A symbiotic association between a fungus and an alga is called a lichen. The fungus forms most of the lichen’s body, or thallus, while the algal partner—called the photobiont—produces carbohydrates through photosynthesis.
The fungal filaments protect the alga from drying out and help collect water and minerals from rain, dust, and the substrate. Some lichens instead partner with cyanobacteria, traditionally called blue-green algae, and a few contain both an alga and cyanobacteria.
Lichens are not plants, mosses, or single organisms in the ordinary sense. They are composite life forms with their own shapes, colors, and chemistry. Common growth forms include crustose lichens that cling tightly to rock, foliose leaf-like forms, and branching fruticose forms.
A common mix-up is mycorrhiza, which is also a fungus-based symbiosis but involves a fungus and plant roots. Lichens can survive in extreme habitats and are important pioneers on bare rock, where they help begin soil formation.