Astrothelium rubrocrystallinum

Astrothelium rubrocrystallinum is a species of corticolous (bark-dwelling), crustose lichen in the family Trypetheliaceae, first described in 2016. It is found in Brazil.

Taxonomy
The species Astrothelium rubrocrystallinum was formally described by the lichenologists André Aptroot and Marcela Eugenia da Silva Cáceres in 2016. The type specimen was collected in Mata do Junco, Santa Luzia do Itanhy, Sergipe, Brazil, at an elevation of approximately 150 m, on 15 April 2011, by the second author.

Description
The thallus of Astrothelium rubrocrystallinum is, smooth, somewhat shiny, and continuous, covering areas up to 5 cm in diameter and about 0.2 mm thick. It is olive-brown in colour, surrounded by a black line approximately 0.2 mm wide, and does not induce gall formation on the host bark. The ascomata are roughly spherical, measuring 0.30–0.45 mm in diameter, mostly found in groups or lines of 2–5 in poorly distinguished. These are mostly distinctly raised above the thallus and are of thallus colour. Inside, there is a dense layer of red crystals about 0.2 mm wide around the ascoma wall, visible as dark through the hyaline cortex around the ostiole. The wall is black all around, up to 50 μm thick. Ostioles are apical, not fused, flat, black, and surrounded by an ochraceous ring of about 0.2 mm, which sometimes has a grey margin. The is inspersed with hyaline oil globules. Asci contain 8 each. Ascospores are hyaline, contains three septa, and have a shape. They measure 22–27 by 7–9 μm and have rounded ends and diamond-shaped. They are not surrounded by a gelatinous layer. were not observed to occur in this species.

Astrothelium rubrocrystallinum belongs to the Astrothelium annulare group and is characterised by the copious red crystals in a thick layer around the ascomata. It is most closely related to Astrothelium buckii, but differs in having shorter with typical dimensions of 37–47 by 14–16 μm.

Chemistry
The thallus surface of Astrothelium rubrocrystallinum is UV negative. The crystals in the medulla react K+ (purple). Thin-layer chromatography analysis reveals the presence of an unidentified anthraquinone substance.

Habitat and distribution
This species is found on the smooth bark of trees in the Atlantic Forest and at the time of its publication was known only from Brazil.