Astrothelium globosum is a species of lichen in the family Trypetheliaceae. This species is characterised by its immersed ascomata (fruiting bodies) and a thallus consisting of nearly spherical warts, along with distinctive 3-septate ascospores.
Astrothelium globosum was formally described as a new species in the journal The Lichenologist in 2016. The type specimen was collected in, Mata do Crasto, Brazil, on the bark of a tree, in March 2012.
The thallus of Astrothelium globosum is, consisting of closely appressed, nearly spherical warts measuring 0.3–0.9 mm in diameter. These warts form a pale greyish-green cover over areas up to 3 cm in diameter and are approximately 0.9 mm thick. The thallus has a thick hyaline and does not induce gall formation on the host bark. are pyriform, measuring 0.2–0.4 mm in diameter, and usually appear in groups of 2–5, immersed in the bark and visible from above by black ostioles between the thallus warts. The ascomata wall is and up to 50 μm thick. Ostioles are eccentric and brown. The is not inspersed, and are 8 per ascus, hyaline, 3-septate, long ellipsoid, 35–40 by 11–13 μm, with diamond-shaped and surrounded by a gelatinous sheath. were not observed in this species.
This species is distinguished by a unique thallus consisting of almost spherical warts, an unusual three-dimensional feature for a crustose lichen. It is most closely related to Astrothelium simplex, which is differentiated by its longer ascospores.
The thallus surface of Astrothelium globosum is UV−, and the thallus medulla is K−. Thin-layer chromatography, a technique used to identify chemical substances, tests revealed no secondary substances.
Astrothelium globosum is found on the smooth bark of trees in lowland Atlantic rainforest. At the time of its original publication, it was known to occur only in Brazil.