British distribution: Throughout Britain, at least as a lichen photobiont.
The genus Trebouxia is best known as a major lichen photobiont (photosynthetic partner in the lichen thallus), though it can also occur free-living on surfaces such as tree bark. One free-living British species, T. arboricola Puymaly, is known - as a common terrestrial alga on soil, tree trunks, wooden fences, walls and the like (see John et al., 2002) - but some Trebouxia occurrences may represent overlooked associations with leprose (powdery) lichens or perhaps 'escapes' from lichen associations. The examples on this page are of Trebouxia as the photobiont in Parmelia sulcata Taylor, a common lichen of trees and rocks.
Trebouxia is a genus of uncertain taxonomy. It is not clear if all lichen photobionts that are described as "trebouxioid" belong to a single genus, nor is it at all clear as to how many species should be recognised. The genus was formerly split into two genera, Trebouxia and "Pseudotrebouxia", but recent research has indicated that the supposed differences in reproduction are invalid. On the other hand, molecular studies are indicating that the genus should be split in other ways. There is useful discussion by Friedl & Büdel in Nash (1996).
The current concept of the genus is that of a unicellular, usually spherical alga, containing a single, large stellate chloroplast ('star-like'), which itself contains a single, central pyrenoid. Reproduction is by autospores (non-motile spores produced within a parent cell, developing the same shape as the parental cell before release) and (but rarely in nature) by biflagellate zoospores.
Trebouxia is substantially the most widespread photobiont in lichens, occurring in many different lichen orders, in Britain and in the world in general. They are not obligately associated with single species of lichenised fungi, since they can be grown independently in culture, and it is also known that a given lichen may form associations with different Trebouxia species.
It remains a matter of debate as to whether the symbiotic lichen association represents true mutual benefit to both the fungal partner, the mycobiont, and the algal partner, the phycobiont, or whether the fungus should be considered as parasitic. Certainly the fungus benefits from the carbohydrates synthesised through photosynthesis by the alga, whereas the extent to which the fungus enhances the nutrient supply to the alga is less definite. It is clear, however, that Trebouxia is able to occur in abundance throughout the world in its rôle as the alga within the majority of lichens.
Photographs: Kindrogan, Perthshire, April 2002.