15 - Fungal sequestration, mobilization and transformation of metals and metalloids

Summary

Introduction

The soil environment is complex and infinitely variable (Metting, 1992). The fate of metal contaminants is similarly diverse and dependent on many factors, such as mineral composition and organic content (Krosshavn, Steinnes & Varskog, 1993), and is mediated by physicochemical processes (Zhu & Alva, 1993). Metal contaminants in the soil undergo complex interactions with both organic and inorganic components, and many studies have shown that the basic inorganic aluminosilicate clays, silts, sands and other mineral components are important metal sequestrants (Farrah & Pickering, 1976, 1977; Kuo & Baker, 1980; Harter, 1983; Krosshavn et al., 1993). It is important to recognize, however, that the soil organic component, which contains both living organisms and their decay or metabolic products, also exerts a strong influence on metal retention.

Fungi exist in soils primarily as saprotrophic degraders of organic matter, and also as pathogens of plants and in mycorrhizal associations with plant root systems. Fungi are ubiquitous soil microorganisms, predominant in acidic soils, often comprising the largest pool of biomass (including bacteria, microalgae, actinomycetes, protozoa, nematodes, earthworms and other invertebrates) and organic products under these conditions (Metting, 1992). This, combined with their high surface area to mass ratio, ensures that fungal–metal interactions are of primary importance in the organic soil environment (see Colpaert & Van Tichelin, Chapter 9). This is especially true of acidic soil conditions where metals are more likely to be speciated into soluble and more mobile forms (Hughes & Poole, 1991) and where metal ion/fungal/mineral interactions are more likely to occur due to the predominance of the fungal component of the biota in such soils.