Biology of the lichen symbiosis

Lichens may undergo cycles of dehydration-desiccation-rehydration several times a day and this, combined with high light irradiances, can determinate a strong oxidative stress. With the aim of evaluating how lichens with different photobionts and ecology cope with this stress, lobes from Flavoparmelia caperata, Lobaria pulmonaria, Peltigera leucophlebia and Peltigera praetextata were submitted to four combinations of light and drought and their chlorophyll a fluorescence and pigments were analyzed. Light was the environmental factor with the most negative effects in all the four species, more than desiccation or prolonged periods in darkness. F. caperata was the most resistant species, tolerating moderately high light regardless of its hydration, while the other three species showed more vulnerability to the light in the dry state. Chlorophyll and antioxidant concentrations were not influenced by the treatments and were considered as constitutively expressed. All the three species with green algae seemed to use the xanthophyll cycle when submitted to desiccation and/or high light, but to different extents. Lobaria pulmonaria used it massively when illuminated in the hydrated state, probably also for photoprotection. This is likely to be the reason why it performed better than when illuminated in the dry state. Nevertheless, F. caperata was able to perform better without this extra photoprotection and further investigation is needed to understand the molecular mechanisms underlying its tolerance. To verify if the photobionts share the same mechanisms of their lichen counterparts, the experiment was reiterated with two of them, Trebouxia gelatinosa (photobiont of F. caperata) and Symbiochloris reticulata (photobiont of L. pulmonaria), the latter being studied for the first time from the physiological point of view. The most resistant species was T. gelatinosa, as its performance was comparable to that of its lichenized counterpart. Major differences were the inability to use the xanthophyll cycle and the presence of repair mechanisms upon rewetting. S. reticulata, instead, suffered consistently each treatment and its recovery was always incomplete. When metabolically active, it degraded a large part of its chlorophylls probably to minimize the damage in case of light exposure, but it was not able to rebuild them upon recovery. So, we hypothesized that when isolated it is desiccation tolerant to a lesser extent. We can conclude that lichenization can influence the physiology of photobionts in very different ways, as T. gelatinosa would be able to survive unharmed out of a lichen thallus, while S. reticulata acquires important protective mechanisms from photo-oxidative stress, becoming able to colonize environments that otherwise would kill it as free-living.