Bioavailability of Trace Elements as Related to Mycorrhizal Fungi

Mycorrhizal fungi can directly affect bioavailability of trace elements by an effect on the free metals in soil solution (immobilization by adsorption, absorption, and accumulation) and indirectly by modifying root exudation or by affecting solubilization of metal-bearing minerals. Mycorrhizae have the capacity to protect their host plants when the metal uptake is excessive. Metal uptake decreases from ericoid to ectomycorrhizae (CECM) and arbuscular mycorrhizae (AM) fungi. This is the result of pH changes in soil and thus metal availability, but it is also related to morphology and biomass of the fungal structure of mycorrhizae.

The ectomycorrhizae are exceptional in this respect because they form a dense and thick sheet of fungal tissue (the mantle) that covers the root surface completely. It seems that any ion entering the root must pass the fungal mantle. In other words, ectomycorrhizae have the ability to "filter" ions that enter the plant, which the other types of mycorrhizae do not have. Mycorrhizal fungi are part of the rhizosphere, so their metal sorption capacity is a fundamental issue to researchers concerning the fate of metals in the rhizosphere. On one hand, the contact between metal ions and the hyphae in soil is the first interaction in mycorrhizal metal transport and the processes that take place at the hyphae surface affect the fate of trace elements in the rhizosphere. On the other hand, sorption of metals on mycorrhizal mycelium may be extensive and limit the amount of metals taken up by the fungi.

In a recent work on cation exchange capacity and Pb sorption in ectomycorrhizal fungi, Marschner et al. [69] found that the electron-dense lead deposits on the surface of ectomycorrhizal fungus with the highest Pb sorption capacity contained molar equivalents of P. The sorption characteristics of a fungus may differ between fungi in vitro and in symbiosis with a plant; Marschner et al. have found higher sorption [64]. Colpaert and Assche [70] showed heavy metal uptake and accumulation by ectomycorrhizal fungi, using axenic cultures, where the metals were added as soluble salts. Under these conditions, the fungal-soil concentration ratios were around 200 and 80 for Cd, and 40 and 30 for Zn for nontolerant and metal-tolerant isolates of Suillus bovines, respectively.

Gast et al. [71] studied the heavy metals in mushrooms and their relationship with soil characteristics and they found large differences between metals with very high accumulation for Cd, exclusion for Pb, and a narrower range of concentrations for Zn and Cu. These workers suggested a regulation of uptake for essential elements and concluded that species differences — not soil factors — are the primary determinants of metal levels in fungi.

Ectomycorrhizal fungi can increase the bioavailability of heavy metals in the rhizosphere by solubilizing minerals containing metals such as rock phosphates [72]. Mycorrhizal fungi associated with plant roots in symbiosis affect plant root exudation quantitatively and modify the composition of root exudates containing carbohydrate, amino acids, and aliphatic acids [73]. Leyval and Ber-thelin [74] proposed that the modification of the composition of root exudates by mycorrhizal fungi influence the bioweathering of minerals in the rhizosphere and the availability of metals in the mycorrhizosphere.

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