Astrothelium octoseptatum is a species of corticolous (bark-dwelling) lichen in the family Trypetheliaceae. Found in Brazil, it was formally described as a new species in 2016 by André Aptroot and Marcela Cáceres. The type specimen was collected by the authors in the Parque Natural Municipal de Porto Velho (Porto Velho, Rondônia), in a low-altitude rainforest. The lichen has a smooth and somewhat shiny, pale greenish-grey thallus that lacks a prothallus and covers areas of up to in diameter. The ascomata are pear-shaped () and typically occur in groups of two to five, usually immersed in the bark tissue. The lichen contains lichexanthone, a substance that causes the surface of the pseudostroma to glow yellow when lit with a high-wavelength UV light. The species epithet octoseptatum refers to the ascospores, which usually have eight septa that divide the spore into distinct compartments.
See also List of lichens of Brazil
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