Introduction

Fungi are of fundamental importance as decomposer organisms and plant symbionts (mycorrhizas) and can comprise the largest pool of biomass (including other microorganisms and invertebrates) in the soil (Wain-wright, 1988; Metting, 1992). They can be dominant in acidic conditions, where the mobility of toxic metals may be increased (Morley et al., 1996), and this, combined with their explorative filamentous growth habit and high surface area to mass ratio, ensures that fungi are integral bioactive components of major environmental cycling processes for metals and other elements including carbon, nitrogen, sulfur and phosphorus (Gadd & Sayer, 2000). There are examples where fungal isolates from soils with high metal contents exhibit higher metal tolerance than isolates from agricultural soils (Amir & Pineau, 1998), while adaptive and constitutive mechanisms of metal resistance are well known in free-living (Gadd, 1993a; Gadd & Sayer, 2000) and mycorrhizal fungi (Meharg & Cairney, 2000). Metals and their compounds, derivatives and radionuclides, interact with fungi in a variety of ways depending on the metal species, organism and environmental conditions, while fungal metabolism can dramatically influence speciation and, therefore, mobility and toxicity (Gadd, 1993a; Gadd & Sayer, 2000). Antagonistic effects between different metal species may also be a significant phenomenon in free-living (Amir & Pineau, 1998) and symbiotic fungi (Hartley et al., 1997). Solubilization mechanisms, for example complexation with organic acids, other metabolites and siderophores, can mobilize metals into forms available for cellular uptake and leaching from the system (Francis, 1994).