Nearly all land plants depend on symbiotic mycorrhizal fungi. Two main types of mycorrhizas include endomycor-rhizas where the fungus colonizes the interior of host plant root cells (e.g., ericoid and arbuscular mycorrhizas) and ectomycorrhizas where the fungus is located outside the plant root cells. Mycorrhizal fungi are involved in proton-and ligand-promoted metal mobilization from mineral sources, metal immobilization within biomass, and extracellular precipitation of mycogenic metal oxalates.
Biogeochemical activities of mycorrhizal fungi lead to changes in the physicochemical characteristics of the root environment and enhanced weathering of soil minerals resulting in metal cation release. It has been shown that ectomycorrhizal mycelia may respond to the presence of different soil silicate and phosphate minerals (apatite, quartz, potassium feldspar) by regulating their growth and activity, for example, colonization, carbon allocation, and substrate acidification.
During their growth, mycorrhizal fungi often excrete low molecular weight carboxylic acids (e.g., malic, succi-nic, gluconic, and oxalic) contributing to the process of 'heterotrophic leaching'. In podzol E horizons under European coniferous forests, the weathering of hornblendes, feldspars, and granitic bedrock has been attributed to oxalic, citric, succinic, formic, and malic acid excretion by ectomycorrhizal hyphae. Ectomycorrrhizal hyphal tips could produce micro- to millimolar concentrations of these organic acids. Ectomycorrhizal fungi (Suillus granu-latus and Paxillus involutus) can release elements from apatite and wood ash (K, Ca, Ti, Mn, and Pb) and accumulate them in the mycelia.
Ericoid mycorrhizal and ectomycorrhizal fungi can dissolve a variety of cadmium-, copper-, zinc-, and lead-bearing minerals including metal phosphates. Mobilization of phosphorus is generally regarded as one of the most important functions of mycorrhizal fungi. An experimental study of zinc phosphate dissolution by the ectomycorrhizal association of Paxillus involutus with Scots pine (Pinus sylvestris) demonstrated that phosphate mineral dissolution, phosphorus acquisition, and zinc accumulation by the plant depended on the mycorrhizal status of the pines, the zinc tolerance of the fungal strain and the phosphorus status of the environment.
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