What's so interesting about lichen?
Well, did you know that lichens are super sensitive to air pollution, and so they help indicate pollution problems to scientists? They also break down rocks to create soil where there previously was none, creating conditions for plant habitation. A number of pharmaceutical compounds come from lichens. Do you know what lichen is?
Lichen is a composite organism. It's composed of both a fungus and, depending on the lichen, an alga or a bacterium.
Lichenology is a discipline in and of itself, but there aren't all that many lichenologists, and so there are still a lot of unanswered questions concerning the relationship between the organisms making up lichen. There is evidence of symbiosis. In the case of algae, it conducts photosynthesis to produce food for itself and for the fungal partner. The fungal partner, in turn, protects the algae from drying out, providing it with water, and in some cases, minerals.
It sounds symbiotic, but there's also evidence of parasitism. For starters, while the algae and bacteria partners can exist on their own in nature, the fungal partner cannot. In the case of algae/fungus partnerships, the fungal partner destroys algae cells in the process of exchanging nutrients with it. The lichen survives this only because algae reproduce cells faster than they're destroyed.