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Reflectance from dry and wet thalli of Physcia aipolia and Xanthoria parietina before and after acetone rinsing. Each curve shows the average of five measurements
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Lichens, representing mutualistic symbioses between photobionts and mycobionts, often accumulate high concentrations of secondary compounds synthesized by the fungal partner. Light screening is one function for cortical compounds being deposited as crystals outside fungal hyphae. These compounds can non-destructively be extracted by 100% acetone fr...
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... more transparent after acetone rinsing, evidenced by a substan- tially greener appearance compared to the grey wet control thalli (Fig. 3). Thus, the green photobiont layer below the upper cortex became visible after removal of atranorin. The darker colour of hydrated atranorin-deficient thalli was clearly documented in the reflectance spectra (Fig. 4a). Control thalli, on the other hand, showed only a slight decrease in reflectance after wetting (Fig. 4a). Xanthoria parietina subjected to acetone rinsing also became greener, but not darker when hydrated (Solhaug and Gauslaa ...
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... the grey wet control thalli (Fig. 3). Thus, the green photobiont layer below the upper cortex became visible after removal of atranorin. The darker colour of hydrated atranorin-deficient thalli was clearly documented in the reflectance spectra (Fig. 4a). Control thalli, on the other hand, showed only a slight decrease in reflectance after wetting (Fig. 4a). Xanthoria parietina subjected to acetone rinsing also became greener, but not darker when hydrated (Solhaug and Gauslaa ...
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... spectra from hydrated P. aipolia ( Fig. 4a) and X. parietina (Fig. 4b) showed that acetone-rinsed P. aipolia had substantially reduced reflectance compared to controls throughout the measured spectrum, whereas rinsed X. parietina exhibited increased reflectance at blue wavelengths. In the air-dry state, P. aipolia showed only Fig. 2 Schematic illustration of screening ...
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... spectra from hydrated P. aipolia ( Fig. 4a) and X. parietina (Fig. 4b) showed that acetone-rinsed P. aipolia had substantially reduced reflectance compared to controls throughout the measured spectrum, whereas rinsed X. parietina exhibited increased reflectance at blue wavelengths. In the air-dry state, P. aipolia showed only Fig. 2 Schematic illustration of screening efficiency calculation. It is assumed ...
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... by acetone rinsing Fig. 3 A thallus of Physcia aipolia divided into two parts. The right part is acetone rinsed and the left represents the untreated control piece. The wet state is displayed in the upper part of the photo, whereas the lower part shows a mirror image of the same thallus after drying small changes as a result of acetone rinsing (Fig. 4a), whereas the reflectance from X. parietina was greatly increased in the blue spectral region (Fig. ...
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... rinsed and the left represents the untreated control piece. The wet state is displayed in the upper part of the photo, whereas the lower part shows a mirror image of the same thallus after drying small changes as a result of acetone rinsing (Fig. 4a), whereas the reflectance from X. parietina was greatly increased in the blue spectral region (Fig. ...
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... high solar radiation concur with periods of desiccation during which photobionts are susceptible to excess irradiances ( Gauslaa and Solhaug 1996). Therefore, reflection by the cortex and absorption by secondary compounds located in the cortex are two alternative photoprotective mechanisms in lichens. The low reflectance from control thalli (Fig. 4b) shows that parietin functions as a solar radiation-absorbing cortical screening compound. Contrary to X. parietina, the reflectance in hydrated control thalli of P. aipolia was significantly higher than in the compound-deficient rinsed thalli (see Fig. 4a). Therefore, the colourless secondary compound atranorin present in P. aipolia ...
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... alternative photoprotective mechanisms in lichens. The low reflectance from control thalli (Fig. 4b) shows that parietin functions as a solar radiation-absorbing cortical screening compound. Contrary to X. parietina, the reflectance in hydrated control thalli of P. aipolia was significantly higher than in the compound-deficient rinsed thalli (see Fig. 4a). Therefore, the colourless secondary compound atranorin present in P. aipolia controls screens excessive light by causing increased reflection. The reflection from control thalli stays at a constantly elevated level throughout the visible and near infrared spectrum (Fig. 4a) and is probably a result of Mie scattering from atranorin ...
