The thallus of Astrothelium xanthosordithecium is slightly shiny and olivaceous green, spreading up to 110 cm (43 in) in diameter and about 0.1 mm thick. It closely follows the substrate without forming a prothallus, an initial growth phase or border seen in other lichens. Reproductive structures (ascomata) are pear-shaped and fused, forming within pseudostromata, a tissue structure that houses multiple ascomata. These pseudostromata are often whitish, round to lobate, and can be 1–3 mm in diameter and 0.9–1.4 mm high. Each pseudostromatum typically has a single group of fused ascomata, thus a single ostiole (opening), which is black and eccentric (off-centre).[2]
The internal tissue, or hamathecium, contains tiny brownish oil globules. The lichen produces eight spores per ascus, which are clear (hyaline), dividided into four compartments by septa (3-septate), and measure 34–37 by 10–12 μm. These spores are long-ellipsoid in shape and do not have a surrounding gelatinous sheath. No pycnidia (structures for asexual reproduction) were observed.[2]
Chemically, both the thallus and pseudostromata show a yellow fluorescence under ultraviolet light. Thin-layer chromatography confirmed the presence of lichexanthone, a secondary metabolite that contributes to the lichen's unique characteristics.[2]
Astrothelium xanthosordithecium is closely related to Astrothelium sordithecium, sharing the habitat and the brownish hamathecium inspersion. However, it is distinguishable by its chemical composition, specifically the presence of lichexanthone, which sets it apart from any other species within Astrothelium.[2]
