Exploiting saprotrophic soil microfungi in bioremediation of DDT: stress tolerance, oxidative stress and antioxidants

DDT was extensively used worldwide as an organochlorine insecticide to control agricultural pests and vectors of several insect-borne human diseases; its use was banned in the early 70's in most industrialized countries due to toxicological concerns (1, 2). However, due to its persistence in the environment, accumulation in the food chain and long-distance transport in the upper atmosphere, residues still remain in environmental compartments becoming long-term sources of exposure affecting organisms (1, 2). Several studies describe indigenous fungi as a promising tool for bioremediation of historically contaminated soils by hazardous chemicals (3, 4). In this research we have isolated 51 saprotrophic fungal species from DDT-contaminated agricultural soils in Poland. Among these species, 24 were selected to test their tolerance (Fig. 1) to high concentration of DDT. In particular, Trichoderma harzianum Rifai and Rhizopus stolonifer (Ehrenb.) Vuill. (Fig. 2) were tested to evaluate fungal oxidative stress responses induced by the presence of the xenobiotic. Two species were chosen on the base of their sporulation capacity, diametric growth, biomass production and tolerance observed when growing on DDT enriched media. The production of reactive oxygen species (ROS) and the activity of major antioxidant enzymes, namely superoxide dismutase (SOD), catalase (CAT), glutathione s-transferase (GST) and peroxidase (PRX) were used to evaluate the oxidative stress in the fungal cells (Fig. 3). Two tolerance indexes based on fungal growth and mycelial biomass (Rt:Rc; TI) showed a high capacity of the two strains to tolerate high DDT concentration in the medium. On the other hand, high level of DDT promoted ROS formation in comparison to the untreated control. The specific activity of SOD increased in T. harzianum while the CAT activity increased in R. stolonifer. Lastly, the GST and PRX activity increased in both species. Our results suggest that the increased ROS production due to DDT induced a differential expression of ROS scavenging enzymes by the two fungal strains, which, in any case, allowed them to tolerate the xenobiotic and produce a strong growth.