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... significantly higher than in the compound-deficient rinsed thalli (see Fig. 4a). Therefore, the colourless secondary compound atranorin present in P. aipolia controls screens excessive light by causing increased reflection. The reflection from control thalli stays at a constantly elevated level throughout the visible and near infrared spectrum (Fig. 4a) and is probably a result of Mie scattering from atranorin crystals on the hyphal interphases. Mie scattering is caused by particles larger than the wavelength of light and has a low wavelength dependency (see e.g. Björn 2002). The effect of acetone rinsing had minor impacts on reflectance in air-dry P. aipolia thalli, whereas the ...
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... has been tested for compounds located outside the medullary hyphae, but with negative results (Lange et al. 1997). However, medullary hyphae are covered with the strongly water-repellents hydrophobins that are apparently not present in cortical hyphae ( Scherrer et al. 2002). The low screening efficiency of atranorin crystals in air-dry thalli (Fig. 4a) is consistent with a hydrophobic function of this ...
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... In addition to lichenization emerging properties encompassing anhydrobiosis, poikilohydry, antioxidant and DNA repair activity, we indicated pigmentation as a key feature for survivability under ultraviolet stress (Kranner et al., 2005;Wynn-Williams et al., 2002). Specifically, parietin crystals upper cortex layer and hyphal matrix may be identified as one of the effective photo-shielding structures to protect the photobiont and its photosystems (Heber et al., 2006;Solhaug et al., 2010). However, further analyses should be performed to enlighten the combined role of photoprotection provided by parietin together to other pigments, possibly involved in lichens' photo-shielding (Beckett et al., 2021). ...
... To assess the effect of lichen substances on desiccation tolerance, substances were removed using the "acetone rinsing" technique [13,21]. Briefly, before acetone rinsing, thalli were initially left overnight over silica gel to ensure they were completely dry. ...
... While they are rather insoluble and occur as crystals on the surface of the mycobiont hyphae (see Section 1), lichen substances are nevertheless spatially well-positioned to intercept ROS, such as H 2 O 2 , on the surface of the surrounding fungal tissues that otherwise may diffuse from the mycobiont to the photobiont during stress. An alternative explanation could be that lichen substances increase thallus reflectance [21,24], potentially reducing any harmful effects of light during desiccation. While light can certainly increase the harmful effects of desiccation on poikilohydric organisms [14,25], here, desiccation and rehydration were deliberately carried out in the dark or under conditions of low laboratory light. ...
In lichens, secondary metabolites have been shown to protect against biotic stresses such as pathogen attacks and grazing, and abiotic stresses such as ultraviolet (UV) and high photosynthetically active radiation (PAR). Lichen secondary metabolites are known to have strong antioxidant activity, and while theoretically they may have roles in tolerance to other abiotic stresses, these roles remain largely unclear. Here, we used the acetone rinsing method to harmlessly remove most of the secondary metabolites from the thalli of six lichen species. This enabled us to compare the effects of desiccation on thalli with and without the presence of secondary metabolites. Results showed that in general, the presence of lichen substances reduces the effects of desiccation stress. For all species, substances significantly improved the photosystem two (PSII) activity of the photobiont during either desiccation or rehydration. In the mycobiont, in four of the six species, the presence of substances reduced membrane damage, which was assessed by measuring ion leakage during rehydration following desiccation. However, in one species, secondary metabolites had no effect, while in another the presence of substances increased membrane damage. Nevertheless, it seems clear that in addition to their more established roles in protecting lichens against pathogen attacks and grazing, lichen substances can also play a role in aiding desiccation tolerance.
... Photosynthetic organisms need light to grow but too much light can be dangerous (Demmig-Adams et al. 1990) by forming reactive oxygen species (ROS) that cause damage (Foyer 2018). To avoid photodamage of lichens, excess light can be avoided by cortical screening of underlying photobionts (Solhaug et al. 2010). In all photosynthetic organisms, absorbed excess light must either be dissipated in a safe way or ROS produced must be detoxified with various antioxidant systems (Jung and Niyogi 2006). ...
