Lichen photobionts can be hardened to photoinhibition by pretreatment with light

Lichens often grow in microhabitats where they receive more light than they are capable of using to fix carbon. Unless regulated, this excess energy can end up activating oxygen, thereby forming reactive oxygen species (ROS). These ROS can damage the photosynthetic apparatus and other cellular components, causing photoinhibition and photo-oxidative stress. Tolerance to high light theoretically can be achieved in a variety of ways, but for lichen photobionts, our knowledge of the precise mechanisms involved is rather fragmentary. Here, we show that tolerance to photoinhibition in the cephalolichen Crocodia aurata could be increased by pretreating thalli at a moderate light intensity for 48 h. Increased tolerance was correlated with increased ability to avoid oxidative stress by dissipating the excess energy as heat, here assessed by measuring non-photochemical quenching (NPQ). Increased tolerance only occurred when thalli were pretreated hydrated. The same pretreament did not increase tolerance in the cyanolichen Sticta fuliginosa, which grows in similar habitats to Crocodia, suggesting that cephalo- and cyanobacterial lichens may require different pretreatment conditions. Similarly, tolerance was not increased in collections of the chlorolichen Ramalina celastri from exposed habitats, although additional experiments showed that fluorescence parameters in Ramalina can display considerable plasticity. It seems likely that in sun populations tolerance is fully expressed and cannot be further increased. However, the ability to harden Crocodia to photoinhibitory stress could provide a foundation for more detailed investigations into the mechanism of photoprotection in lichen photobionts such as the type of NPQ or the role of antioxidant enzymes.

Automated summary

The study shows that pretreatment at a moderate light intensity can increase tolerance to photoinhibition in cephalolichen, Crocodia aurata. Increased tolerance is correlated with the ability to dissipate excess energy as heat, thereby avoiding oxidative stress.