Occurrence and possible roles of melanic pigments in lichenized ascomycetes

Many species of lichenized ascomycetes are capable of synthesizing darkly colored melanin pigments by polymerizing phenolic compounds. Available data suggest that different clades of lichens produce various kinds of melanins, with N₂-fixing Peltigeralean lichens producing N-rich DOPA or “eumelanins” and lichens from other orders producing N-poor “allomelanins.” In general, melanic lichens seem to be more common in environments with high levels of abiotic stress such as polar and montane regions, but they are by no means restricted to these habitats. Here, we review the occurrence of melanins in lichens, their chemical structure, methods of quantification and biological roles. Good evidence exists that melanins in lichens protect the mycobiont from high UV, and the photobiont against high PAR. However, given their many unique properties, it seems likely that melanins help lichens to survive a range of biotic and abiotic stresses.