Photobiont-dependent humidity threshold for chlorolichen photosystem II activation

Laboratory and field measurements of water relations, photosynthetic parameters, and hydration traits in macrolichens in a tropical lower montane rainforest in Thailand

Ecophysiological studies of lichens in tropical Asia are rare, and additional studies can increase the understanding of lichen life in this region. The main aim of this study was to observe the relationships between water availability and photosynthetic parameters, as well as hydration trait parameters, in macrolichens during the rainy and dry seasons in a tropical forest. A total of 11 lichen species growing in a lower montane rainforest in Thailand were collected and studied. The results clearly showed that the specific thallus mass (STM), net photosynthetic rate (Pn), the potential quantum yield of primary photochemistry (Fv/Fm), chlorophyll content, and carotenoid content of almost all lichens were lower in the dry season than in the rainy season. Field measurements in the dry season revealed that only the foliose chlorolichen Parmotrema tinctorum was metabolically active and exhibited slight carbon assimilation. In the rainy season, all lichens started their photosynthesis in the early morning, reached maximal values, declined, and ceased when the thalli desiccated. The photosynthetically active period of the lichens was approximately 2-3 h in the morning, and the activities of the cyanolichens ended approximately 30 min after the chlorolichens. The hydration trait parameters, including the STM, maximal water content (WCmax), and water holding capacity (WHC), were greater in the cyanolichens. In addition, the maximal Pn (Pnmax) and optimal water content (WCopt) for Pn were also greater in the cyanolichens, but the maximal Fv/Fm (Fv/Fmmax) was lower. The cyanolichens compensated for their inability to use humid air to restore photosynthesis by having higher water content and storage, higher photosynthetic rates, and longer photosynthetically active periods. This study provides additional insights into lichen ecophysiology in tropical forests that can be useful for lichen conservation.

Symbionts out of sync: Decoupled physiological responses are widespread and ecologically important in lichen associations

A core vulnerability in symbioses is the need for coordination between the symbiotic partners, which are often assumed to be closely physiologically integrated. We critically re-examine this assumed integration between symbionts in lichen symbioses, recovering a long overlooked yet fundamental physiological asymmetry in carbon balance. We examine the physiological, ecological, and transcriptional basis of this asymmetry in the lichen Evernia mesomorpha. This carbon balance asymmetry depends on hydration source and aligns with climatic range limits. Differences in gene expression across the E. mesomorpha symbiosis suggest that the physiologies of the primary lichen symbionts are decoupled. Furthermore, we use gas exchange data to show that asymmetries in carbon balance are widespread and common across evolutionarily disparate lichen associations. Using carbon balance asymmetry as an example, we provide evidence for the wide-ranging importance of physiological asymmetries in symbioses.

Interactions of moisture and light drive lichen growth and the response to climate change scenarios: experimental evidence for Lobaria pulmonaria

BACKGROUND AND AIMS

There is growing interest in the functional ecology of poikilohydric non-vascular photoautotrophs (NVPs), including 'cryptogamic' bryophytes and lichens. These organisms are structurally important in many ecosystems, contributing substantially to ecosystem function and services, while also being sensitive to climate change. Previous research has quantified the climate change response of poikilohydric NVPs using predictive bioclimatic models with standard climate variables including precipitation totals and temperature averages. This study aimed for an improved functional understanding of their climate change response based on their growth rate sensitivity to moisture and light.

METHODS

We conducted a 24-month experiment to monitor lichen hydration and growth. We accounted for two well-known features in the ecology of poikilohydric NVPs, and exemplified here for a structurally dominant lichen epiphyte, Lobaria pulmonaria: (1) sensitivity to multiple sources of atmospheric moisture including rain, condensed dew-formation and water vapour; and (2) growth determined by the amount of time hydrated in the light, driving photosynthesis, referred to as the Iwet hypothesis.

KEY RESULTS

First, we found that even within an oceanic high-rainfall environment, lichen hydration was better explained by vapour pressure deficit than precipitation totals. Second, growth at a monthly resolution was positively related to the amount of time spent hydrated in the light, and negatively related to the amount of time spent hydrated in the dark.

CONCLUSIONS

Using multimodel averaging to project growth models for an ensemble of future climate change scenarios, we demonstrated reduced net growth for L. pulmonaria by the late 21st century, explained by extended climate dryness and lichen desiccation for periods when there is otherwise sufficient light to drive photosynthesis. The results further emphasize a key issue of photoperiodism when constructing functionally relevant models to understand the risk of climate change, especially for poikilohydric NVPs.

