Atranorin Fluorescence: A Possible Photoprotective Role in Lichen Defense Against UV and Blue–Violet Radiation

Epiphytic lichens are naturally subjected to highly fluctuating light and must protect their algal symbionts from harmful short wavelengths while facilitating efficient photosynthesis. Confocal microscopy confirmed that the upper cortices of Hypogymnia physodes and H. bitteri thalli contained colorless secondary metabolites atranorin and chloroatranorin, which were able to protect lichens against excessive UV and blue–violet radiation in two ways. The first way screens algae through visible light reflection; the second way is realized through the absorption of UV-A and blue–violet radiation and fluorescence emission in the yellow–green spectrum. Acetone rinsing removed atranorin and chloroatranorin crystals from the cortical surface, thus reducing the reflectance of both H. physodes and H. bitteri. Nevertheless, fluorescence in H. physodes remained largely unaffected due to the deep-layer localization of atranorin within the upper cortex. However, melanized H. bitteri exhibited weak fluorescence either before or after acetone rinsing, despite containing atranorin and chloroatranorin at levels comparable to H. physodes. Within the heavily melanized lower cortical layer of H. bitteri, young areas contained significant amounts of atranorin and chloroatranorin. A reduction in their content resulted in a compensatory enhancement of non-photochemical quenching in algae. This phenomenon suggests that atranorin and chloroatranorin contribute to the photoprotection of lichens in yet another way through light reflectance and through fluorescence in a photosynthetically safe manner.