Astrothelium is a large genus of corticolous (bark-dwelling) lichens in the family Trypetheliaceae. The genus is characterized by a corticate thallus and diverse ascomata structures, which can be simple, aggregated, or forming . Astrothelium is also notable for the walls of its ascomata, the so-called (i.e., tightly interwoven) arrangement of cells in these walls, and various forms of, transparent spores.
The genus was circumscribed in 1824 by German botanist Franz Gerhard Eschweiler, with Astrothelium conicum assigned as the type species.
The thallus, or the vegetative body of Astrothelium, is, meaning it is covered by a protective . The ascomata, which are the fruiting bodies containing the reproductive spores, can appear in various forms such as simple, aggregated, or forming . These structures often differ in their composition and colour, and can be used as to distinguish between species. The ascomata can range from being immersed within the thallus to prominently displayed on the surface. The ostioles, or small openings that allow the release of spores, can be located at the apex or eccentrically on the ascomata and may appear as either simple or fused structures.
Astrothelium is also notable for its ascomatal wall, which is made up of hyphal cells and is typically carbonized. The wall features a, a term used to describe a tightly interwoven arrangement of cells.
Within the, the tissue that houses the asci (spore-producing structures), the cells are either clear or filled with oil droplets. The, which are the sexual spores responsible for reproduction, are and hyaline in appearance. They can also exhibit a variety of forms, including transversely septate or, reflecting their segmented or multi-cellular nature.
, Species Fungorum accepts 267 species in Astrothelium.