Microbial Degradation

Soil samples containing PCBs have been found to contain microorganisms that metabolize the PCBs. Different degradative pathways have been observed which proceed in the presence and absence of molecular oxygen.

8.7.4.1 Aerobic Biodégradation

A variety of microbial types, most of which are members of the genus Pseudomonas, degrade PCBs in the presence of oxygen. They attack the more lightly chlorinated PCBs (mono- to tetrachlorinated) to give phenolic products by a pathway that is similar to the microbial oxidation of nonchlorinated aromatic compounds. The metabolic pathways used by these diverse microorganisms for PCB destruction are similar (Figure 8-3). This suggests that the degradation is catalyzed by the same group of enzymes and thus has the same genetic basis in all the microorganisms. Since it is unlikely that the same degradative pathway evolved separately in each genetic type, it appears likely that these unrelated microorganisms obtained the ability to degrade PCBs by gene transfer. These mobile genes have made it possible for microorganisms living in the presence of PCBs to gain a selective advantage over the others by utilizing the energy and carbon present in PCBs for their own metabolic processes.

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bphC

FIGURE 8-3 Degradation of biphenyl (bph) and chlorobiphenyls (clx) by the 2,3-dioxygenase pathway in Pseudomonas strain LB400. Redrawn from F. J. Mondello, J. Bacteriol., 171, 1725 (1989). Used by permission of the American Society for Microbiology.

bphC

Clx

FIGURE 8-3 Degradation of biphenyl (bph) and chlorobiphenyls (clx) by the 2,3-dioxygenase pathway in Pseudomonas strain LB400. Redrawn from F. J. Mondello, J. Bacteriol., 171, 1725 (1989). Used by permission of the American Society for Microbiology.

The genes encoding the enzymes responsible for the aerobic degradation of PCBs have been isolated, cloned, and incorporated into microorganisms such as E. coli. These genetically engineered microorganisms have the same ability to degrade lightly chlorinated PCBs as the ones found in the soil samples. It may be possible to design microorganisms that degrade PCB's faster than those that occur naturally.

The bulk of the PCBs present in the environment are present in the sediments of lakes, rivers, and oceans where there is little or no oxygen. In some lakes, like Lake Ontario, where there is little homogenization of sediments, there are close correlations between the U.S. sales of PCBs and the concentrations of PCBs in sediment cores (Figure 8-4). Since sediments are the principal repository for PCBs, anaerobic degradative pathways are likely to be of greater importance than aerobic ones.

Studies of the degradation of PCBs in the sediments of the Hudson River in New York resulted in the discovery of microorganisms that degrade PCBs in the absence of oxygen. These microorganisms degrade the more highly chlorinated PCBs (> tetrachloro) to the mono-, di-, and trisubstituted isomers. This reductive dehalogenation results in the substitution of a hydrogen for the chloro group on the aromatic ring. Similar reductive dechlorinations have been observed in the environment with other chlorinated aromatic compounds, as shown earlier in reaction (8-23). Anaerobic microorganisms reduce the chloro groups from the highly substituted biphenyls to give lightly chlorinated biphenyls, and the aerobic microorganisms destroy the lightly substituted biphenyls. In one proposed bioremediation procedure, PCB-containing soil is

8.7.4.2 Anaerobic Biodégradation

FIGURE 8-4 Correlation of PCBs in Lake Ontarico core samples with PCB sales in the United States: E-30 (triangles) and C-32 (circles) are cores from two different sites in Lake Ontario. Redrawn from D. L. Swockhamer and S. J. Ehrenreich, in Organic Contaminants in the Environment. Environmental Pathways and Effects, K. C. Jones, ed., Elsevier Applied Science, London. Copyright © 1991.

FIGURE 8-4 Correlation of PCBs in Lake Ontarico core samples with PCB sales in the United States: E-30 (triangles) and C-32 (circles) are cores from two different sites in Lake Ontario. Redrawn from D. L. Swockhamer and S. J. Ehrenreich, in Organic Contaminants in the Environment. Environmental Pathways and Effects, K. C. Jones, ed., Elsevier Applied Science, London. Copyright © 1991.

to be incubated first with anaerobic microorganisms in the absence of oxygen and then with aerobes in the presence of oxygen. This should result in the destruction of the PCBs in sediments, while requiring a minimal energy input to the process.

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