Screening of white-rot fungi for their ability to mineralize polycyclic aromatic hydrocarbons in soil

Abstract

Soil samples from an agricultural field contaminated with 10 ppm 14C-benz(a)anthracene in glass tubes were brought into contact with cultures of wood-rotting fungi, precultivated on wheat straw substrate. Forty-five strains of white-rot fungi and four brown-rot fungi were tested for their ability to colonize the soil and to mineralize 14C-benz(a)anthracene to 14CO2 within a 20-week incubation time. Twenty-two white-rot fungi and all brown-rot fungi were unable to colonize the soil. Twenty-three strains of white-rot fungi, all belonging to the genus Pleurotus, colonized the soil. During the experiment the non-colonizing fungi and their substrate disintegrated more and more to a nonstructured pulp from which water diffused into the soil. The same phenomenon was observed in the control which contained only straw without fungus and contaminated soil. In samples with colonizing fungi the substrate as well as the mycelia in the soil remained visibly unchanged during the entire experiment. Surprisingly, most samples with fungi not colonizing the soil and the control without fungus liberated between 40 and 58% of the applied radioactivity as 14CO2 whereas the samples with the colonizing fungi respired only 15-25% as 14CO2. This was 3-5 times more 14CO2 than that liberated from the control (4.9%) which contained only contaminated soil without straw and fungus. A similar result was obtained with selected colonizing and noncolonizing fungi and soil contaminated with 10 ppm 14C-pyrene. However, in pure culture studies in which 14C-pyrene was added to the straw substrate, Pleurotus sp. (P2), as a representative of the colonizing fungi, mineralized 40.3% of the added radioactivity to 14CO2. The noncolonizing fungi Dichomitus squalens and Flammulina velutipes liberated only 17.2 or 1.7%, respectively, as 14CO2. These results lead to the hypothesis that the native soil microflora stimulated by the formed products of straw lysis is responsible for high degradation rates found with noncolonizing fungi.