Abstract
Plant roots and their symbiotic and free-living microbial populations alter the physico-chemical characteristics of the rhizosphere making it different from bulk soil and affecting toxic metal speciation. The mechanisms by which fungi and plants obtain phosphate are of particular interest since solubilization of inorganic phosphates can result in release of associated metals. Zinc phosphate was of least toxicity and the most easily solubilized by axenic ericoid and ectomycorrhizal fungi among tested insoluble cadmium-, copper-, lead- and zinc-containing minerals. Solubilization of toxic metal minerals was related to metal tolerance of the fungal species. Zinc phosphate solubilization by Paxillus involutus/Pinus sylvestris ectomycorrhizal associations and protection of host plants against toxic metal mobilized from mineral depended on fungal tolerance and the phosphorus status in the growth matrix. Zinc, copper and lead mobilized from minerals were oxygen-coordinated within the fungal/ectomycorrhizal biomass. “Heterotrophic leaching” of toxic metal(s) from insoluble minerals combined acidification and ligand-promoted dissolution mediated by different organic acid anions: if oxalic acid was produced, precipitation of metal oxalates resulted.
