A lichen is a symbiotic relationship between fungus and green (or blue-green) algae. Algae are a polyphyletic category of organisms. This indicates that the species in this category are not necessarily related and have no common ancestor. They do, however, have one thing in common: they are eukaryotes that can photosynthesise and have chlorophyll as their principal pigment, although they lack other morphoanatomical characteristics seen in vascular plants. Phycology is the scientific study of algae. Blue-green algae is mentioned in a few places. However, because they are prokaryotes, they are not classified as algae by other agencies.
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