Epiphytes as leading indicators of climate and other changes. A commentary on 'Interactions of moisture and light drive lichen growth and the response to climate change scenarios - experimental evidence for Lobaria pulmonaria'

This article comments on:

Martine Borge and Christopher J. Ellis, Interactions of moisture and light drive lichen growth and the response to climate change scenarios: experimental evidence for Lobaria pulmonaria, Annals of Botany, Volume 134, Issue 1, 3 July 2024, Pages 43–57 https://doi.org/10.1093/aob/mcae029

Acquisition of green algal photobionts enables both chlorolichens and chloro-cyanolichens to activate photosynthesis at low humidity without liquid water

Cyanobacteria require liquid water for photosynthesis, whereas green algae can photosynthesise with water vapour alone. We discovered that several Lobaria spp. which normally have cyanobacteria as the sole photobiont, in some regions of the trans-Himalayas also harboured green algae. We tested whether green algal acquisition was: limited to high elevations; obtained from neighbouring chloro-Lobaria species; enabled photosynthesis at low humidity. Lobaria spp. were collected from 2000 to 4000 m elevation. Spectrophotometry quantified green algal abundance by measuring chlorophyll b (absent in cyanobacteria). Thalli cross-sections visually confirmed green algal presence. We sequenced gene regions: Lobaria (ITS-EF-1α-RPB2), green algae (18S-RBC-L) and Nostoc (16S). Phylogenetic analysis determined myco-photobiont associations. We used a custom closed-circuit gas exchange system with an infrared gas analyser to measure CO2 exchange rates for desiccated specimens at 33%, 76%, 86% and 98% humidity. Cross-sections revealed that the photobiont layers in putative cyano-Lobaria contained both cyanobacteria and green algae, indicating that they should be considered chloro-cyanolichens. Chloro-Lobaria had no visible cephalodia nor cyanobacteria in the photobiont layer. Chloro-Lobaria and chloro-cyano-Lobaria had comparable levels of chlorophyll b. Chloro-Lobaria usually contained Symbiochloris. Chloro-cyano-Lobaria mainly associated with Parachloroidium and Nostoc; infrequently with Symbiochloris, Apatococcus, Chloroidium, Pseudochlorella, Trebouxia. Sequences from two green algal genera were obtained from within some thalli. Desiccated specimens of every Lobaria species could attain net photosynthesis with light exposure and 33% humidity. CO2 exchange dynamics over a five-day period differed between species. At all elevations, chloro-cyano-Lobaria spp. had abundant green algae in the photobiont layer, but green algal strains mostly differed to those of chloro-Lobaria spp. Both chloro-Lobaria and chloro-cyano-Lobaria were capable of conducting photosynthesis without liquid water. The data strongly suggest that they attained positive net photosynthesis.

Old-growth forest versus generalist lichens: Sensitivity to prolonged desiccation stress and photosynthesis reactivation rate upon rehydration

Most epiphytic lichens demonstrate high specificity to a habitat type, and sensitive hygrophilous species usually find shelter only in close-to-natural forest complexes. Some of them are considered as old-growth forest and/or long ecological continuity indicators. To evaluate general links between the narrow ecological range and physiological traits, two distinct sets of model lichens, i.e., old-growth forest (Cetrelia cetrarioides (Duby) W.L. Culb. & C.F. Culb., Lobaria pulmonaria (L.) Hoffm., Menegazzia terebrata (Hoffm.) A. Massal.), and generalist (Flavoparmelia caperata (L.) Hale, Hypogymnia physodes (L.) Nyl., Parmelia sulcata Taylor) ones, were examined in terms of sensitivity to long-term desiccation stress (1-, 2-, and 3-month) and photosynthesis activation rate upon rehydration. Desiccation tolerance and response rate to rehydration are specific to a given ecological set of lichens rather than to a particular species. Noticeable delayed and prompt recovery of high photosynthetic activity of photosystem II (PSII) characterize these sets, respectively. At the same time, although a decrease in the potential quantum yield of PSII in lichen thalli with a relative water content (RWC) at the level of 25% was observed, the efficiency remained at a very high level for all species, regardless of habitat preferences. Among the examined lichens, the fluorescence emission parameters for F. caperata were the fastest toward equilibrium upon rehydration, both after a shorter and a longer period of desiccation stress. In contrast to generalist lichens, retrieving of photosynthesis after 3-month desiccation failed in old-growth forest lichens. In the long term, prolonged rainless periods and unfavorable water balance in the environment predicted in the future may have a severely limiting effect on hygrophilous lichens during growing season (also in the sense of species associations) and, at the same time, promote the development of generalists.

