ring is then cleaved through the assistance of another dioxygenase at either of two locations. The opened ring is then further metabolized by well-known pathways. PCB congeners containing several chlorine groups, especially in the 2 (ortho) or 3 (meta) positions, can block the first oxygenation of a PCB. Other researchers (Bedard and Haberl 1990) have shown that a 3,4-dioxygenase might also be effective, thus permitting somewhat broader susceptibility of PCB congeners to aerobic biodegradation. Because chlorine atoms on the PCB ring effectively block the action of the oxygenating enzymes, only PCBs with relatively few chlorine atoms are readily susceptible to aerobic biodegradation.
Anaerobic biotransformation of PCBs is significantly different from aerobic biodegradation (Tiedje et al. 1993) and is most effective with more highly chlorinated PCBs. Under anaerobic conditions, PCBs are transformed by reductive dehalogenation. Here, a chlorine atom is removed from the molecule and substituted with hydrogen. Reductive dehalogenation of organic molecules has become recognized in recent years as a general process that is effective for dehalogenating a variety of halogenated organic compounds, from pesticides and many aromatic compounds, such as PCBs, to aliphatic compounds, such as chlorinated solvents (Holliger et al. 1998; Tiedje et al. 1993). In such reductions, the haloorganic serves as an electron acceptor, the role that oxygen serves under aerobic conditions. Such dehalogenation might be a fortuitous event brought about by enzymes that are designed for other purposes. In this case, the process is called a cometabolic one. However, in some cases, organisms can use the electron-accepting potential of halo-organics in energy metabolism, in which case the process is called dehalorespiration (Holliger et al. 1998) or simply halorespiration. Dehalorespiration has been demonstrated in the case of biotransformation of chlorobenzoates (Dolfing and Tiedje 1987) and tetrachloroethene (Holliger et al. 1993). The electron donor for these transformations frequently is molecular hydrogen (H2), which is commonly formed as an intermediate in normal anaerobic fermentation of complex organic materials. In other cases, simple organic compounds, such as acetate or lactate, might serve as the electron donor. Dehalorespiration tends to be a more efficient dehalogenating process because the organisms can grow catalytically by the process; reaction rates tend to be higher, and the use of available electron donors is more efficient. Whether cometabolic or through dehalorespiration, reductive dehalogenation generally requires the presence of other organic matter, such as decaying vegetable matter, to provide the electron donor required for the process to occur.