Astrothelium bireagens

The lichen forms a crust-like body (thallus) on bark, up to 5 cm (2 in) across and 200–500 μm thick. The surface is blistered or puffed-up (bullate) and light olive-green to olive-yellow, with a thick outer skin (cortex) over a loose algal layer and a thick inner tissue (medulla) containing many grayish crystals. The algal photosynthetic partner (photobiont) is from the green algal genus Trentepohlia.^[2]

The fruiting bodies (perithecia) are prominent and exposed, brownish black, and may be fused sideways into grouped structures (pseudostromatic groupings). Individual perithecia are 0.8–1.2 mm in diameter and 0.4–0.6 mm high, with a flattened top; the pore-like opening (ostiole) is sometimes multiple (2–3). Internally, each fruiting body has 5–10 chambers that merge into a single ostiole. The asci contain eight colorless, 4-celled (3-septate) ascospores with thick cross-walls (distosepta), measuring 17–22 × 6–8 μm; the spores do not stain with iodine (non-amyloid, I−). Thin, branched sterile filaments (paraphyses) are present in the tissue surrounding the asci (hamathecium).^[2]

The species is known from Colombia (Guainía Department), where it has been collected in the municipality of Inírida, including at 150–180 m (490–590 ft) elevation. It was found growing on bark in natural savanna vegetation over white sand, and the type collection was made near the La Ceiba indigenous community; an additional specimen was reported from the Caño Vitina community.^[2]