Modelling the carbon balance in bryophytes and lichens: Presentation of PoiCarb 1.0, a new model for explaining distribution patterns and predicting climate-change effects

PREMISE

Bryophytes and lichens have important functional roles in many ecosystems. Insight into their CO2 -exchange responses to climatic conditions is essential for understanding current and predicting future productivity and biomass patterns, but responses are hard to quantify at time scales beyond instantaneous measurements. We present PoiCarb 1.0, a model to study how CO2 -exchange rates of these poikilohydric organisms change through time as a function of weather conditions.

METHODS

PoiCarb simulates diel fluctuations of CO2 exchange and estimates long-term carbon balances, identifying optimal and limiting climatic patterns. Modelled processes were net photosynthesis, dark respiration, evaporation and water uptake. Measured CO2 -exchange responses to light, temperature, atmospheric CO2 concentration, and thallus water content (calculated in a separate module) were used to parameterize the model's carbon module. We validated the model by comparing modelled diel courses of net CO2 exchange to such courses from field measurements on the tropical lichen Crocodia aurata. To demonstrate the model's usefulness, we simulated potential climate-change effects.

RESULTS

Diel patterns were reproduced well, and the modelled and observed diel carbon balances were strongly positively correlated. Simulated warming effects via changes in metabolic rates were consistently negative, while effects via faster drying were variable, depending on the timing of hydration.

CONCLUSIONS

Reproducing weather-dependent variation in diel carbon balances is a clear improvement compared to simply extrapolating short-term measurements or potential photosynthetic rates. Apart from predicting climate-change effects, future uses of PoiCarb include testing hypotheses about distribution patterns of poikilohydric organisms and guiding conservation strategies for species.

The effect of the water source on niche partioning of chlorolichens and cyanobacteria-implications for resilience?

MAIN CONCLUSION

Microclimate determines lichens and cyanobacteria distribution in the Negev, with lichens and cyanobacteria inhabit dewy and dewless habitats, respectively. Lichens experiences more frequent and extensive environmental fluctuations than cyanobacteria. The spatial partitioning of chlorolichens (eukaryotes) and cyanobacteria (prokaryotes) are intriguing, especially following recent intense search for extraterrestrial life. This is especially relevant for deserts, where both lithobionts are thought to use rain and dew but may differ in their resilience to environmental extremes and fluctuations. Following the different spatial distribution of lithobionts in a south-facing slope of the Negev Highlands (with cyanobacteria-inhabiting rocks and chlorolichen-inhabiting cobbles), measurements of temperature, non-rainfall water (NRW) and biomass were carried out within the drainage basin aiming to test the hypotheses that (i) cobble-inhabiting lichens may access more water (through NRW) and may be subjected to more extensive environmental fluctuations of temperature and water than bedrock-inhabiting cyanobacteria, and (ii) will therefore have a greater contribution to the ecosystem productivity. In contrast to cyanobacteria, cobble-inhabiting chlorolichens were found to access NRW (up to 0.20 mm of daily amounts in comparison to < 0.04 mm of the cyanobacteria) and to experience higher fluctuations of temperatures (up to 4.1 °C higher and 5.3 °C lower). With lichens and cyanobacteria inhabiting dewy and dewless habitats, respectively, NRW was found responsible for contributing 6.8-fold higher organic carbon to the lithobiontic community. At this site, chlorolichens experience more extensive environmental fluctuations than cyanobacteria, possibly indicating a higher tolerance for environmental fluctuations. These observations may assist in the interpretation of the abiotic conditions responsible for past or present lithobiontic life on Mars.

Diversity and assembly of lichens and bryophytes on tree trunks along a temperate to boreal elevation gradient

Based on hypotheses related to environmental filtering vs. stochastic community assembly, we tested taxon-specific predictions regarding the relationships of alpha diversity, beta diversity and species composition of epiphytic macrolichens and bryophytes with elevation and the lateral gradient on trees (the different sides of the tree bole related to aspect and trunk inclination) at Parc national du Mont-Mégantic in Southeastern Québec, Canada. For lichens on firs, increasing elevation was associated with increasing alpha diversity, and a marked shift in community composition, at the scale of whole trees. In contrast, for bryophytes on maples, tree inclination and the lateral gradient had the strongest effects: more inclined trees had greater whole-tree alpha diversity and stronger within-tree contrasts in composition between the upper and lower bole surfaces. For lichens on maples, whole-tree alpha diversity showed a weak, negative relationship with inclination, and beta diversity increased slightly with elevation. Our results are consistent with theories predicting greater alpha diversity in more favorable environments (for lichens: high elevation with high relative air humidity and lower temperatures; for bryophytes: upper surfaces of tree boles with liquid water available), but support was weak for the prediction of greater beta diversity in more favorable environments. Overall, the important predictors of epiphytic cryptogam diversity vary more among the species of tree host (maple vs. fir) than focal taxa (lichens vs. bryophytes), with patterns likely related to different effects of water, temperature, and competition between lichens and bryophytes.