... A slower way in which lichens acclimate to high light is by the synthesis of light-screening fungal pigments, e.g., the dark light-absorbing melanin in melanic species and the pale light-reflecting usnic acid (McEvoy et al. 2007a, b) in usnic species. Such pigments protect the symbiotic photobiont by screening excess photosynthetically active radiation (PAR) and ultraviolet radiation (Solhaug et al. 2010). Fungal pigments are induced by UV-B (Solhaug et al. 2003;McEvoy et al. 2006) and boosted by photosynthates McEvoy et al. 2006) and are thus moderators optimizing lichen growth rates along natural sun-shade gradients (Gauslaa and Goward 2020). ...
... Φ PSII was measured from 0 to 450 μmol photons m −2 s −1 in late summer, and from 0 to 1250 μmol photons m −2 s −1 in spring samples, using a red light ImagingPAM M-series fluorometer (Heinz Walz GmbH, Effeltrich, Germany). The Abs parameter is assumed to be 0.85 in green leaves, but is hard to estimate in lichens in which it is lower due to screening pigments (Solhaug et al. 2010). We assessed apparent ETR (ETR App ) setting Abs = 1. ...
The mat-forming fruticose lichens Cladonia stellaris and Cetraria islandica frequently co-occur on soils in sun-exposed boreal, subarctic, and alpine ecosystems. While the dominant reindeer lichen Cladonia lacks a cortex but produces the light-reflecting pale pigment usnic acid on its surface, the common but patchier Cetraria has a firm cortex sealed by the light-absorbing pigment melanin. By measuring reflectance spectra, high-light tolerance, photosynthetic responses, and chlorophyll fluorescence in sympatric populations of these lichens differing in fungal pigments, we aimed to study how they cope with high light while hydrated. Specimens of the two species tolerated high light equally well but with different protective mechanisms. The mycobiont of the melanic species efficiently absorbed excess light, consistent with a lower need for its photobiont to protect itself by non-photochemical quenching (NPQ). By contrast, usnic acid screened light at 450–700 nm by reflectance and absorbed shorter wavelengths. The ecorticate usnic species with less efficient fungal light screening exhibited a consistently lower light compensation point and higher CO2 uptake rates than the melanic lichen. In both species, steady state NPQ rapidly increased at increasing light with no signs of light saturation. To compensate for less internal shading causing light fluctuations with a larger amplitude, the usnic lichen photobiont adjusted to changing light by faster induction and faster relaxation of NPQ rapidly transforming excess excitation energy to less damaging heat. The high and flexible NPQ tracking fluctuations in solar radiation probably contributes to the strong dominance of the usnic mat-forming Cladonia in open lichen-dominated heaths.
... Our study involved the lichen species Xanthoria parietina (L.) Th. Fr.-already tested in space-like and Mars-simulated conditions-that has proven to have high survival performances due to poikilohydry and anhydrobiosis features, hyphal matrix, thallus structure and lichen secondary substances (Solhaug and Gauslaa 2004;Solhaug et al. 2010;Gauslaa et al. 2012Gauslaa et al. , 2017Lorenz et al. 2022Lorenz et al. , 2023. Lorenz et al. (2023) provide a mainstream photosystem II (PSII) activity analysis using F V /F M as a photochemical indicator. ...
Xanthoria parietina survivability in Mars-like conditions was supported by water-lysis efficiency recovery and antioxidant content balancing with ROS production after 30 days of exposure.
