A lichen is not two organisms. In many, it is three

For a century a lichen was a fungus farming an alga. In 2016 a second fungus, a yeast, was found living in the cortex of many common lichens, and its abundance tracks how the lichen looks.

A lichen has no single species name that is honest. The name belongs to the fungal partner, usually an ascomycete, which builds the body and shelters a green alga or a cyanobacterium that feeds it. That partnership is old, real, and still the core. Toby Spribille and colleagues, trying to explain why two lichens with the same named fungus looked different, found basidiomycete yeasts embedded in the outer cortex. The yeasts were not occasional contaminants. They were specific to the lichen, present across continents, and more abundant in the forms with a particular surface. The layer everyone had drawn as the fungus's own skin contained a second fungus.

The finding was in macrolichens of one broad group, on six continents, not in every stain on every rock. Some lichens may still be the classic pair. Many common ones are not. A century of sections missed the yeast because it does not look like a tenant if you expect tenants to be large. The awe is not that nature is more complicated than a diagram. It is that the diagram was of a thing people hold in one hand, and a third organism was in the part their thumb was touching.