Lichen is not one organism, but two organisms that live together and help each other – a fungus and an algae in a symbiotic relationship.
Interesting, isn’t it. They are found on tree barks, exposed rock, and biological soil crust. Lichens are used by humans as food and as sources of medicine and dye.
For many years, scientists classified them as a single organism – but the association of fungi with algae or cyanobacteria became evident with advent of microscopy. There is still some discussion on how to classify lichens.
Read more about them on https://www.britannica.com/science/lichen.