Cladonia grayi

Cladonia grayi, commonly known as Gray's cup lichen or Gray's pixie cup, is a species of fruticose lichen in the family Cladoniaceae. It is characterised by small, leaf-like squamules forming its primary thallus and distinctive upright podetia (5–15 mm tall) that develop into goblet-shaped cups. The species contains several unique lichen substances, primarily grayanic acid, which causes it to glow light blue under ultraviolet light, and has also been found to produce protective pyrrolopyrazine compounds. Its photobiont is the green algal species Asterochloris glomerata.

The fungal component shows distinctive growth responses during development that occur only when encountering compatible algal cells, with the fungus growing extremely slowly at ≤1 cm per year in culture. Though found worldwide, C. grayi is most abundant in Arctic and temperate regions of the Northern Hemisphere. It typically grows on acidic substrates including rotting wood, organic soil layers, and bare ground, showing particular abundance in pine forests across a range of humidity conditions. Two chemical variants (chemotypes) have been documented: one containing only grayanic acid and 4-O-demethylgrayanic acid, and another that additionally contains substances of the fumarprotocetraric acid complex.