The fungus Beauveria caledonica was highly tolerant to toxic metals and solubilized cadmium, copper, lead, and zinc minerals, converting them into oxalates. This fungus was found to overexcrete organic acids with strong metal-chelating properties (oxalic and citric acids), suggesting that a ligand-promoted mechanism was the main mechanism of mineral dissolution. Our data also suggested that oxalic acid was the main mineraltransforming agent. Cadmium, copper, and zinc oxalates were precipitated by the fungus in the local environment and also in association with the mycelium. The presence of toxic metal minerals often led to the formation of mycelial cords, and in the presence of copper-containing minerals, these cords exhibited enhanced excretion of oxalic acid, which resulted in considerable encrustation of the cords by copper oxalate hydrate (moolooite). It was found that B. caledonica hyphae and cords were covered by a thick hydrated mucilaginous sheath which provided a microenvironment for chemical reactions, crystal deposition, and growth. Cryoscanning electron microscopy revealed that mycogenic metal oxalates overgrew parental fungal hyphae, leaving a labyrinth of fungal tunnels within the newly formed mineral matter. X-ray absorption spectroscopy revealed that oxygen ligands played a major role in metal coordination within the fungal biomass during the accumulation of mobilized toxic metals by B. caledonica mycelium; these ligands were carboxylic groups in copper phosphate-containing medium and phosphate groups in pyromorphite-containing medium.Fungi, which are a major and often dominant component of the microbiota in soils and mineral substrates, are important as decomposer organisms, animal and plant symbionts and pathogens, and spoilage organisms of natural and synthetic materials, and they play an important role in biogeochemical cycles of elements (7,17,18). Certain fungal processes solubilize metals from minerals and bound locations, thereby increasing metal bioavailability, whereas other fungal processes immobilize metals and reduce their bioavailability (7, 18). Flexible mycelial growth strategies and the ability to produce and exude organic acids, protons, and other metabolites make fungi important biological weathering agents of natural rock, minerals, and building materials (7,16,18). As mineral components contain considerable quantities of metals, as well as other elements which are biologically unavailable, the influence of such processes on metal mobility are of economic and environmental significance and may be important in the treatment or natural attenuation of contaminated soil (17).Metal mobilization by fungi can occur as a result of several mechanisms, including acidolysis (proton promoted), complexolysis (ligand promoted), reductive mobilization, and the mycelium functioning as a sink for soluble metal species (8). These processes include proton efflux and the production of siderophores [for Fe(III)], but in many strains leaching occurs due to the production of primary and secondary me...