Lichens

AbstractLichens are most appropriately viewed as self‐supporting ecosystems formed by the interaction of a fungus, one or more photosynthetic partners, and an indeterminate number of other microscopic organisms – with the fungal partner providing the overall physical structure and scientific name used to refer to them. They were the first associations to which the term symbiosis was applied and are mutualisms as all partners benefit by enabling new habitats to be colonised. Their different morphologies, structures and ecophysiological relationships are introduced, and their dispersal and establishment discussed. Ecological roles range from other organisms that are dependent on them for food or camouflage, to involvement in rock weathering. Special attention is given to their use as biodindicators of air pollutants, habitat disturbance and climate change. Current economic and traditional uses are summarised, and attention is drawn to the numerous natural products produced some of which have potential pharmaceutical applications.Key ConceptsNames given to lichens strictly refer only to the fungal component and the fungi involved are integrated into the overall system of fungal classification.The fungi involved are mainly ascomycetes, dispersed through around 40 different orders, and many include fungi other than those that form lichens.Single genera can include species that form lichens and ones with different biologies.Some single species are facultative lichen‐formers, sometimes producing a lichen and sometimes not, or starting as invaders of other lichens and then becoming independent.Lichen symbioses were well‐established by the Devonian, when land plants first evolved, but long before flowering plants arose in the Cretaceous.The photosynthetic partners in lichens, green algae or cyanobacteria, are less diverse and different ones can associate with a single fungal partner.Specialised bacteria, most unknown outside lichens, are now recognised as an integral part of lichen associations.Lichens are largely dependent on nutrients and moisture from the air and long‐lived, making them valuable as biodindicators of air pollutants and climate change.Lichens have important roles in global geochemical cycles, especially carbon.The fungal partners produce over 800 natural products, many of which have potential pharmaceutical applications.