Introduction

Fungi are chemoheterotrophic organisms, ubiquitous in subaerial and subsoil environments, and important as decomposers, animal and plant symbionts and pathogens, and spoilage organisms of natural and man-made materials (Gadd, 1993, 1999; Burford et al., 2003a). A fungal role in biogeochemical cycling of the elements (e.g. C, N, P, S, metals) is obvious and interlinked with the ability to adopt a variety of growth, metabolic and morphological strategies, their adaptive capabilities to environmental extremes and their symbiotic associations with animals, plants, algae and cyanobacteria (Burford et al., 2003a; Braissant et al., 2004; Gadd, 2004). Fungal polymorphism and reproduction by spores underpin successful colonization of different environments. Most fungi exhibit a filamentous growth habit, which provides an ability for adoption of either explorative or exploitative growth strategies, and the formation of linear organs of aggregated hyphae for protected fungal translocation (see Fomina et al., 2005b). Some fungi are polymorphic, occurring as both filamentous mycelium and unicellular yeasts or yeast-like cells, e.g. black meristematic or microcolonial fungi colonizing rocks (Sterflinger, 2000; Gorbushina et al., 2002, 2003). Fungi can also grow inside their own parental hyphae utilizing dead parts of the colony under the protection of parental cell walls (Gorbushina et al., 2003). The ability of fungi to translocate nutrients through the mycelial network is another important feature for exploring heterogeneous environments (Jacobs et al., 2002, 2004; Lindahl & Olsson, 2004).

The earliest fossil record of fungi in terrestrial ecosystems occurred during the Ordovician period (480 to 460 MYBP) (Heckman et al., 2001).