Degradation of xenobiotics by white rot fungi.

White rot fungi such as P. chrysosporium degrade the nonrepeating, nonstereoselective, insoluble polymer lignin under conditions of nutrient limitation. The attack on lignin principally involves extracellular peroxidases (ligninases) and hydrogen peroxide. Hydroxyl radicals may also make a significant contribution. The ligninolytic system lends itself to the degradation of xenobiotics, since these often have limited solubility in water and are not readily available in soil to intracellular metabolism. A nonspecific attack should proceed at a rate independent of the target's concentration and the fungal system would be expected to remediate soil contaminated with a mixture of compounds. This contrasts with the need for induction and problems with simultaneous metabolism encountered with bacterial inoculation. The P. chrysosporium system has been found active against such diverse substrates as DDT, lindane, PCBs, TNT and crystal violet, with substantial mineralization in many cases. Some like biphenyl and triphenylmethane dyes are structurally related to lignin substructures while others bear groups such as nitro (TNT) or halogen (PCP) that are absent from the natural polymer. The fate of transformed targets varies: pentachlorophenol is incorporated into soil organic matter as a result of fungal ligninase action, whereas highly lipophilic Aroclor PCBs are converted to water-soluble metabolites. Normally less toxic intermediates are generated: for example, with benzo[a]pyrene, mutagenic arene oxides do not appear in the white rot fungal system. In certain cases, purified ligninases were also active in degrading pollutants such as PCP, benzo[a]pyrene or triphenylmethane dyes. Methods of optimizing ligninase activity in fungal reactors have been described, such as the addition of surfactants and veratryl alcohol to the medium. It remains to be seen how molecular biology can provide further advances in maximizing the bioremediating activity of white rot fungi applied to contaminated soil.