Fungal PAH-metabolites resist mineralization by soil microorganisms

This study investigated the mineralization of water-soluble polycyclic aromatic hydrocarbon (PAH) metabolites produced by the soil fungus Cunninghamella elegans. Eleven soil fungi were screened for their ability to metabolize ¹⁴C-phenanthrene, ¹⁴C-fluoranthene, and ¹⁴C-pyrene into water-soluble compounds. Eight fungi produced water-soluble metabolites from all or some of the PAHs. The composition of the water-soluble PAH-metabolites from the most effective solubilizer C. elegans was analyzed by an ultraperformance liquid chromatograph interfaced to a quadrupole time-of-flight mass spectrometer. Thirty-eight metabolites were detected. All of 34 identified metabolites were sulfate-conjugated. The mineralization of ¹⁴C-metabolites, produced by C. elegans, was compared to mineralization of the parent ¹⁴C-PAHs in soil slurries. It was hypothesized that the increased bioavailability and metabolic activation of the metabolites would increase mineralization in soil slurries compared to mineralization of the parent PAHs. Unexpectedly, the mineralization of the ¹⁴C-metabolites was in all cases extremely slow compared to the mineralization of the parent ¹⁴C-PAHs. Slow ¹⁴C-metabolite mineralization was not caused by metabolite toxicity, neither was cometabolic mineralization of ¹⁴C-metabolites stimulated by the presence of active PAH-degraders. High water solubility, low lipophilicity, and extremely slow mineralization of the metabolites indicate a potential problem of leaching of fungal PAH-metabolites to the groundwater.