Lichens, I think, deserve a page of their own, alongside plant and animal symbionts. After all, neither algae or cyanobacteria count as plants, and the relationship between them, the fungi, and yeast warrants a closer look.
To understand lichens you need to understand what they are, and what they're made of. Lichens are composite organisms, a unique form of life that consists of 2 (or more) species that have basically fused into a new organism, with its own adaptations and survival strategies. Lichens always contain a mycobiont and a photobiont, as well as potential a yeast or bacterial partner alongside those two. The fungal portion, or mycobiont, makes up the majority of the lichen, with fungal cells vastly outnumbering all other cells. Its role is
One thing that sets lichens apart from every other mutualism I've covered is the level of contact between symbionts. While there are many other relationships where one cannot survive without the other, or where the hyphae of a fungus permeates the cells of a plant, but lichen are even closer than that.
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