Xanthoria parietina (L.) Th. Fr. is a widespread lichen showing tolerance against air pollutants and UV-radiation. It has been tested under space-like and Mars-like conditions resulting in high recovery performances. Hereby, we aim to assess the mechanisms at the basis of the thalli resilience against multiple space stress factors. Living thalli of X. parietina were exposed to simulated Martian atmospheric conditions (Dark Mars) and UV radiation (Full Mars). Then, we monitored as vitality indicator the photosynthetic efficiency, assessed by in vivo chlorophyll emission fluorescence measurements (FM; FV/F0). The physiological defense was evaluated by analyzing the thalli antioxidant capacity. The drop of FM and FV/F0 immediately after the exposure indicated a reduction of photosynthesis. After 24 h from exposure, photosynthetic efficiency began to recover suggesting the occurrence of protective mechanisms. Antioxidant concentrations were higher during the exposure, only decreasing after 30 days. The recovery of photosynthetic efficiency in both treatments suggested a strong resilience by the photosynthetic apparatus against combined space stress factors, likely due to the boosted antioxidants at the beginning and their depletion at the end of the exposure. The overall results indicated that the production of antioxidants, along with the occurrence of photoprotection mechanisms, guarantee X. parietina survivability in Mars-like environment.
... The results demonstrated that screening techniques differ depending on the chemical since parietin absorbs light while atranorin reflects it. Finally, in terms of photoprotective action, parietin has a poor in vitro SPF value when compared to homosalate, a commercial sunscreen chemical, with values of 1.9 and 3.9, respectively (Solhaug et al. 2010). The anthraquinones haematommone and russulone were identified in the red colour apothecia of Haematomma stevensiae and were interestingly detected in the mycelia of its grown mycobiont as a reaction to a UV light of 365 nm exposure, like in the earlier lichexanthone investigation. ...
Environmental change as well as the continued increase in UV radiation (UVR; 280–400 nm) has a significant impact on terrestrial and aquatic ecosystems. The majority of sun-exposed species are negatively affected by solar UVR. This may be the strong reason behind the evolution of photoprotectants like phenylpropanoids, flavonoids, mycosporines, scytonemin, mycosporine-like amino acids (MAAs), parietin, xanthophyll, phycobiliproteins etc. It is mainly found in cyanobacteria, algae, fungi, lichens and other non-flowering lower plants. These photoprotectors have their own evolutionary significance. To reduce photochemical damage, carotenoids are being considered as UV-protective additives. Carotenoids are major light-harvesting and photoprotective components of the photosynthetic apparatus, and work as quencher of singlet oxygen species. Many other UV-absorbing compounds are also known for their multifunctional capabilities, such as flavonoids, which have antioxidative and antibacterial potentials. The study of these photoprotectants has resulted in the identification of new sunscreen classes and their distribution throughout various microbes and non-flowering lower plants. These natural photoprotectants play an important role in different forms such as scytonemin and MAAs operate as the third line of defence in cyanobacteria, mycosporines are critical for UV-induced photodamage in fungi. Lichens possess a wide range of primary and secondary metabolites, in which MAAs and parietin have major photoprotective effects. Moreover, these photoprotectants aid in diffusing heat from absorbed radiation without generating reactive oxygen species (ROS). This chapter deals with the evolutionary significance of photoprotectors and their possible modes of action.
... The accumulation of protective secondary metabolites in the thallus is considered an adaptive response to harsh environmental conditions (Solhaug et al. 2010). However, a 40% decrease in the content of phenolics in C. islandica exposed to UV-B irradiation was observed (Boustie et al. 2011), which could explain the lower yield in the tested subalpine samples. ...
Lichens are supra-organismal symbiotic systems found in most environments. Environmental factors, such as temperature, altitude, precipitation, UV irradiation, or pathogens, significantly influence the physiology of lichens, and thus their secondary metabolism. The thalli of the same lichen species from different
environments exhibit variation in the production of secondary metabolites and protective pigments. We selected two populations of the lichen Cetraria islandica from habitats differing in altitude, temperature, and precipitation. Then we compared their antioxidative and antibacterial activity. The lichen thalli were
divided into two parts: the upper parts were exposed to light and the lower parts hidden from extensive radiation. The results show that the thalli from harsh alpine environments have higher 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) radical scavenging activity suggesting better tolerance to oxidative stress. On
the other hand, the individuals from milder montane environments generally produce more secondary metabolites, leading to increased antibacterial activity of the extracts. The extracts of C. islandica containing fumarprotocetraric and
paraconic acids exhibit inhibitory effects against gram-positive bacteria (e.g. Staphylococcus aureus) and some lower activity against gram-negative bacteria (e.g. Escherichia coli).
