Astrothelium trematum is a species of corticolous (bark-dwelling), crustose lichen in the family Trypetheliaceae, first described in 2018. Found only in Brazil, it is characterised by its lack of lichexanthone and pigments, and distinctive .
Astrothelium trematum was formally described by lichenologists Klaus Kalb and André Aptroot in 2018. The type specimen was collected near Caraguatatuba, São Paulo, Brazil, in a plantation of Theobroma cacao. The specimen was collected by the first author on 9 March 1980. The specific epithet trematum refers to the distinct ring around the ostiole that separates the ostiole from the thallus.
The thallus of Astrothelium trematum is dull, up to 0.3 mm thick, and pale olivaceous green, with a thick and a containing hyaline crystals. It is not surrounded by a . Ascomata are spherical to, 0.6–0.9 mm in diameter, solitary in warts. The is low hemispherical, almost completely covered by a thallus layer, about 1–1.5 mm in diameter, and without pigmentation. Ostioles are apical, consisting of exposed, black ascoma wall, about 0.2 mm wide, surrounded by a whitish ring of approximately 0.3 mm in diameter. The is not . number eight per ascus, and are hyaline with 5 to 7 septa, measuring 95–135 by 25–33 μm. They have a long ellipsoid shape with internal diamond-shaped . were not observed.
The thallus of Astrothelium trematum is UV−, C−, K−, KC−, and P−. Thin-layer chromatography analysis did not detect any secondary metabolites.
Astrothelium trematum was found on tree bark in a cocoa plantation, and at the time of its original publication, was known only to occur in Brazil.