Astrothelium stromatofluorescens

The lichen has a smooth, somewhat shiny, pale ochraceous-green thallus with a cortex surrounded by a thin (0.2 mm) black prothallus line. The ascomata are spherical, measuring 0.2–0.5 mm in diameter, and arranged in groups of 7 to 50 in pseudostromata (a stroma in which fungal cells and bits of host tissue are mixed). The ascospores, which number eight per ascus, are hyaline, spindle-shaped (fusiform) with rounded edges, and measure 20–23 by 7–9 μm. The spores have 3 transverse septa that form diamond-shaped cavities (lumina), and are ensheathed in a gelatinous layer up to 6 μm thick.^[1]

The pseudostromata contain lichexanthone, a lichen product that causes these structures to fluoresce a yellow colour when lit with a long-wavelength UV light. A similar species, Astrothelium phlyctaena, is distinguished by having lichexanthone in both the pseudostromata and the thallus.^[1]

Astrothelium stromatofluorescens is only known to occur in Brazil and Bolivia. In addition to the type locality in Rondônia, it has also been recorded in Amazonas.^[2] In Bolivia, it was found in the Ríos Blanco y Negro Wildlife Reserve (Santa Cruz).^[3]