Introduction

Fungi comprise a significant proportion of the soil microbial community as decomposer organisms and plant symbionts (mycorrhizas), playing fundamental roles in carbon mineralization and other biogeochemical cycles (Wainwright, 1988), and are often dominant in acidic soils where toxic metals may he speciated into mobile forms (Morley et al., 1996). Anthropogenic activities, including fossil fuel combustion, mineral mining and processing, and production of industrial effluents and sludges, biocides and preservatives (Gadd & Griffiths, 1978; Gadd, 1992), release a variety of toxic metal species into aquatic and terrestrial ecosystems and this can have significant effects on the biota as well as resulting in metal transfer to higher organisms, plants and animals(Wainwright & Gadd, 1997). Metals and their compounds can interact with fungi in various ways depending on the metal species, organism and environment, while metabolic activity can also influence speciation and mobility. Certain mechanisms may mobilize metals into forms available for cellular uptake or leaching from the system, e.g. complexation with citric acid, other metabolites and siderophores (Francis, 1994). Metalsmay also be immobilized by, for example, sorption onto cell components, exopolymers, transport and intracellular sequestration or precipitation, both intra- and extracellular (Morley & Gadd, 1995; Sayer & Gadd, 1997). The apparently opposing phenomena of metal solubilization and immobilization are key components of biogeochemical cycles for toxic metals, whether indigenous or introduced into a given location, since both mobility and toxicity can be affected.