Transformations of toxic zinc minerals by mycorrhizal fungi can profoundly affect the availability and mobility of zinc in the mycorrhizosphere of plants. Zinc containing minerals with a low solubility (phosphates, oxides, carbonates, …) are a permanent source of free Zn ions in soil solution. This bio-available zinc fraction may prevent or hinder the establishment of a vegetation cover on the most polluted soils around Zn smelters. Only adapted plant species and their associated symbiotic micro-organisms can survive in these toxic environments. With the new research project, we will investigate whether ectomycorrhizal fungi, the normal root symbionts of trees, have the potential to reduce the availability of zinc in the rhizosphere. Ectomycorrhizal fungi exude organic acids in the mycorrhizosphere of their host plants in order to release the important nutrient phosphate from poorly soluble Ca, Al, Fe phosphates. This process might reduce the availability of the metal ions through the formation of biogenic metal minerals with an extremely low solubility. Released free metal ions can precipitate as insoluble metal oxalates (Zn, Pb, Cu oxalate). The specific objective of the study is an analysis of the organic acid production in zinc tolerant and non-tolerant isolates of the ectomycorrhizal fungus Suillus luteus. The solubilisation of Zn phosphate and the formation of Zn oxalates and other biogenic Zn minerals by this fungal species will be investigated in detail. This work is relevant to improve phytoremediation technologies for metal-contaminated land as well as nutrient dynamics in the rhizosphere.