The dual nature of lichen organisms was first proposed in 1869 by the Swiss botanist Simon Schwendener. Soon afterwards an imaginative Scottish priest described the dual relationship as ‘the unnatural union between a captive algal damsel and a tyrant fungal master’! This remark had a great effect on the Scottish psyche that has lasted to this day. See Scottish Lichenology.
Lichens are the result of a physiological relationship between a fungus and a photosynthesising partner termed the photobiont. The photobiont is either green algae or bacteria that use blue-green pigment to photosynthesise; such bacteria are called cyanobacteria.
The photobiont supplies food in the form of carbohydrate to the fungal partner; the fungal or mycobiont partner provides a home and some nutrients for the photobiont. Working together they take on the form and functionality of lichen. In the case of the photobiont being a green algae, when both are separated and grown separately, they form an amorphous mass unlike the original lichen form. This enforces the idea that the partnership is one of equality and not, as some writers have suggested, that the algae is prisoner to the fungus.
An interesting element of the symbiotic relationship is that in each lichen species the mycobiont is different, whereas the photobiont is one of a few algae or cyanobacteria. Because of the individuality of each fungus to a lichen, the naming of lichens is derived from the fungus. Most of the fungal partners come from the Class Ascomycetes. The photobiont is frequently one of the following:
Green algae: Trebouxia, Myrmecia, Stichococcus, Heterococcus and Trentepohlia.
Cyanobacteria: Stigonema, Chroococcus, Nostoc, Gloeocapsa and Scytonema.