... A particular mechanism for tolerance to long-term light stress in lichens is the synthesis of light-screening secondary metabolites in the upper cortex, such as brown melanins in L. pulmonaria and C. aurata and the orange pigment parietin in X. parietina; these pigments can increase tolerance to photoinhibition (Mafole et al., 2019b;Solhaug et al., 2010). In L. pulmonaria, melanic pigments apparently adjust the light received by the photobiont beneath the screening upper cortex to compensate for variations in tree canopy openness (Gauslaa & Goward, 2020). ...
The photosynthetic apparatus of lichen photobionts has been well-characterised by chlorophyll fluorescence analysis (e.g. by pulse amplitude modulation; PAM), which provides a proxy of the activity of photosystem II (PSII) and its antenna. However, such kinetics are unable to directly characterise photosystem I (PSI) activity and the associated alternative electron pathways that may be involved in photoprotection. Instead, PSI can be probed in vivo by near-infrared absorption, measured at the same time as standard chlorophyll fluorescence (e.g. using the WALZ Dual PAM). Here, we used the Dual PAM to investigate cyclic electron flow and photoprotection in a range of mostly temperate lichens sampled from shaded to more open microhabitats. Sun species displayed lower acceptor side limitation of PSI (Y[NA]) early in illumination when compared to shade species, indicative of higher flavodiiron-mediated pseudocyclic electron flow. In response to high irradiance, some lichens accumulate melanin, and Y[NA] was lower and NAD(P)H dehydrogenase (NDH-2)-type cyclic flow was higher in melanised than pale forms. Furthermore, non-photochemical quenching (NPQ) was higher and faster relaxing in shade than sun species, while all lichens displayed high rates of photosynthetic cyclic electron flow. In conclusion, our data suggest that 1) low acceptor side limitation of PSI is important for sun-exposed lichen; 2) NPQ helps shade species tolerate brief exposure to high irradiance; and 3) cyclic electron flow is a prominent feature of lichens regardless of habitat, although NDH-2-type flow is associated with high light acclimation. This article is protected by copyright. All rights reserved.
... The mycobiont produces such pigments to reduce the photoinhibitory stress for its photosynthetic partner Solhaug 2001, 2004) and to optimize growth across natural light gradients (Gauslaa and Goward 2020). Light screening not only involves cortical pigments (Solhaug et al., 2010), but also reflectance from surfaces of air spaces inside dry cortices and external structures (e.g., pruina, hairs, calcium oxalate crystals). Such structures become exposed after surface water has evaporated and they contribute to the increased opaqueness of lichen cortices during drying. ...
... At the same time, increased reflectance and thus brightness during desiccation is a strategy to reduce photoinhibition and heat load in lichens during sunny weather (Sancho et al., 1994;McEvoy et al., 2007). Species-specific light-absorbing (e.g., melanin, parietin) or light-reflecting cortical pigments (e.g., atranorin, usnic acid) further adjust the solar radiation reaching the photobionts (Solhaug et al., 2010;Phinney et al., 2019). Even though the interaction of hydration and spectral properties profoundly shapes lichen functioning (Phinney et al., 2022), such relationships are not well studied, anddue to species-specific screening pigments and photobiont associationsthere is a need to study these interactions across a range of species with different light requirements. ...
