Symbiosis in lichens

[1][2][3] 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 adopted this mode of life.

[4] The autotrophic symbionts occurring in lichens are a wide variety of simple, photosynthetic organisms commonly and traditionally known as “algae”.

[5][4][3][2] A lichen is a combination of fungus and/or algae and/or cyanobacteria that has a very different form (morphology), physiology, and biochemistry than any of the constituent species growing separately.

The algae or cyanobacteria benefit their fungal partner by producing organic carbon compounds through photosynthesis.

The largest number of lichenized fungi occur in the Ascomycota, with about 40% of species forming such an association.

[9] Approximately 100 species of photosynthetic partners from 40 genera and 5 distinct classes (prokaryotic: Cyanophyceae; eukaryotic: Trebouxiophyceae, Phaeophyceae, Chlorophyceae) have been found to associate with the lichen-forming fungi.

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.

Another cyanolichen group, the jelly lichens (e.g., from the genera Collema or Leptogium) are large and foliose (e.g., species of Peltigera, Lobaria, and Degelia.

Many of these characterize the Lobarion communities of higher rainfall areas in western Britain, e.g., in the Celtic Rainforest.

[14] Throughout lichenization (and after its completion), the fungus and the alga continue to exchange different chemical signals.