (PCBs). For example, Dehalococcoides sp. CBDB1 has shown broad metabolic activity utilizing pentachlorophenol (PCP), tetrachlorophenols, trichlorophenols, hexachlorobenzene, pentachlorobenzene, and tetrachlorobenzenes as growth supporting electron acceptors. Dehalococcoides have also been detected in PCB impacted sediment and transformation of numerous PCBs containing 5 or more chlorine substituents has been demonstrated for strain CBDB1
For more information on Dehalococcoides please visit the following links Pollutions Engineering Article - Inside the Black Box Loeffler Laboratory at Georgia Tech |
