INTRODUCTION

The most important perceived environmental roles of fungi are as decomposer organisms, plant pathogens and symbionts (mycorrhizas, lichens), and in the maintenance of soil structure through their filamentous growth habit and production of exopolymers. However, a broader appreciation of fungi as agents of biogeochemical change is lacking and, apart from obvious connections with the carbon cycle, they are frequently neglected within broader microbiological and geochemical research contexts. While the profound geochemical activities of bacteria and archaea receive considerable attention, especially in relation to carbon-limited and/or anaerobic environments (see elsewhere in this volume), in aerobic environments fungi are of great importance, especially when considering rock surfaces, soil and the plant root-soil interface (Gadd, 2005a). For example, mycorrhizal fungi are associated with ∼ 80 % of plant species and are involved in major mineral transformations and redistributions of inorganic nutrients, e.g. essential metals and phosphate, as well as carbon flow. Free-living fungi have major roles in the decomposition of plant and other organic materials, including xenobiotics, as well as mineral solubilization (Gadd, 2004). Lichens (a symbiosis between an alga or cyanobacterium and a fungus) are one of the commonest members of the microbial consortia inhabiting exposed subaerial rock substrates, and play fundamental roles in early stages of rock colonization and mineral soil formation.