Biodegradation of Dioxin-Related Compounds: A Review

Abstract

A number of bacteria have been isolated, as well as genetically constructed, that are able to transform dioxin-related compounds including mono- and dichlorinated dibenzo-p-dioxins, dibenzofurans, and diphenyl ethers. Invariably, dioxygenases are the primary degradative enzymes involved in the transformations. In most cases, these dioxygenases operate regioselectively and attack their substrates at an ether bond-carrying carbon and an adjacent carbon (the angular position), which prompts the irreversible cleavage of otherwise recalcitrant aryl ether bonds. Arising metabolites typically are mineralized to carbon dioxide, water, and inorganic salts. Some angular dioxygenases have been studied biochemically, resulting in information that may help to expand their substrate ranges to include tri-, tetra-, and other poychlorinated dioxins. Theoretically, all toxic congeners are susceptible to biocatalysis because they all possess an unsubstituted carbon adjacent to an ether bridge. Bacteria harboring these enzymes represent a promising tool for the future bioremediation of contaminated soils. Feasibility studies have shown that hydrophobic dioxins are bioavailable and rapidly degraded in soils from concentrations of 10 ppm to levels in the low ppb range by laboratory-cultured bacteria. Even lower treatment endpoints may be achievable by using these bacteria in concert with methods to increase the bioavailability of the pollutants.