Pyrenula guyanensis is a species of corticolous (bark-dwelling) crustose lichen in the family Pyrenulaceae. The species forms a pale yellowish-gray crust with a slightly warted surface and produces dark fruiting bodies (perithecia) in distinct warts up to 1.2 mm wide. It contains lichexanthone, a substance that causes the thallus to fluoresce yellow under ultraviolet light, and grows on tree bark in low open woodlands and savannah scrub on nutrient-poor sandy soils.
Pyrenula guyanensis was described as new by Harrie Sipman and André Aptroot in 2013. The holotype was collected by Sipman on near the northeast edge of the upper plateau in BolÃÂvar, Venezuela; the elevation was about . The specific epithet reflects its occurrence in the Guayana/Guyana Highlands.
This species forms a pale yellowishâÂÂgray crust several centimeters across that has a slightly warted surface and often a thin black border. A very thin around 5 üm thick encloses the thallus, which lacks white spots and develops within the outer bark (it is ). The perithecia occur in distinct warts that can be 0.8âÂÂ1.2 mm wide and are somewhat constricted at the base. Each perithecium is more or less spherical (subglobose), up to about 1 mm wide, with a heavily wall 100âÂÂ200 üm thick and a black pore at the apex. The is densely sprinkled with tiny, 1âÂÂ3 üm wide colorless droplets. The asci are long and slender (about 150 üm by 15 üm). Each ascus bears eight pale grayâÂÂbrown ascospores in a single row. These spores have three crossâÂÂwalls (septa), measure 14âÂÂ20 üm long and 7âÂÂ10 üm wide, and have lensâÂÂshaped central chambers and more rounded end chambers. They do not develop a thick inner wall at the tips. The thallus contains lichexanthone, which causes it to fluoresce yellow when lit with long-wavelength ultraviolet light.
The corticolous (bark-dwelling) Pyrenula guyanensis is known only from the Guyana Highlands and adjacent areas in Guyana and Venezuela, where it occurs in low, open woodland and savannah scrub on nutrient-poor white sandy soils. No additional location were reported by Aptroot in his 2021 world key to the genus.