Astrothelium macrostomum is a species of corticolous (bark-dwelling), crustose lichen in the family Trypetheliaceae. It is found in Brazil, Guyana, and Venezuela.
The species was formally described as new to science in 2016 by the Dutch lichenologist André Aptroot. The type collection was made by Harrie Sipman in, Minas Gerais, Brazil, on 16 September 1997.
The thallus of Astrothelium macrostomum is, smooth, somewhat shiny, and continuous, covering areas up to 7 cm in diameter and less than 0.1 mm thick. It appears olive-green to olive-grey, typically lacking a, and induces gall formation on the host bark, causing the lower bark or cambium layer to swell and erupt through the upper bark.
Ascomata are (pear-chaped), about 0.6–1.2 mm in diameter, usually aggregated in groups of 2–5, and mostly immersed in the bark tissue. The wall is, up to approximately 80 μm thick. Ostioles are eccentric and fused, with a flat, white- appearance, surrounded by a whitish zone. The does not contain oil lobules. Asci each contain eight . These ascospores are hyaline, 5–7-septate,, measuring 65–85 by 16–19 μm, with pointed ends and diamond-shaped, not surrounded by a gelatinous layer. were not observed to occur in this species.
The thallus surface of Astrothelium macrostomum is UV−, while the medulla is K−. The ostiolar region has a UV+ (yellow) reaction. Thin-layer chromatography indicates the presence of lichexanthone, a xanthone compound that fluoresces when lit with a long-wavelength UV light.
Astrothelium macrostomum is similar to Astrothelium eustomum, but can be differentiated by its 5–7-septate, as opposed to the 3–5-septate and smaller ascospores of A. eustomum. Astrothelium diplocarpoides shares some similar characteristics, but differs in having an inspersed and the presence of lichexanthone throughout the thallus.
Astrothelium macrostomum grows on the smooth bark of trees within rainforests. It is known to occur in Brazil, Guyana, and Venezuela.