Safely valves for photosynthesis in lichens
Lichens are classic desiccation tolerant or “poikilohydric” organisms. A particular problem for lichens is that they retain chlorophyll during desiccation, and therefore continue to absorb light. Continued light absorption while carbon fixation is inhibited by desiccation can result in the production of reactive oxygen species (ROS). The first line of defence is to dissipate excess energy as heat, which occurs in various ways, including the classical xanthophyll cycle. Continued stress will overwhelm thermal dissipation, and lichens need to possess excellent ROS scavenging systems. Nevertheless, eventually “photoinhibition” can occur. The “PSII repair cycle”, occurring in chloroplasts and in cyanobacteria, involves degrading and re-synthesizing the D1 protein, a key component of photosystem II. In addition to these biochemical methods of photoprotection, lichen mycobionts respond to high light by synthesizing cortical sun-screening secondary metabolites. They screen solar radiation by absorptance (e.g. parietin or vulpinic acid) or by reflectance (atranorin). In some lichens, high light induces the synthesis of dark brown melanic pigments which also screen high light.
Mafole T.C. et al. Tolerance to photoinhibition within a lichen species higher in melanised thalli // Photosynthetica (2019) 57: 96-102. https://doi.org/10.32615/ps.2019.008.
Mafole T.C. et al. Melanisation in the old forest lichen Lobaria pulmonaria (L) Hoffm. reduces the efficiency of photosynthesis // Fungal Ecology (2017) 29: 103-110. DOI: 10.1016/j.funbio.2017.03.005