Buellia is a genus of mostly lichen-forming fungi in the family Caliciaceae. The fungi are usually part of a crustose lichen.[2] In this case, the lichen species is given the same name as the fungus. But members may also grow as parasites on lichens (lichenicolous).[2] The algae in the lichen (the photobiont partner) is always a member of the genus Trebouxia.[2]
Lichens in the genus are commonly called disc lichens,[3] or button lichens.[4]: 229 The genus has a widespread distribution and contains about 450 species.[5]
Genetic studies indicate that the genus Amandinea and Buellia may be the same,[6] although this is not widely accepted.[7]
Genus Buellia consists of lichens that are either lichenised or occasionally lichenicolous. This means they typically form a symbiotic relationship with a photosynthetic partner, or in some cases, parasitise other lichens. Most Buellia species are independent, forming their own visible thallus (lichen body), but some start as parasites and eventually develop independent thalli.[8]
The thallus of Buellia species is crustose, meaning it forms a crust-like layer. Its appearance can range from continuous to patchy, and it may be cracked or broken into small, tile-like segments (areolate). In some cases, the thallus may develop into small scales or lobes. Often, there is a prothallus, which is a preliminary growth stage that typically appears dark and may form a thin outline or a more extensive network between the areoles. The surface of the thallus can be colourless or various shades of brown, yellow, or green, and it may be smooth, rough, or covered in a fine powder (pruinose). The margins of the thallus can be clearly defined or blend seamlessly into the substrate.[8]
The primary photobiont (symbiotic partner conducting photosynthesis) in Buellia is usually a species of Trebouxia. The medulla, or inner tissue, is typically white but may contain yellow or orange pigments. Calcium oxalate crystals, which are colourless and needle-shaped, are often present in the medulla.[8]