Lichens are really fungi that exist in facultative or obligate symbioses with one or more photosynthesizing partners, and play an important role in many biogeo-chemical processes. The symbiotic lichen association with algae and/or cyanobacteria, where photosynthetic symbionts provide a source of carbon and surface protection from light and irradiation, is one of the most successful means for fungi to survive in extreme subaerial environments. Lichens are pioneer colonizers of fresh rock outcrops, and were possibly one of the earliest life forms. The lichen symbiosis formed between the fungal partner (mycobiont) and the photosynthesizing partner (algal or cyanobacterial photobiont) enables lichens to grow in practically all surface terrestrial environments: an estimated 6% of the Earth's land surface is covered by lichen-dominated vegetation. Globally, lichens play an important role in the retention and distribution of nutrient (e.g., C and N) and trace elements, in soil formation, and in rock weathering. Lichens can readily accumulate metals such as lead (Pb), copper (Cu), and others of environmental concern, including radionuclides, and also form a variety of metal-organic biominerals, especially during growth on metal-rich substrates. On copper sulfide bearing rocks, precipitation of copper oxalate (moolooite) can occur within the lichen thallus.
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