Metal mobilization from rocks, minerals, soil, and other substrates can be achieved by protonolysis, respiratory carbon dioxide resulting in carbonic acid formation, chelation by excreted metabolites and Fe(iii)-binding siderophores, and methylation which can result in volatilization. In addition, other excreted metabolites with metal-complexing properties, for example, amino acids, phenolic compounds, and organic acids, may also be involved. Fungal-derived car-boxylic acids can play an integral role in chemical attack of mineral surfaces and these provide a source ofprotons as well as a metal-chelating anion. Oxalic acid can act as a leaching agent for those metals that form soluble oxalate complexes, including Al and Fe. Solubilization phenomena can also have consequences for mobilization of metals from toxic metal containing minerals, for example, pyromorphite (Pb5(PO4)3Cl), contaminated soil, and other solid wastes. Fungi can also mobilize metals and attack mineral surfaces by redox processes: Fe(iii) and Mn(iv) solubility is increased by reduction to Fe(ii) and Mn(ii), respectively. Reduction of Hg(ii) to volatile elemental Hg(0) can also be mediated by fungi.
The removal of metals from industrial wastes and byproducts, low-grade ores and metal-bearing minerals by fungal 'heterotrophic leaching' is relevant to metal recovery and recycling and/or bioremediation of contaminated solid wastes. Although fungi need a source ofcarbon and aeration, they can solubilize metals at higher pH values than thioba-cilli and so could perhaps become important where leaching with such bacteria is not possible and in bioreactors. Leaching of metals with fungi can be effective although a high level of organic acid production may be necessary. Other possible applications of fungal metal solubilization are the removal of unwanted phosphates, and metal recovery from scrap electronic and computer materials.
The ability of fungi, along with bacteria, to transform metalloids has been utilized successfully in the biore-mediation of contaminated land and water. Selenium methylation results in volatilization, a process which has been used to remove selenium from the SanJoaquin Valley and Kesterson Reservoir, California, using evaporation pond management and primary pond operation.
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