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Photosynthetic organisms have evolved a great variety of mechanisms to optimize their use of sunlight. Some of the clearest examples of adaptations can be seen by comparing photosynthesis in different species and in different individuals of the same species that grow under high and low light levels. While the adaptations of sun and shade higher plants have been relatively well studied, much less information is available on the photobionts of lichenized Ascomycetes. An important adaptation that can protect photosynthetic organisms from the potentially harmful effects of excess light is non-photochemical quenching (NPQ); NPQ can dissipate unused light energy as heat. Here we used chlorophyll fluorescence to compare the induction and relaxation of NPQ and the induction of electron transport (rETR) in collections of the same lichen species from exposed and from more shaded locations. All species have trebouxioid photobionts and normally grow in more exposed microhabitats but can also be readily collected from more shaded locations. Shade forms display generally higher NPQ, presumably to protect lichens from occasional rapid increases in light that occur during sunflecks. Furthermore, the NPQ of shade forms relaxes quickly when light levels are reduced, presumably to ensure efficient photosynthesis after a sunfleck has passed. The maximal relative electron transport rate is lower in shade than sun collections, probably reflecting a downregulation of photosynthetic capacity to reduce energy costs. We also compared collections of pale and melanized thalli from three species of shade lichens with Symbiochloris as their photobiont. Interestingly, NPQ in melanized thalli from slightly more exposed microhabitats induced and relaxed in a way that resembled shade rather than sun forms of the trebouxioid lichens. This might suggest that in some locations melanization induced during a temporary period of high light may be excessive and could potentially reduce photosynthesis later in the growing season. Taken together, the results suggest that lichen photobionts can flexibly adjust the amount and type of NPQ, and their levels of rETR in response to light availability.
In our earlier work, we demonstrated that the oxidases tyrosinase (TYR), laccase (LAC), and a heme peroxidase (POX) occur widely in lichens. Here we report on the occurrence of another oxidoreductase enzyme, quinone reductase (QR) (EC 22.214.171.124). While QR has been reported to occur widely in other organisms, there is currently no information on QR activities in lichens. Here we present a survey of QR activity in 14 species of lichens. Results demonstrate that QR activity is readily detectable in all lichen species tested. However, activities vary greatly, with ‘jelly’ lichens in the genera Collema and Leptogium having the highest activities. QR, LAC and POX are all believed to have a role in extracellular hydroxyl radical production. However, in this study no correlation was found between the activities of these enzymes and the rates at which hydroxyl radicals were produced. Possible roles for QR in lichen biology are discussed.
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.
Some lichenized ascomycetes synthesize melanic pigments in their upper cortices when exposed to ultraviolet light and high solar radiation. Our previous work showed that melanized chloro- and cyanolichens from both high light and more shaded habitats were less photoinhibited than pale ones during controlled exposure to high light. However, protection from high light might not necessarily be the consequence of just sun-screening by melanins in upper cortices. An inherent problem with earlier experiments was that the photobionts of melanized thalli might have received more light than those beneath pale cortices. The photobionts may therefore have possessed other light-induced tolerance mechanisms that gave protection from photoinhibition. Here, we aimed to test directly the inherent tolerance of lichen photobionts to photoinhibition. The method involved removing the lower cortices and medullas of three lichen species, Cetraria islandica, Crocodia aurata and Lobaria pulmonaria, and exposing the photobionts to light from below. Results confirmed that most of the improvement in tolerance to photoinhibition in melanized lichens derives from fungal melanization in the upper cortex. However, in C. islandica, the most heavily melanized species, algae from melanized thalli possessed a significantly higher tolerance to photoinhibition than those from pale thalli, suggesting that photobionts can also adapt themselves to high light.
Between 16–21 August 2004, 250 lichenologists gathered in Tartu, Estonia for the Fifth Symposium of the International Association for Lichenology (IAL5). The meeting comprised six principal lecture sessions and associated posters. This issue of The Lichenologist presents papers deriving from contributions to Session 3: Genes, Physiology and Structure. This session probably had the broadest scope of all those comprising the symposium because it essentially encompassed all aspects of the general biology of the lichen thallus. The response to this topic from prospective authors and presenters was excellent and the final programme included fourteen oral presentations and sixteen posters; of this total of thirty contributions, eight are published here. Inevitably, several of the oral and poster presentations were already submitted to, or destined for, alternative journals or were considered too preliminary to warrant publication at this stage. In particular, there were several excellent “molecular” contributions dealing with such topics as breeding systems, genetic diversity, microbial diversity within thalli, and the genetic control of secondary metabolite production, but none were available or offered as papers to this volume.
This review discusses the production of reactive oxygen species (ROS) by lichens and their possible roles. All organisms produce ROS, and production is increased by many abiotic and biotic stresses. Intracellular ROS production is generally considered to be harmful, and a variety of enzymic and non-enzymic scavenging systems exist to detoxify them. However, extracellular ROS formation has been suggested to play ‘positive roles’, particularly in the response of organisms to stress. Given their high stress tolerance, it is rather surprising that studies on extracellular ROS production by lichens have just started. Surveys of a wide range of lichens have shown that constitutively high rates of extracellular superoxide production occur in the Suborder Peltigerineae, but production appears to be absent in other groups. In some members of the Peltigerineae ROS production is stimulated by desiccation and wounding. It seems probable that the enzymes that produce the superoxide are laccases, based on first the types of substrates that lichens can break down, and second the dependence of the breakdown of these substrates on pH, temperature and the presence of inhibitors. While much more work is needed, we suggest that physiological roles of extracellular ROS production will be found to include defence against pathogens, melanization, and lignin breakdown.