Interactions of Fungi with Heavy Metals in the Soil Environment

Metals may be present in soils as free metal ions, complexes with organic matter, or they may be chemically precipitated into insoluble compounds, such as oxalates, carbonates, and hydroxides. The degree of toxicity of the metal to organisms depends upon its relative availability (solubility) within the soil solution. This availability is dependent upon a number of edaphic factors, such as soil pH, Eh, organic matter, and clay content. Soil microfungi are able to tolerate higher Cd concentrations in the presence of clay or montmorillonite than in kaolinite, and are less sensitive at higher pH levels (Babich and Stotzky 1977). The biogenic factors affecting metal availability include solubilization (leaching), immobilization by co-precipitation with biogenic compounds, biosorption, and bioaccumulation (Berthelin et al. 1995; Gadd 2007).

Fungi are able to restrict the entry of toxic metal species into cells by (1) extracellular metal sequestration - binding the metal to siderophores or other fungi-derived metabolites; (2) binding it to the cell wall and wall-associated components, and; (3) reducing its uptake by intracellular chelation or sequestration. The above defense mechanisms act simultaneously; the mycorrhizal fungus Paxillus involutus is able to produce oxalate that binds some extracellular metals. The heavy metals that are not bound come into contact with mycelium and are localized in or near the cell wall, in the vacuoles, and in the cytoplasm. For Cd, its distribution among the three biomass components listed above was 50%, 20%, and 30%, respectively (Blaudez et al. 2000). Similar detoxification systems may act both intra- and extra-cellularly: in the case of nickel immobilized by Aspergillus niger, Ni oxalate crystals have been documented in the extracellular fraction as well as in the cell wall and cytoplasm (Magyarosy et al. 2002).

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