<P> About 90% of all known lichens have a green alga as a symbiont . "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 . </P> <P> The term "Trebouxioid" refers to members of the Trebouxia algae or other algae that resemble them: a clorococcoid green algae photobiont in the genus Trebouxia . Algae that resemble members of the Trebouxia are presumed to be in the class Trebouxiophyceae and go by the same descriptive name (Trebouxioid). Trebouxia was once included here, but is now considered to be in a separate class Trebouxiophyceae . </P> <P> Although the photobionts are almost always green algae (chlorophyta), sometimes the lichen contains a blue - green alga instead (cyanobacteria, not really an alga), and sometimes both types of photobionts are found in the same lichen . </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 . </P>

How is the association between fungi and algae beneficial to each other