Photoprotection in lichens: adaptations of photobionts to high light

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Abstract
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.

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