130 Elena Fernández-Miranda Cagigal
It has been demonstrated that several fungi, including mycorrhizal fungi,
are involved in metal immobilization and accumulation within biomass by
binding metals to cell walls or pigments, by intracellular uptake or by
extracellular precipitation of mycogenic toxic metal oxalates and carbonates
(Fomina et al., 2005). Leading in this case to the occasional generation of
precipitation or crystallization in the cell wall of evaporable minerals such as
gypsum (CaSO4 2H2O) and iron oxides (magnetite, Fe3O4) (Gorbushina et
al., 2002). Furthermore, several fungal species are capable of mobilizing heavy
metals by means of chelation, the release of the metabolites, siderophores,
methylation of metals and organometallics or redox reactions that can result in
volatilization (Gadd, 2007; Harms et al., 2011).
Each species of fungus could use one or more of these mechanisms, with
varying responses to metal exposure. Thus, while species such as Candida
glabata or Schizosaccharomyces pombe in the Cd presence produce
phytochelatines and metallothionein (Ow et al., 1994), in Paxillus involutus
these molecules rarely appear, while and glutathione is abruptly increased
(Courbot et al., 2004).
Moreover, mycorrhizae can increase phytoextraction, either directly or
indirectly, increasing metal accumulation on the ground, or through increased
biomass production (Fernández et al., 2008). Despite the capacity for tolerance
and accumulation of different species of fungi seems to be an intrinsic
character, in many cases it has been observed that species or ecotypes isolated
from contaminated areas respond better to the presence of heavy metals (Gadd,
2007).
Therefore, an extensive knowledge on the diversity of fungal communities
existing in the roots and rhizosphere of plants colonizing highly polluted areas
is paramount for their potential use in restoration programs.
FUNGAL ASSOCIATIONS
The dependency of trees from mycorrhizal fungi has been long
established, especially where soil characteristics are adverse. Numerous papers
have demonstrated that if degraded areas are reforested using mycorrhized
trees, the survival and development rates achieved are superior to those
achieved with trees without mycorrhizal fungi (Duñabeitia et al., 2004;
Meharg & Cairney, 2000).
The two most widespread types of mycorrhizal association are arbuscular
mycorrhizal (AM) and ectomycorrhizal (ECM). It is known that AM establish