Degradation of benzo[a]pyrene by mitosporic fungi and extracellular oxidative enzymes

The involvement of extracellular oxidative enzymes (laccase, lignin peroxidase and manganese-dependent peroxidase) in the degradation of benzo[a]pyrene, a high molecular weight polycyclic aromatic hydrocarbon, by three mitosporic fungi (Deuteromycetes) isolated from polluted soils was examined. These fungal strains were found to have different abilities to degrade benzo[a]pyrene: relative degradation percentages per unit biomass for Trichoderma viride, Fusarium solani and Fusarium oxysporum were approximately 39, 17 and 8, respectively. No peroxidase activities were detected in any of the fungal strains. Laccase activities were measured in F. solani and F. oxysporum cultures, but the specific activities assessed were similar in both whether in the presence or absence of benzo[a]pyrene. Extracellular laccase and peroxidase activities were not induced by benzo[a]pyrene. Moreover, laccase activities of F. oxysporum, a poor benzo[a]pyrene degrader, were fivefold higher than in F. solani, a better benzo[a]pyrene degrader. The use of a laccase inhibitor, sodium azide, did not decrease benzo[a]pyrene degradation but the laccase activity was inhibited by 50%. The fact that T. viride degrades benzo[a]pyrene more efficiently, without any detectable laccase activity, confirms this result. In conclusion, in these three fungal strains, no apparent correlation between degradation percentage and the tested ligninolytic enzymes production could be shown in our culture conditions.