Lichen ecophysiology in a changing climate

Lichens are one of the most iconic and ubiquitous symbioses known, widely valued as indicators of environmental quality and, more recently, climate change. Our understanding of lichen responses to climate has greatly expanded in recent decades, but some biases and constraints have shaped our present knowledge. In this review we focus on lichen ecophysiology as a key to predicting responses to present and future climates, highlighting recent advances and remaining challenges. Lichen ecophysiology is best understood through complementary whole-thallus and within-thallus scales. Water content and form (vapor or liquid) are central to whole-thallus perspectives, making vapor pressure differential (VPD) a particularly informative environmental driver. Responses to water content are further modulated by photobiont physiology and whole-thallus phenotype, providing clear links to a functional trait framework. However, this thallus-level perspective is incomplete without also considering within-thallus dynamics, such as changing proportions or even identities of symbionts in response to climate, nutrients, and other stressors. These changes provide pathways for acclimation, but their understanding is currently limited by large gaps in our understanding of carbon allocation and symbiont turnover in lichens. Lastly, the study of lichen physiology has mainly prioritized larger lichens at high latitudes, producing valuable insights but underrepresenting the range of lichenized lineages and ecologies. Key areas for future work include improving geographic and phylogenetic coverage, greater emphasis on VPD as a climatic factor, advances in the study of carbon allocation and symbiont turnover, and the incorporation of physiological theory and functional traits in our predictive models.

Little time left. Microrefuges may fail in mitigating the effects of climate change on epiphytic lichens

Climate change is already causing considerable reductions in biodiversity in all terrestrial ecosystems. These consequences are expected to be exacerbated in biomes that are particularly exposed to change, such as those in the Mediterranean, and in certain groups of more sensitive organisms, such as epiphytic lichens. These poikylohydric organisms find suitable light and water conditions on trunks under the tree canopy. Despite their small size, epiphytic communities contribute significantly to the functionality of forest ecosystems. In this work, we surveyed epiphytic lichen communities in a Mediterranean area (Sardinia, Italy) and hypothesized that 1) the effect of microclimate on lichens at tree scale is mediated by the functional traits of these organisms and that 2) micro-refuge trees with certain morphological characteristics can mitigate the negative effects of future climate change. Results confirm the first hypothesis, while the second is only partially supported, suggesting that the capability of specific trees to host specific conditions may not be sufficient to maintain the diversity and ecosystem functionality of lichen communities in the Mediterranean.

Dew and fog as possible evolutionary drivers? The expansion of crustose and fruticose lichens in the Negev is respectively mainly dictated by dew and fog

The expansion of crustose lichens in the Negev is principally determined by dew and that of fruticose lichens by fog. Crustose and fruticose lichens are largely adapted to dew and fog, respectively. Although crustose and fruticosea lichens were shown to efficiently use dew and fog, the link between their expansion and the occurrence of dew and fog has never been shown experimentally. This is also the case for the Negev Desert Highlands, where (i) dewless habitats were not inhabited by lichens and (ii) an increase in fruticose lichens with high-altitude fog-prone areas was noted, leading us to hypothesize that the expansion of crustose and fruticose lichens is mainly linked to dew and fog, respectively. Experiments aiming to compare the non-rainfall water (NRW) were conducted. We used cloths attached to 7 cm-high cobbles to mimic crustose lichens (MCL), cloths placed horizontally aboveground to evaluate the amount of NRW without the presence of the cobble (CoP), cloths attached to a wire scaffold mimicking fruticose lichens (MFL), and cloths attached to glass plates (CPM) that served as a reference. Substrate temperatures were compared to the dew point temperature. In addition, sprinkling experiments, which mimicked fog under variable wind speeds (0.9, 1.4, 3.3 and 5.7 m s^-1), were also conducted. NRW followed the pattern: MCL ≈ CPM > CoP >  > MFL. While MCL yielded substantially higher amounts of NRW (0.09 mm) in comparison to MFL (0.04 mm) during dew events, similar amounts were obtained by both substrates (0.15-0.16 mm) following fog. However, fog interception increased substantially with wind speed. The findings may explain the expansion of crustose lichens in extreme deserts benefiting mainly from dew (but also fog), and the proliferation of fruticose lichens in fog-prone areas, especially when accompanied by high-speed winds. While (mainly) high proliferation of crustose lichens may serve as bioindicators for dew in extreme deserts, fruticose lichens may serve as bioindicators for fog.

