The lichen cushion: A functional perspective of color and size of a dominant growth form on glacier forelands.

Mat-forming lichens dominating high-latitudinal habitats vary in color and geometry. Widespread species are light greenish yellow (usnic acid) and reflect solar radiation, whereas melanic species absorbing most solar wavelengths are spatially more restricted. Color thereby influences lichens' energy budget and thus their hydration and photosynthetically active periods. By using well-defined cushions from early successional stages on glacier forelands - three melanic(m) and three usnic(u) mat-forming lichens with hair-like branches (Alectoria ochroleuca(u), Gowardia nigricans(m)), hollow terete branches (Cladonia uncialis(u), Cetraria muricata(m)), and flat branches (Flavocetraria nivalis(u), Cetraria islandica(m)) - we quantified hydration traits and analyzed how color and cushion size affect water loss rate (WLR) and duration of active periods. Main findings: 1) WLR declined with cushion size and was highest in melanic lichens. 2) Active periods were longer for usnic than for melanic lichens and increased with size in all groups. 3) Size, color, and taxon nested in color significantly influenced WLR and duration of active periods in linear mixed models. 4) Hair lichen cushions had shorter active periods than growth forms with terete or flat branches due to their more open canopy architecture and lower water holding capacity (WHC). 5) WHC measured for isolated branches highly underestimated WHC for intact cushions.

Functional trade-off of hydration strategies in old forest epiphytic cephalolichens.

Although water is essential for photosynthetic activation in lichens, rates of vapor uptake and activation in humid air, which likely influence their niche preferences and distribution ranges, are insufficiently known. This study simultaneously quantifies rehydration kinetics and PSII reactivation in sympatric, yet morphologically and functionally distinct cephalolichens (Lobaria amplissima, Lobaria pulmonaria, Lobaria virens). High-temporal resolution monitoring of rehydrating thalli by automatic weighing combined with chlorophyll fluorescence imaging of maximal PSII efficiency (FV/FM) was applied to determine species-specific rates of vapor uptake and photosynthetic activation. The thin and loosely attached growth form of L. pulmonaria rehydrates and reactivates faster in humid air than the thick L. amplissima, with L. virens in between. This flexible hydration strategy is consistent with L. pulmonaria's wide geographical distribution stretching from rainforests to continental forests. By contrast, the thick and resupinate L. amplissima reactivates slowly in humid air but stores much water when provided in abundance. This prolongs active periods after rain, which could represent an advantage where abundant rain and stem flow alternates with long-lasting drying. Understanding links between morphological traits and functional responses, and their ecological implications for species at risk, is crucial to conservation planning and for modelling populations under various climate scenarios.

Photobiont-dependent humidity threshold for chlorolichen photosystem II activation.

MAIN CONCLUSION: Photobiont type influences the relative humidity threshold at which photosystem II activates in green algal lichens. Water vapor uptake alone can activate photosynthesis in lichens with green algal photobionts. However, the minimum relative humidity needed for activation is insufficiently known. The objective of this study was to quantify the humidity threshold for photosystem II (PSII) activation in a range of chlorolichen species associated with photobionts from Trebouxiaceae, Coccomyxaceae and Trentepohliaceae. These lichens exhibit distribution, habitat and substrate patterns that are likely coupled to their efficiency in utilizing water vapor at lower levels of relative humidity (RH) for photosynthesis. Using chlorophyll fluorescence imaging during water uptake from humid air of 25 species of chlorolichens representing the above photobiont groups, we monitored PSII activation within controlled chambers with constant RH at five levels ranging from 75.6 to 95.4%. The results demonstrate clear photobiont-specific activation patterns: the trentepohlioid lichens activated PSII at significantly lower RH (75.6%) than trebouxioid (81.7%) and coccomyxoid (92.0%) lichens. These responses are consistent with a preference for warm and sheltered habitats for trentepohlioid lichens, with cool and moist habitats for the coccomyxoid lichens, and with a more widespread occurrence of the trebouxioid lichens. Within each photobiont group, lichen species exposed to marine aerosols in their source habitats seemed to be activated at lower RH than lichens sampled from inland sites. High osmolyte concentration may therefore play a role in lowering a photobiont's activation threshold. We conclude that photobiont type influences water vapor-driven photosynthetic activation of lichens, thereby shaping the ecological niches in which they occur.

Why chartreuse? The pigment vulpinic acid screens blue light in the lichen Letharia vulpina.

MAIN CONCLUSION: Chlorophyll fluorescence, infrared gas exchange and photoinhibition data consistently show that vulpinic acid in L. vulpina functions as a strong blue light screening compound. The cortical lichen compounds, parietin, atranorin, usnic acid and melanins are known to screen photosynthetically active radiation (PAR), thereby protecting the underlying photobionts. The role of the toxic UV-/blue light-absorbing vulpinic acid in lichen cortices is poorly documented. By comparing controls with acetone-rinsed Letharia vulpina thalli (75% reduced vulpinic acid concentration), we aimed to test PAR screening by vulpinic acid. We exposed such thalli to blue, green and red irradiance, respectively, and recorded light quality-specific light saturation curves of CO2 uptake, quantum yields of CO2 uptake (QYCO2) and effective quantum yields of PSII (ΦPSII). We also quantified light quality-dependent photoinhibition after 4-h exposure to 400 µmol photons m-2 s-1. In controls, the greatest high light-induced reductions in CO2 uptake and ΦPSII, as well as the strongest photoinhibition [lowered maximal quantum yield of PSII (Fv/Fm)], occurred in red light, followed by green, and was low in blue light. Removal of vulpinic acid significantly exacerbated photoinhibition, reduced ΦPSII, and increased QYCO2 in blue light. By contrast, acetone rinsing had no or weak effects in green and red lights. Comparing control with acetone-rinsed thalli, blue light screening was estimated at 69% using ΦPSII data and 49% using QYCO2. To compensate for the 25% residual vulpinic acid left after rinsing, we repeated the screening estimation by comparing responses in blue and red lights. This resulted in 88% screening using ΦPSII data and 77% using QYCO2. The consistent responses in all photosynthetic parameters support the hypothesis that vulpinic acid functions as a blue light screen in L. vulpina.