There is a need for non-invasive monitoring of temporal and spatial variation in hydration and photosynthetic activity of red-listed poikilohydric autotrophs. Here, we simultaneously recorded kinetics in RGB-colors (photos), reflectance spectra, water content, maximal (F V /F M), and effective quantum yield of PSII (Φ PSII) during desiccation in foliose lichens differing in cortical characteristics and photobionts. The spectral absorbance peaks of chlorophyll a, phycocyanin, and phycoerythrin were clearly displayed at high hydration levels. Brightness and total RGB colors of the lichens strongly increased during desiccation. The normalized difference vegetation index (NDVI) efficiently estimated hydration level and Φ PSII-a proxy for lichen photosynthesis-in all species, including threatened old forest lichens. Color and reflectance indices based on green wavelengths gave good estimates of water content in cephalo-and chlorolichens, but not in cyanolichens with a wider range of photosynthetic pigments. Due to species-specific characteristics, species-wise calibration is essential for non-invasive assessments of lichen functioning.
... Their distinct yellowish-green color is attributable partly to chlorophyll and partly to the faintly yellow usnic acid. Such pale pigments screen solar radiation by reflectance (Solhaug et al. 2010) thereby reducing solar radiation-induced warming (Kershaw 1975) and prolonging photosynthesis after hydration (Phinney et al. 2022). By contrast, the dark cortex of the melanic genus Bryoria (Fig. 6B) absorbs most incident solar radiation and thereby prevents photoinhibitory damage to the underlying photosynthetic algae (Färber et al. 2014). ...
Hair lichens are distinctive for their capillary growth and typically arboreal occurrence, especially in temperate and boreal forests. They consist of two morphogroups based on cortical pigments, i.e., a brown-black group with fungal melanin and a pale yellow-green group with usnic acid. Here we test the hypothesis that these morphogroups are ecologically distinct and thus appropriately regarded as functional groups. We examine their respective horizontal occurrence in the lower canopy of 60-year-old conifer forests on a 250 m tall volcanic cone in south-central British Columbia. Trees on open south-facing slopes and near the summit were found to support mainly melanic hair lichens (Bryoria and Nodobryoria), whereas more densely spaced trees on N-facing slopes and at the base had higher cover values of usnic lichens (especially Alectoria sarmentosa and Ramalina thrausta). The cover of melanic hair lichens was strongly correlated with canopy openness but not for their usnic counterparts. We suggest that investment in light-absorbing melanic pigments is an extreme form of specialization for high light, favoring persistence in dry, sun-exposed canopies of otherwise cool forests. By contrast, the cortex of pendent usnic hair lichens appears to facilitate optimum light transmission to underlying photobionts in shaded sites, though at the cost of sensitivity to light in open habitats, especially in rather dry regions.
... The hyphal matrix with the thallus structure and lichen substances may be involved in the lichen survivability. The secondary lichen substance parietin-deposited in the thallus' upper cortex-is known for its blue-light, UVB and UVC screening properties 67 , occurring more efficiently in anhydrobiosis or desiccated state 27,67 because of the better thermal dissipation mechanism of light energy 68 . Carotenoids also may have played a crucial role in the photobiont's photoprotection. ...
Xanthoria parietina (L.) Th. Fr. is a widely spread foliose lichen showing high tolerance against UV-radiation thanks to parietin, a secondary lichen substance. We exposed samples of X. parietina under simulated Martian conditions for 30 days to explore its survivability. The lichen's vitality was monitored via chlorophyll a fluorescence that gives an indication for active light reaction of photosynthesis, performing in situ and after-treatment analyses. Raman spectroscopy and TEM were used to evaluate carotenoid preservation and possible variations in the photobiont's ultrastructure respectively. Significant differences in the photo-efficiency between UV irradiated samples and dark-kept samples were observed. Fluorescence values correlated with temperature and humidity day-night cycles. The photo-efficiency recovery showed that UV irradiation caused significant effects on the photosynthetic light reaction. Raman spectroscopy showed that the carotenoid signal from UV exposed samples decreased significantly after the exposure. TEM observations confirmed that UV exposed samples were the most affected by the treatment, showing chloroplastidial disorganization in photobionts' cells. Overall, X. parietina was able to survive the simulated Mars conditions, and for this reason it may be considered as a candidate for space long-term space exposure and evaluations of the parietin photodegradability.
