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<br /> pentachlorobenzene(PeCB),tetrachlorobenzene(TeCB)isomers,and trichlorobenzene(TCB)isomers has been well documented[29],
<br /> although biodegradation of individual compounds and isomers varies between isolates. For example,Dehalococcoides strain CBDB1
<br /> reductively dechlorinats HCB,PeCB,all three TeCB isomers,1,2,3-TCB,and 1,2,4-TCB[9,301. Dehalobium chlorocoercia DF-1 has been
<br /> shown to be capable of reductive dechlorination of HCB, PeCB, and 1,2,3,5-TeCB [31]. The dichlorobenzene (DCB) isomers and
<br /> chlorobenzene (CB)were considered relatively recalcitrant under anaerobic conditions. However,new evidence has demonstrated
<br /> reductive dechlorination of DCBs to CB and CB to benzene[32]with corresponding increases in concentrations of Dehalobacter spp.
<br /> [33].
<br /> Reductive Dechlorination-Chlorinated Phenols: Pentachlorophenol (PCP) was one of the most widely used biocides in the
<br /> U.S. and despite residential use restrictions, is still extensively used industrially as a wood preservative. Along with PCP, the
<br /> tetrachlorophenol and trichlorophenol isomers were also used as fungicides in wood preserving formulations. 2,4-Dichlorophenol
<br /> and 2,4,5-TCP were used as chemical intermediates in herbicide production(e.g. 2,4-D)and chlorophenols are known byproducts
<br /> of chlorine bleaching in the pulp and paper industry. While the range of compounds utilized varies by strain, some Dehalococ-
<br /> coides isolates are capable of reductive dechlorination of PCP and other chlorinated phenols. For example, Dehalococcoides strain
<br /> CBDB1 is capable of utilizing PCP, all three tetrachlorophenol (TeCP) congeners, all six trichlorophenol (TCP) congeners, and
<br /> 2,3-dichlorophenol(2,3-DCP).PCP dechlorination by strain CBDB1 produces a mixture of 3,5-DCP,3,4-DCP,2,4-DCP,3-CP,and 4-CP
<br /> [34]. In the same study,however,Dehalococcoides strain 195 dechlorinated a more narrow spectrum of chlorophenols which included
<br /> 2,3-DCP,2,3,4-TCP,and 2,3,6-TCP,but no other TCPs or PCP.Similar to Dehalococcoides,some species and strains of Desulfitobacterium
<br /> are capable of utilizing PCP and other chlorinated phenols. Desulfitobacterium hafniense PCP-1 is capable of reductive dechlorination
<br /> of PCP to 3-CP [35]. However,the ability to biodegrade PCP is not universal among Desulfitobacterium isolates. Desulfitobacterium
<br /> sp.strain PCE1 and D.chlororespirans strain Co23,for example,can utilize some TCP and DCP isomers,but not PCP for growth[2,36].
<br /> Reductive Dechlorination-Chlorinated Propanes: Dehalogenimonas is a recently described bacterial genus of the phylum Chlo-
<br /> roflexi which also includes the well-known chloroethene-respiring Dehalococcoides[23]. The Dehalogenimonas isolates characterized to
<br /> date are also halorespiring bacteria,but utilize a rather unique range of chlorinated compounds as electron acceptors including chlo-
<br /> rinated propanes (1,2,3-TCP and 1,2-DCP) and a variety of other vicinally chlorinated alkanes including 1,1,2,2-tetrachloroethane,
<br /> 1,1,2-trichloroethane,and 1,2-dichloroethane[23].
<br /> Aerobic-Chlorinated Ethene Cometabolism: Under aerobic conditions, several different types of bacteria including methane-
<br /> oxidizing bacteria (methanotrophs), and many benzene, toluene, ethylbenzene, xylene, and (BTEX)-utilizing bacteria can
<br /> cometabolize or co-oxidize TCE, DCE, and vinyl chloride [37]. In general, cometabolism of chlorinated ethenes is mediated
<br /> by monooxygenase enzymes with "relaxed' specificity that oxidize a primary (growth supporting) substrate (e.g. methane)
<br /> and co-oxidize the chlorinated compound (e.g.TCE). QuantArray®-Chlor provides quantification of a suite of genes encoding
<br /> oxygenase enzymes capable of co-oxidation of chlorinated ethenes including soluble methane monooxygenase (sMMO). Soluble
<br /> methane monooxygenases co-oxidize a broad range of chlorinated compounds [38-41] including TCE,cis-DCE,and vinyl chloride.
<br /> Furthermore, soluble methane monooxygenases are generally believed to support greater rates of aerobic cometabolism [40].
<br /> QuantArray®-Chlor also quantifies aromatic oxygenase genes encoding ring hydroxylating toluene monooxygenase genes (RMO,
<br /> RDEG), toluene dioxygenase (TOD) and phenol hydroxylases (PHE) capable of TCE co-oxidation [42-46]. TCE or a degradation
<br /> product has been shown to induce expression of toluene monooxygenases in some laboratory studies[43,471 raising the possibility
<br /> of TCE cometabolism with an alternative(non-aromatic)growth substrate. Moreover,while a number of additional factors must be
<br /> considered,recent research under ESTCP Project 201584 has shown positive correlations between concentrations of monooxygenase
<br /> genes (soluble methane monooxygenase, ring hydroxylating monooxygenases, and phenol hydroxylase) and the rate of TCE
<br /> degradation[48].
<br /> Aerobic-Chlorinated Ethane Cometabolism: While less widely studied than cometabolism of chlorinated ethenes, some chlori-
<br /> nated ethanes are also susceptible to co-oxidation.As mentioned previously,soluble methane monooxygenases(sMMO)exhibit very
<br /> relaxed specificity. In laboratory studies,sMMO has been shown to co-oxidize a number of chlorinated ethanes including 1,1,1-TCA
<br /> and 1,2-DCA[38,40].
<br /> Aerobic-Vinyl Chloride Cometabolism: Beginning in the early 1990s, numerous microcosm studies demonstrated aerobic ox-
<br /> idation of vinyl chloride under MNA conditions without the addition of exogenous primary substrates. Since then, strains of
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