Lichens often grow in microhabitats where they are exposed to severe abiotic stresses such as desiccation and temperature extremes. They are also often exposed to levels of light that are greater than lichen photobionts can use in carbon fixation. Unless regulated, excess energy absorbed by the photobionts can convert ground state oxygen to reactive oxygen species (ROS). These ROS can attack the photosynthetic apparatus, causing photoinhibition and photo-oxidative stress, reducing the ability of the photobionts to fix carbon. Here, we outline our current understanding of the effects of high light on lichens and the mechanisms they use to mitigate or tolerate this stress in hydrated and desiccated states. Tolerance to high light can be achieved first by lowering ROS formation, via synthesizing light screening pigments or by thermally dissipating the excess light energy absorbed; second, by scavenging ROS once formed; or third, by repairing ROS-induced damage. While the primary focus of this review is tolerance to high light in lichen photobionts, our knowledge is rather fragmentary, and therefore we also include recent findings in free-living relatives to stimulate new lines of research in the study of high light tolerance in lichens.

Photosynthesis was compared in two cyanobacterial lichens (Lobaria hallii and Peltigera praetextata) and two green algal lichens (Lobaria pulmonaria and Peltigera leucophlebia) exposed to red, green or blue light. Cyanolichens had substantially lower photosynthetic CO₂ uptake and O₂ evolution than the green algal lichens in blue light, but slightly higher photosynthesis in red and green light. The effective quantum yield of photosystem (PS) II (Φ_PSII) decreased with increasing red and green light for all species, but in blue light this response occurred in green algal lichens only. Cyanolichen Φ_PSII increased with increasing blue light at low irradiances, but decreased at stronger exposures. However, after adding red light the efficiency of blue light for photosynthetic O₂ evolution increased by 2.4 times. Because phycobilisomes associated with PSII have a low blue light absorption, our results are consistent with blue light absorption mainly by Chl in PSI. Thereby, unequal allocation of excitation energy between PSII and PSI results in low cyanolichen photosynthesis under blue light. This is new knowledge in the science of lichenology with important implications for e.g. the reliability of using Chl fluorometers with blue light for cyanolichens.

Chlorophyllafluorescence is often used to estimate various types of damage in lichens. In order to optimize the output and improve interpretations of such measurements the protocol for pretreatment and measuring is important. To study the effects of measurement conditions, the lichensLobaria pulmonaria,L. scrobiculata,Xanthoria parietinaandParmelia sulcatawere first stressed by high light intensities at 600 or 1000 µmol photons m⁻²s⁻¹for 4 h. Then various conditions during recovery or pretreatment were used to optimize the detection of more lasting damage. Recovery from photoinhibition was incomplete in darkness, whereas light as low as 0·2 or 1·0 µmol m⁻²s⁻¹resulted in complete recovery if the recovery period was long enough. Additionally, low intensity light given for1·5 h after one day in darkness caused rapid and complete recovery. In conclusion, before measuring maximal PSII efficiency (F_v/F_m) with chlorophyll fluorescence, it is important to let lichens recover in low intensity light and not in darkness, to optimize recovery from photoinhibition; dark adaptation can only be recommended if the photoinhibition status of the lichens is of interest.