Seasonal Changes in the Photosynthetic Activity of Terrestrial Lichens and Mosses in the Lichen Scots Pine Forest Habitat

Photosynthetic activity is one of the most important metabolic processes that can be quickly and easily studied in the field. It can be used for identifying the environmental factors affecting ecosystem balance, as any stressor influencing metabolic and physiological processes will have a measurable effect on photosynthesis. The aim of this study was to measure the photosynthetic activity of selected lichens and mosses and investigate its changes resulted from diurnal and seasonal variability. We studied two lichens (Cladonia mitis Sandst and Cladonia uncialis (L.) Weber ex F.H. Wigg.) and two mosses (Pleurozium schreberi (Willd. ex Brid.) Mitt. and Dicranum scoparium (L.) Hedw.). Samples were collected in the area of lichen Scots pine forest of the “Bory Tucholskie” National Park. Our study revealed that the photosynthetic activity of cryptogams depended on species, season, time of the day, and water availability. Cladonia species, which are the main component of lichen Scots pine forests, have higher photosynthetic activity than Pleurozium schreberi, which represents species of fresh coniferous forests. Photosynthetic activity increased from spring through summer and reached the highest values in autumn. It was also higher in soaked samples collected in the morning and afternoon compared to noon. Despite the water access, noon samples still showed the lowest activity. This can result from natural changes in humidity during the day to which cryptogams are well-adapted.

Functional Traits in Lichen Ecology: A Review of Challenge and Opportunity

Community ecology has experienced a major transition, from a focus on patterns in taxonomic composition, to revealing the processes underlying community assembly through the analysis of species functional traits. The power of the functional trait approach is its generality, predictive capacity such as with respect to environmental change, and, through linkage of response and effect traits, the synthesis of community assembly with ecosystem function and services. Lichens are a potentially rich source of information about how traits govern community structure and function, thereby creating opportunity to better integrate lichens into 'mainstream' ecological studies, while lichen ecology and conservation can also benefit from using the trait approach as an investigative tool. This paper brings together a range of author perspectives to review the use of traits in lichenology, particularly with respect to European ecosystems from the Mediterranean to the Arctic-Alpine. It emphasizes the types of traits that lichenologists have used in their studies, both response and effect, the bundling of traits towards the evolution of life-history strategies, and the critical importance of scale (both spatial and temporal) in functional trait ecology.

Apparent electron transport rate - a non-invasive proxy of photosynthetic CO2 uptake in lichens

During desiccation, both apparent electron transport rate (ETR_app) and photosynthetic CO₂ uptake peak when external water has evaporated. External water, causing suprasaturation, weakens the strong correlation between ETR_app and CO₂ uptake. Lichens are poikilohydric organisms passively regulated by ambient conditions. In theory, apparent electron transport rate (ETR_app), estimated by photosystem II yield measured in light (Φ_PSII), is a proxy of photosynthetic CO₂ uptake. Hydration level, however, is a complicating factor, particularly during suprasaturation that strongly reduces CO₂ diffusion. Here, the cephalolichen Lobaria pulmonaria and two chlorolichens Parmelia sulcata and Xanthoria aureola were excessively hydrated before photosynthetic CO₂ uptake and Φ_PSII using imaging fluorescence tools were simultaneously measured while drying at 200 µmol photons m^-2 s^-1. CO₂ uptake peaked when hydration had declined to a level equivalent to their respective internal water holding capacity (WHC_internal) i.e., the water per thallus area after blotting external water. CO₂ uptake and ETR_app in all species were highly correlated at hydration levels below WHC_internal, but weaker at higher hydration (chlorolichens) or absent (cephalolichen). Yet, at a specimen level for the two chlorolichens, the correlation was strong during suprasaturation. The CO₂ uptake-ETR_app relationship did not differ between measured species, but may vary between other lichens because the slope depends on cortical transmittance and fraction of electrons not used for CO₂ uptake. For new lichen species, calibration of ETR_app against CO₂ uptake is therefore necessary. At intrathalline scales, Φ_PSII during drying initially increased along thallus margins before reaching maximum values in central portions when hydration approached WHC_internal. WHC_internal represents the optimal hydration level for lichen photosynthesis. In conclusion, ETR_app is an easily measured and reliable proxy of CO₂ uptake in thalli without external water but overestimates photosynthesis during suprasaturation.

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 (F_V/F_M) 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.