<P> The photosynthetic component of a lichen is called the photobiont or phycobiont . Sometimes the photobiont is a green algae (chlorophyta), sometimes a blue - green alga (cyanobacteria, not really an alga), and sometimes both . The layer of tissue containing the cells of the photobiont is called the "photobiontic layer". </P> <P> "Clorococcoid" means a green alga (Chlorophyta) that has single cells that are globose, which is common in lichens . This was once classified in the order Chlorococcales, which one may find stated in older literature, but new DNA data shows many independent lines of evolution exist among this formerly large taxonomic group . Chlorococcales is now a relatively small order and may no longer include any lichen photobionts . Trebouxia, once included here, is now considered to be in a separate class, Trebouxiophyceae . The term "trebouxioid" refers to members of this class or algae resembling them . </P> <P> "Trebouxioid" means a clorococcoid green algae photobiont is in the genus Trebouxia, or resembles a member of that genus, and is therefore presumably a member of the class Trebouxiophyceae . </P> <P> A cyanolichen is a lichen with a cyanobacterium as its main photosynthetic component (photobiont). Many cyanolichens are small and black, and have limestone as the substrate . 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 . These lichen species are grey - blue, especially when dampened or wet . Many of these characterize the Lobarion communities of higher rainfall areas in western Britain, e.g., in the Celtic Rainforest . </P>

Some fungal species live in permanent mutually dependent relationship with blue green algae