Published online by Cambridge University Press: 05 June 2012
Introduction
Lichens are composite organisms including at least one fungus (mycobiont) and an alga or cyanobacterium (photobiont) living in a mutualistic symbiosis (Hawksworth & Honegger 1994). The lichen symbiosis may involve multiple and different bionts, especially photobionts, at various stages of its life history (Hawksworth 1988; Jahns 1988). Lichenised fungi are ecologically obligate biotrophs acquiring carbon from their photobionts (Honegger 1997). Lichens colonize bark, rocks, soil and various other substrata, and occur in all terrestrial ecosystems, covering more than 6% of the Earth's land surface. They are dominant in Arctic and Antarctic tundra regions where they form the key component of ecosystem processes, as a part of global biogeochemical cycles and also the food chain. Arctic and sub-Arctic lichen heaths are readily visible from space using remote sensing techniques and the effects of ‘point source’ smelters in creating ‘industrial barrens’ and emission reductions leading to recovery of Cladonia-rich heaths are well-documented (Tømmervik et al. 1995, 1998, 2003).
Lichens play a major role in plant ecology and the cycling of elements, such as C, N, P, heavy metals and radionuclides (Knops et al. 1991; Nash 1996b) and are extremely tolerant of ionising radiation (Brodo 1964). Lichens contribute to soil formation and stabilization (Jones 1988). Lichenized fungi may well be ancestral to fruit-body forming ascomycetes (Eriksson 2005). There are around 13,500 known species and 18,000–20,000 estimated worldwide (Sipman & Aptroot 2001).
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