Metal and Metalloid Transformations

Fungi can transform metals, metalloids (elements with properties intermediate between those of metals and non-metals, e.g., arsenic, selenium, and tellurium), and organometallic compounds by reduction, methylation, and dealkylation, again processes of environmental importance since transformation of a metal(loid) may modify its mobility and toxicity. For example, methylated selenium derivatives are volatile and less toxic than inorganic forms while reduction of metalloid oxyanions, such as selenite or tellurite to amorphous elemental selenium or tellurium respectively, results in immobilization and detoxification. The mechanisms by which fungi (and other microorganisms) effect changes in metal speciation and mobility are survival determinants but also components of biogeochem-ical cycles for metals, and many other associated elements including carbon, nitrogen, sulfur, and phosphorus.

Metals and their compounds interact with fungi in various ways depending on the metal species, organism and environment, while fungal metabolism also influences metal speciation and mobility. Many metals are essential for life, for example, Na, K, Cu, Zn, Co, Ca, Mg, Mn, and Fe, but all can exert toxicity when present above certain threshold concentrations. Other metals, for example, Cs, Al, Cd, Hg, and Pb, have no known biological function but all can be accumulated by fungi. Metal toxicity is affected by environmental conditions and the chemical behavior of the particular metal species in question. Despite apparent toxicity, many fungi survive, grow, and flourish in apparently metal-polluted locations and a variety of mechanisms, both active and incidental, contribute to tolerance. Fungi have many properties which influence metal toxicity including the production of metal-binding proteins, organic and inorganic precipitation, active transport and intracellular compartmentalization, while major constituents of fungal cell walls, for example, chitin and melanin, have significant metal-binding abilities.

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