Improved photoprotection in melanized lichens is a result of fungal solar radiation screening rather than photobiont acclimation

Abstract

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.

Melanisation in Boreal Lichens Is Accompanied by Variable Changes in Non-Photochemical Quenching

“…As a result, the photobionts of melanised thalli may have developed other mechanisms that increased tolerance to photoinhibition. Recently, we compared the importance of melanisation in tolerance to high light with the importance of other tolerance mechanisms by dissecting away the lower cortices and medullas in a range of species [ 16 ]. This enabled us to photo-inhibit photobionts with light from below i.e., without the presence of a melanised upper cortex.…”

Effect of Melanization on Thallus Microstructure in the Lichen Lobaria pulmonaria

“…The lichen L. pulmonaria responds to solar radiation by synthesizing the brown pigment melanin in the paraplectenchyma that forms the upper cortex of the thallus [ 4 ] ( Figure 1 ). Browning can be quantified by measuring the BRI index [ 14 ], which in the present study was on average c. 15 in the more heavily melanized thalli ( Figure 2 ). This value is similar to our earlier measurements of L. pulmonaria from the same site [ 21 ], although sometimes even darker thalli can be collected with BRIs exceeding 40 [ 22 ].…”

“…Reflectance spectra of the upper cortex were recorded on dry, intact lichen discs. Browning reflectance indexes of the lichen thalli were assessed using the integrating sphere method, as described in Beckett et al [ 14 ]. Briefly, an integrating sphere (model ISP-50-REFL, OceanOptics, Eerbeek, The Netherlands) was pressed against the thalli, which were then illuminated with a halogen lamp (model DH2000, OceanOptics) through a 600 µm thick optical fibre connected to the input port of the integrating sphere.…”

Resilience of Epiphytic Lichens to Combined Effects of Increasing Nitrogen and Solar Radiation

“…The activation of such protective mechanisms comes at a cost, as shown by proteomic analysis in thalli of Cladonia portentosa : In thalli exposed to long-term N treatments, the ability to cope with increased N availability was related to an enhanced energetic metabolism [ 67 ]. In contrast, UV protection and tolerance to high solar radiation are provided by specific sunscreen compounds (e.g., usnic acid, phenolics, parietin [ 68 , 69 , 70 ]) and a general capacity to avoid and repair damages of oxidative stress [ 71 ]. As for N protection strategies, both the synthesis of secondary metabolites and antioxidant activities require energy [ 69 ].…”