Lichens aren't two organisms living together -- they're three

Textbooks describe lichen as a simple partnership between a fungus and an alga or cyanobacterium. In 2016 researchers found a second, previously overlooked fungal partner -- a yeast embedded in the outer layer of many lichens -- overturning the 'dual organism' definition biology had taught for over a century.

— Toby Spribille et al., Basidiomycete yeasts in the cortex of ascomycete macrolichens — Science, 2016