This volume focuses primarily on the influence of free-living fungi in biogeochemistry. Lichens, fungi that exist in facultative or obligate symbiosis with one or more photosynthesizing partners, also play an important role in many biogeochemical processes. Pioneer colonizers of fresh rock outcrops, lichens were possibly one of the first life forms to occupy Earth's land surfaces. The unique lichen symbiosis formed between the fungal partner (mycobiont) and the photosynthesizing partner, an alga or cyanobacterium (photobiont), enables lichens to grow in all surface terrestrial environments. These include extreme environments where no other multicellular vegetation can survive, such as the dry Antarctic valleys (Nash, 1996). An estimated 6% of the Earth's land surface is covered by lichen-dominated vegetation.
Globally, lichens play an important biogeochemical role in the retention and distribution of nutrient (e.g. C, N) and trace elements (e.g. Knops et al., 1991; Garty et al., 1995), in soil formation processes (Ascaso et al., 1976; Jones, 1988) and in rock weathering (Hallbauer & Jahns, 1977; Wilson et al., 1981; Wessels & Schoeman, 1988; McCarroll & Viles, 1995; Barker et al., 1997; Lee & Parsons, 1999). Lichens tend to accumulate trace elements such as lead, copper and other heavy metals of environmental concern (see below), including radionuclides (Yliruokanen, 1975; Nieboer & Richardson, 1981; Beckett et al., 1982; Boileau et al., 1982, 1985a, b; Richardson et al., 1985; Fahselt et al., 1995; Haas et al., 1998; McLean et al., 1998; Jacquiot & Daillant, 1999; Purvis et al., 2004).