Chrysothrix citrinella

The lichen has a crustose thallus composed of small, tightly attached microsquamules, ranging from 0.05 to 0.6 mm in diameter, bright citrine yellow in colour. The surface of these microsquamules partly breaks down into soredia, which measure about 40–55 μm in diameter, giving the thallus a distinctly powdery appearance.^[2]

Chemically, C. citrinella contains the compound calycin, identifiable by specific chemical spot tests that produce an orange reaction with potassium hydroxide solution (K+ orange) and para-phenylenediamine (P+ orange). It also has a characteristic dark orange fluorescence under ultraviolet light.^[2]

This species is closely related to Chrysothrix granulosa, Chrysothrix placodioides, and Chrysothrix candelaris, all of which share similar chemistry. However, C. citrinella is distinct due to its microsquamulose thallus that breaks down into regular soredia. Unlike C. granulosa, it is relatively thin and does not easily detach intact from its substrate. It also differs from C. placodioides, which lacks soredia, and from C. candelaris, which lacks a microsquamulose structure.^[2]

At the time of its original publication, Chrysothrix citrinella was only known from its type locality in Maranhão, Brazil, where it grows on sandstone in Cerrado vegetation. It appears to prefer this specific substrate and is locally abundant at this location.^[2]