Photobiont selectivity in the epiphytic lichens Hypogymnia physodes and Lecanora conizaeoides

In two lichen species, Hypogymnia physodes and Lecanora conizaeoides, often used as model organisms for pollution-sensitive and pollution-tolerant epiphytic lichens, respectively, the hypothesis was tested that the toxitolerance of the Trebouxia photobiont limits the tolerance of the entire lichen symbiosis. Being lecanoralean-trebouxioid associations, H. physodes and L. conizaeoides represent the most common type of lichens. Photobionts of both lichen species deriving from microhabitats with varying supply of S and heavy metals were identified using nuclear ITS nrDNA sequencing. The photobiont of L. conizaeoides was identified as T. simplex, whereas the photobiont of H. physodes belongs to an undescribed Trebouxia species, related to T. jamesii subsp. angustilobata and provisionally named as T. hypogymniae Hauck & Friedl ined. Since T. hypogymniae ined. is also known from Lecidea silacea, which is characteristic of rock and slag with high heavy metal content, a high sensitivity of this alga to pollutants is unlikely to be a key factor for the relatively low toxitolerance of H. physodes. Furthermore, the photobiont cannot be crucial for the extremely high toxitolerance of L. conizaeoides, as T simplex is also known from pollution-sensitive lichens of the fruticose genus Pseudevernia. These findings suggest that the photobiont is not generally a key factor determining pollution sensitivity in the most common type of lichen symbiosis. The high specificity for T. simplex in L. conizaeoides in existing populations of L. conizaeoides suggest that already established thalli could be a source of photobiont cells for re-lichenization.