Astrothelium megeustomum is a species of corticolous (bark-dwelling), crustose lichen in the family Trypetheliaceae, first scientifically described in 2016. It is found in Brazil.
The species was formally described in 2016 by André Aptroot and Carlos Fraga. The type specimen was collected in Guaratuba, Paraná, Brazil. Fraga found it growing on tree bark in arboreal restinga on May 8, 2015.
Astrothelium megeustomum has a , smooth, somewhat shiny, and continuous thallus covering areas up to 7 cm in diameter and about 0.2 mm thick. The thallus is pale yellowish-green and not surrounded by a . It does not induce gall formation on the host bark. Ascomata are (pear-shaped), measuring around 0.6âÂÂ1.2 mm in diameter. They are mostly aggregated in groups of 2âÂÂ5 and mostly immersed in the bark tissue below , which are distinctly raised above the thallus and mostly irregular in outline. The wall of the ascomata is and up to 80 üm thick. Ostioles are eccentric, fused, flat, pale brownish, white-, and surrounded by a whitish zone. does not contain oil globules. Asci contain eight each. Ascospores are hyaline, , , measuring 117âÂÂ125 by 17âÂÂ21 üm, with pointed ends and surrounded by a gelatinous layer up to 10 üm thick. The median septum is thickened. were not observed.
The thallus surface of Astrothelium megeustomum is UV negative, and the medulla has no reaction to the potassium hydroxide (KâÂÂ) chemical spot test. The ostiolar region has a UV+ (yellow) reaction. Thin-layer chromatography analysis reveals the presence of lichexanthone, a xanthone substance that fluoresces when lit with a long-wavelength UV light.
This species is found on smooth bark of trees in old-growth forest. At the time of its original publication, it was only known to occur in Brazil.