Solubilization of insoluble inorganic zinc compounds by ericoid mycorrhizal fungi derived from heavy metal polluted sites

Ericoid mycorrhizal fungi increase the ability of their host plants to colonize soils polluted with toxic metals, although the underlying mechanisms are not clearly understood. Two mycorrhizal strains of Oidiodendron maius isolated from contaminated soil were previously shown to tolerate high concentrations of toxic metals. We investigated further the biological mechanisms that may explain metal tolerance, focussing on the interactions between insoluble metal species and extracellular fungal metabolites. In particular, we demonstrate that fungal strains derived from polluted and unpolluted soils mobilize insoluble inorganic zinc compounds to different extents. Strains from polluted soils showed in fact little ability to solubilize Zn from both ZnO and Zn3(PO4)2, whereas strains from unpolluted soils showed a higher solubilization potential. This different behaviour was confirmed when the solubilization abilities of a wider range of fungal strains (25 isolates) was examined. Induction of organic acids (malate and citrate) by the metal compounds was at least in part responsible for metal solubilization. Our results suggest that ericoid mycorrhizal strains from polluted and unpolluted soils may interact differently with metal compounds. We speculate that this may reflect specific strategies to maintain homeostasis of essential metals under different soil conditions.