Introduction

The most important perceived environmental roles of fungi are as decomposer organisms, plant pathogens, sym-bionts (mycorrhizas, lichens), and in the maintenance of soil structure due to their filamentous branching growth habit and exopolymer production. However, a broader appreciation of fungi as agents of biogeochemical change is lacking, and apart from obvious connections with the carbon cycle because of their degradative abilities, they are frequently neglected in contrast to bacteria. A much wider array of metabolic capabilities are found within prokar-yotes and while geochemical activities of bacteria and archaea receive considerable attention, especially in relation to carbon-limited and/or anaerobic environments, fungi are of great importance in aerobic environments. Although fungi can inhabit deep subsurface and anaerobic environments, rather less information is so far available about the biogeochemical transformations they mediate in such locations. While fungi are found in all manner of freshwater and marine ecosystems, the bulk of research has been concerned with decomposition, pathogenicity, and taxonomy. However, the significance of anaerobic and aquatic fungal communities as agents of biogeochem-ical change is probably limited in comparison to other microbiota. It is within the terrestrial aerobic ecosystem that fungi exert their profound influence on biogeochem-ical processes on the biosphere, especially when considering soil, rock, and mineral surfaces, and the plant root-soil interface (Figure 1 and Table 1). For example, symbiotic mycorrhizal fungi are associated with ^80% of plant species and are responsible for major mineral transformations and redistributions of inorganic nutrients, for example, essential metals and phosphate, as well as carbon flow, while free-living fungi have major roles in decomposition of plant and other organic materials, including xenobiotics, as well as mineral transformations (Figure 1 ). Fungi are often dominant members of the soil microflora, especially in acidic environments, and may operate over a wider pH range than many heterotrophic bacteria. Fungi are also major biodeterioration agents of stone, wood, plaster, cement, and other building materials, and it is

Fungi

Organic

Figure 1 Simple model of fungal action on naturally occurring and/or anthropogenically derived organic and inorganic substrates. (1) Organic and inorganic transformations mediated by enzymes and metabolites, for example, H+, CO2, and organic acids, and physicochemical changes occurring as a result of metabolism; (2) uptake, metabolism or degradation of organic substrates; (3) uptake, accumulation, sorption, metabolism of inorganic substrates; (4) production of organic metabolites, exopolymers, and biomass; (5) production of inorganic metabolites, secondary minerals and transformed metal(loid)s; (6) chemical interactions between organic and inorganic substances, for example, complexation and chelation, which can modify bioavailability, toxicity, and mobility. Translocation phenomena may also be associated with the fungal component of this model.

now realized that they are important components of rock-inhabiting microbial communities with significant roles in mineral dissolution and secondary mineral formation. The ubiquity and significance of lichens, a fungal growth form, as pioneer organisms in the early stages of mineral soil formation is well appreciated. The purpose of this article is to outline the important roles of fungi as biogeochemical agents in the biosphere and the significance of these processes for environmental cycling of elements on global and local scales.

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