Reductive dechlorination anaerobic microorganisms

In the environmental compartments, PCBs are degraded mainly hy microorganisms. Mono-, di-, and trichlorobiphenyls are broken down relatively quickly, whereas the more highly chlorinated PCBs are much more persistent. Highly chlorinated PCBs are reductively dechlorinated anaerobically with replacement of Cl by H, and the less-chlorinated products are subject to aerobic breakdown processes. The formation of ortho-dioh is thought to involve arene epoxides (Figure 11). 

Reductive Dehalogenases. Anaerobic microorganisms capable of reductive dechlorination of tetrachloroethylene, trichloroethylene, chlorinated phenols, and other chlorinated aromatics have recently been discovered and studied (10,53). The corrinoid reductase of Sulfurospirillum multivorans has been shown. 

The refractory nature of some pollutants, notably, persistent polyhalogenated compounds, has raised problems of bioremediation of contaminated sites (e.g., sediments and dumping sites). There has been interest in the identification, or the production by genetic manipulation, of strains of microorganisms that can metabolically degrade recalcitrant molecules. For example, there are bacterial strains that can reductively dechlorinate PCBs under anaerobic conditions. 

Under anaerobic conditions, p,p -DDT is converted to p,p -DDD by reductive dechlorination, a biotransfonnation that occurs postmortem in vertebrate tissues such as liver and muscle and in certain anaerobic microorganisms (Walker and Jefferies 1978). Reductive dechlorination is carried out by reduced iron porphyrins. It is carried out by cytochrome P450 of vertebrate liver microsomes when supplied with NADPH in the absence of oxygen (Walker 1969 Walker and Jefferies 1978). Reductive dechlorination by hepatic microsomal cytochrome P450 can account for the relatively rapid conversion of p,p -DDT to p,p -DDD in avian liver immediately after death, and mirrors the reductive dechlorination of other organochlorine substrates (e.g., CCI4 and halothane) under anaerobic conditions. It is uncertain to what extent, if at all, the reductive dechlorination of DDT occurs in vivo in vertebrates (Walker 1974). 

Mikesell MD, S A Boyd (1986) Complete reductive dechlorination and mineralization of pentachlorophenol by anaerobic microorganisms. Appl Environ Microbiol 52 861-865. 

Reductive dechlorination of PCBs is important because it reduces their potential toxicity and persistence. In situ dechlorination of PCBs attributed to microorganisms in the anaerobic sediments has been documented in the Hudson River, Silver Lake (MA), the St. Lawrence River (NY), and New Bedford Harbor (MA) [76,371,451,454-456]. 

Quensen, J.F. Ill, Tiedje, J.M. Boyd, S. A. (1988). Reductive dechlorination of polychlorinated biphenyls by anaerobic microorganisms from sediments. Science, 242, 752-4. 

Van Dort, H. Bedard, D. L. (1991)- Reductive ortho and meta dechlorination of polychlorinated biphenyl congeners by anaerobic microorganisms. Applied and Environmental Microbiology, 57, 1576-8. 

DDT can be reductively dechlorinated to TDE (DDD) by anaerobic liver plus NADPH, dead tissues, and some microorganisms (Figure 8.17). It is not known whether the reaction proceeds enzymatically or nonenzymatically. 

Possible Mechanism for Reductive Dechlorination by Anaerobic Microorganisms 6-5. 1998 Pish Advisories for Polychlorinated Biphenyls... 

Van Doit HM, Bedard DL. 1991. Reductive ortho and meta-dechlorination of a polychlorinated biphenyl congener by anaerobic microorganisms. Appl Environ Microbiol 57 1576-1578... 

DDD is the major product when DDT is incubated anaerobically with microorganisms. This reductive dechlorination has occurred in many but not all of the aerobic studies. Aerobacter aerogenes has been studied aerobically and anaerobically in parallel experiments. In some cases the reaction was similar under both conditions (16 17), but it was suggested that anaerobic conditions increased the yield of DDD (18), In contrast... 

Another promising approach for the detoxification of PCBs is the finding that anaerobic bacteria dechlorinate PCBs reductively [79, 80]. The authors used anaerobic microorganisms from Hudson River sediment and report that, at PCB concentrations of 700 ppm Aroclor, 63 per cent of the total chlorine was removed in 16 weeks, and the proportion of mono- and dichlorobiphenyls increased from 9 to 88 per cent. Dechlorination occurred primarily from the meta and para positions. These results indicate that reductive dechlorination may be an important environmental fate of PCBs, and suggest that a sequential anaerobic-aerobic biological treatment system for PCBs may be feasible. The proton source for the microbial reductive dechlorination of 2,3,4,5,6-pentachlorobiphenyl has been identified by Nies and Vogel [81]. Tlie authors report that the exact mechanism of the electron transfer for the dechlorination of PCBs is imknown however, they could show that the sotirce of tiie hydrogen atom is the proton from water, and that chloride is released from the PCB. 

Herbicides, 2,4-D and 2,4,5-T are structurally related, the latter having an extra chlorine atom at position 5. Unlike 2,4-D, 2,4,5-T is poorly biodegradable, and persists for long periods, hence constituting a pollution problem [155]. Cometabolism of 2,4,5-T by Brevibacterium sp. resulted in the formation of the product, tentatively identified as 3,5-dichlorocatechol. Bacterial cometabohsm of 2,4,5-T was also described by Rosenberg and Alexander [156], who proposed a degradation pathway of 2,4,5-T in soil [157]. Reductive dechlorination of 2,4,5-T by anaerobic microorganisms was described by Suflita et al. [130]. 

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