<P> Living as a symbiont in a lichen appears to be a successful way for a fungus to derive essential nutrients, as about 20% of all fungal species have acquired this mode of life . The fungal partner may be an Ascomycete or Basidiomycete . Common algal partners are Trebouxia, Pseudotrebouxia, or Myrmecia . Common cyanobacterium partners include are Nostoc or Scytonema . </P> <P> The largest number of lichenized fungi occur in the Ascomycota, with about 40% of species forming such an association . Some of these lichenized fungi occur in orders with nonlichenized fungi that live as saprotrophs or plant parasites (for example, the Leotiales, Dothideales, and Pezizales). Other lichen fungi occur in only five orders in which all members are engaged in this habit (Orders Graphidales, Gyalectales, Peltigerales, Pertusariales, and Teloschistales). Lichenized and nonlichenized fungi can even be found in the same genus or species . Overall, about 98% of lichens have an ascomycetous mycobiont . Next to the Ascomycota, the largest number of lichenized fungi occur in the unassigned fungi imperfecti . Comparatively few Basidiomycetes are lichenized, but these include agarics, such as species of Lichenomphalia, clavarioid fungi, such as species of Multiclavula, and corticioid fungi, such as species of Dictyonema . </P> <P> The autotrophic symbionts occurring in lichens are a wide variety of simple, photosynthetic organisms commonly and traditionally known as algae . These symbionts include both prokaryotic and eukaryotic organisms . Approximately 100 species of photosynthetic partners from 40 genera and five distinct classes (prokaryotic: Cyanophyceae; eukaryotic: Trebouxiophyceae, Phaeophyceae, Chlorophyceae) have been found to associate with the lichen - forming fungi . The prokaryotes belong to the Cyanobacteria, whose representatives are often called bluegreen algae . The bluegreen algae occur as symbionts in about 8% of the known lichens . The most commonly occurring genus is Nostoc . The majority of the lichens contain eukaryotic autotrophs belonging to the Chlorophyta (green algae) or to the Xanthophyta (yellow - green algae). About 90% of all known lichens have a green alga as a symbiont . Among these, Trebouxia is the most common genus, occurring in about 20% of all lichens . The second most commonly represented green alga genus is Trentepohlia . Overall, about 100 species are known to occur as autotrophs in lichens . All the algae are probably able to exist independently in nature as well as in the lichen . </P> <P> A particular fungus species and algal species are not necessarily always associated together in a lichen . One fungus, for example, can form lichens with a variety of different algae . The thalli produced by a given fungal symbiont with its differing partners will be similar, and the secondary metabolites identical, indicating that the fungus has the dominant role in determining the morphology of the lichen . Further, the same algal species can occur in association with different fungal partners . Lichens are known in which there is one fungus associated with two or even three algal species . Rarely, the reverse can occur, and two or more fungal species can interact to form the same lichen . </P>

Lichens result from the symbiosis of a fungus and a