Mobilization and Immobilization of Heavy Metals

The availability of metals in soils is - at least in part - also dependent on fungal activity. In podzol E horizons under boreal or mountainous coniferous forests, the weathering of bedrock has been attributed to oxalic, citric, succinic, formic, and malic acid excretion by saprotrophic and mycorrhizal fungi. Ectomycorrhizal fungi can form micropores (3-10 mm) in weatherable minerals, and hyphal tips are able to excrete micro- to millimolar concentrations of these organic acids (Jongmans et al. 1997). Heavy metals can be mobilized during this process as well as during fungal weathering of limestone, sandstone, marble or other minerals (Gadd 2007). The ability to solubilize metals from metal oxides is frequently present among soil micromycetes (e.g., Aspergillus and Penicillium spp.). One-third of 56 soil isolates were able to solubilize either ZnO, Zn3(PO4)2, or Co3(PO4)2, and five strains solubilized all of the compounds (Sayer et al. 1995). In addition, pyromorphite (Pb5(PO4)3Cl) can be solubilized by several organic acid-producing fungi (Sayer et al. 1999). While acidification seems to be the most frequent mechanism of metal solubilization, there are also other mechanisms that involve metal chelation, such as MnO and Zn solubilization by Trichoderma harzianum (Altomare et al. 1999).

The most abundant metal chelator produced by saprotrophic as well as mycor-rhizal fungi is oxalate. The production of oxalic acid by fungi provides a means of immobilizing soluble metal ions or complexes as insoluble oxalates, thus decreasing bioavailability and increasing tolerance of these metals (Dutton and Evans 1996). Recently, the inducible and concentration-dependent production of oxalate by several ectomycorrhizal fungi and two saprotrophic Hypholoma spp. was demonstrated in laboratory experiments (Johansson et al. 2008). Metal oxalates can be formed with Ca, Cd, Co, Cu, Mn, Sr, and Zn. In addition to metal oxalate, metal oxides (e.g., MnO or FeO, desert varnish), metal hydroxides, moolooite (CuC2O4 nH2O), and several other metals can be formed by fungi under certain circumstances (Gadd 2007).

Cell walls of fungi are extremely important in the reduction of metal toxicity. Cell wall binding significantly contributes to metal immobilization, for example in the cases of Cd, Cu, and Ag in Aspergillus niger and Mucor rouxii (Mullen et al. 1992). Even the intact cell walls of many fungal species exhibit high binding capacities for heavy metals (Baldrian 2003; Baldrian and Gabriel 2003b). The binding capacity of the cell wall can be further increased through the production of specific wall-associated compounds, including the polysaccharidic outer hyphal sheath and melanin. Melanins are phenolic molecules, some of which are efficient bioabsorbers of copper, cadmium, lead, zinc, or toxic tin compounds (Fogarty and Tobin 1996; Baldrian 2003). In several fungi, the presence of heavy metals induces melanin production (Caesar-Tonthat et al. 1995). The presence of melanin significantly reduces the toxicity of Cu, Zn, Cd, Pb, and probably also that of the other bivalent metal ions (Fogarty and Tobin 1996). An investigation of metal binding to the mycelial melanin of Armillaria spp. found that the melanized rhizomorphs concentrated Al, Zn, Fe, and Cu ions to levels up to 50-100 times higher than those found in the surrounding soil (Rizzo et al. 1992).

The uptake of metals into the cytoplasm can be caused by the low selectivity of transporters for essential metals. Metal ions with similar chemical properties (e.g., Ca, Cd, and Zn) can use the same active transport system. The intracellular accumulation of Cd in Paxillus involutus was shown to be metabolically mediated, although Cd is not essential for fungi (Blaudez et al. 2000). At higher concentrations, metal uptake can result from cell wall permeabilization by metals such as Cu and Cd (Ross 1993).

In most fungi, the presence of bivalent heavy metals like Cd and Cu induces the production of intracellular binding compounds. These can be divided into two main groups: (1) metal-binding oligopeptides containing cysteine-glutathione (GSH), phytochelatins, and related compounds, and; (2) proteins, such as metallothioneins. Both types of compounds can be produced simultaneously. In strains of Aspergillus sp., GSH was found to be responsible for the binding of arsenic such that strains with different sensitivities to As were found to differ in their levels of GSH production (Canovas et al. 2004). In addition, other Aspergillus strains were found to produce copper-binding proteins (Goetghebeur et al. 1995). In Mucor racemosus on the other hand, Cd (but not Cu or Zn) decreased the level of GSH by the induction of phytochelatin synthesis from GSH (Miersch et al. 2001). The ascomycete Neurospora crassa produces both phytochelatins and metallothionein in response to copper and other heavy metals (Lerch 1980; Kneer et al. 1992).

In the basidiomycete genus Agaricus, a typical metallothionein is produced in response to Cu (Lerch 1980), while cadmium induces the production of a protein of another type, called mycophosphatin (Meisch and Schmitt 1986). Cd-mycophosphatin is also produced in the ectomycorrhizal Boletus edulis, but Cd, Cu, Hg, and Zn also induce the synthesis of phytochelatins (Collin-Hansen et al. 2003, 2007). In Paxillus involutus, Cd increases the levels of intracellular GSH, and both Cu and Cd (but not Zn) also induce metallothionein synthesis (Courbot et al. 2004; Bellion et al. 2007). The complexes of Cd and its binding molecules are sequestered into vacuoles (Ott et al. 2002). It seems that there is no general mechanism of response to heavy metals in basidiomycetes.

Body Detox Made Easy

Body Detox Made Easy

What exactly is a detox routine? Basically a detox routine is an all-natural method of cleansing yourbr body by giving it the time and conditions it needs to rebuild and heal from the damages of daily life and the foods you eat and other substances you intake. There are many different types of known detox routines.

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