How to build a lichen: from metabolite release to symbiotic interplay

Exposing their vegetative bodies to the light, lichens are outstanding amongst other fungal symbioses. Not requiring a pre-established host, 'lichenized fungi' build an entirely new structure together with microbial photosynthetic partners that neither can form alone. The signals involved in the transition of a fungus and a compatible photosynthetic partner from a free-living to a symbiotic state culminating in thallus formation, termed 'lichenization', and in the maintenance of the symbiosis, are poorly understood. Here, we synthesise the puzzle pieces of the scarce knowledge available into an updated concept of signalling involved in lichenization, comprising five main stages: (1) the 'pre-contact stage', (2) the 'contact stage', (3) 'envelopment' of algal cells by the fungus, (4) their 'incorporation' into a pre-thallus and (5) 'differentiation' into a complex thallus. Considering the involvement of extracellularly released metabolites in each phase, we propose that compounds such as fungal lectins and algal cyclic peptides elicit early contact between the symbionts-to-be, whereas phytohormone signalling, antioxidant protection and carbon exchange through sugars and sugar alcohols are of continued importance throughout all stages. In the fully formed lichen thallus, secondary lichen metabolites and mineral nutrition are suggested to stabilize the functionalities of the thallus, including the associated microbiota.

Evolutionary biology of lichen symbioses

Lichens are the symbiotic outcomes of open, interspecies relationships, central to which are a fungus and a phototroph, typically an alga and/or cyanobacterium. The evolutionary processes that led to the global success of lichens are poorly understood. In this review, we explore the goods and services exchange between fungus and phototroph and how this propelled the success of both symbiont and symbiosis. Lichen fungal symbionts count among the only filamentous fungi that expose most of their mycelium to an aerial environment. Phototrophs export carbohydrates to the fungus, which converts them to specific polyols. Experimental evidence suggests that polyols are not only growth and respiratory substrates but also play a role in anhydrobiosis, the capacity to survive desiccation. We propose that this dual functionality is pivotal to the evolution of fungal symbionts, enabling persistence in environments otherwise hostile to fungi while simultaneously imposing costs on growth. Phototrophs, in turn, benefit from fungal protection from herbivory and light stress, while appearing to exert leverage over fungal sex and morphogenesis. Combined with the recently recognized habit of symbionts to occur in multiple symbioses, this creates the conditions for a multiplayer marketplace of rewards and penalties that could drive symbiont selection and lichen diversification.

A Label-Free Proteomic and Complementary Metabolomic Analysis of Leaves of the Resurrection Plant Xerophytaschlechteri during Dehydration

Vegetative desiccation tolerance, or the ability to survive the loss of ~95% relative water content (RWC), is rare in angiosperms, with these being commonly called resurrection plants. It is a complex multigenic and multi-factorial trait, with its understanding requiring a comprehensive systems biology approach. The aim of the current study was to conduct a label-free proteomic analysis of leaves of the resurrection plant Xerophyta schlechteri in response to desiccation. A targeted metabolomics approach was validated and correlated to the proteomics, contributing the missing link in studies on this species. Three physiological stages were identified: an early response to drying, during which the leaf tissues declined from full turgor to a RWC of ~80–70%, a mid-response in which the RWC declined to 40% and a late response where the tissues declined to 10% RWC. We identified 517 distinct proteins that were differentially expressed, of which 253 proteins were upregulated and 264 were downregulated in response to the three drying stages. Metabolomics analyses, which included monitoring the levels of a selection of phytohormones, amino acids, sugars, sugar alcohols, fatty acids and organic acids in response to dehydration, correlated with some of the proteomic differences, giving insight into the biological processes apparently involved in desiccation tolerance in this species.

A metabolomic study of Gomphrena agrestis in Brazilian Cerrado suggests drought-adaptive strategies on metabolism

Drought is the main factor that limits the distribution and productivity of plant species. In the Brazilian Cerrado, the vegetation is adapted to a seasonal climate with long- and short-term periods of drought. To analyze the metabolic strategies under such conditions, a metabolomic approach was used to characterize Gomphrena agrestis Mart. (Amaranthaceae) a native species that grows under natural conditions, in a rock-field area. Roots and leaves material from native specimens were sampled along different seasons of the year and LC–MS and GC–MS analyzed for multiple chemical constituents. The datasets derived from the different measurements were combined and evaluated using multivariate analysis. Principal component analysis was used to obtain an overview of the samples and identify outliers. Later, the data was analyzed with orthogonal projection to latent structures discriminant analysis to obtain valid models that could explain the metabolite variations in the different seasons. Two hundred and eighty metabolites were annotated, generating a unique database to characterize metabolic strategies used to cope with the effects of drought. The accumulation of fructans in the thickened roots is consistent with the storage of carbons during the rainy season to support the energy demand during a long period of drought. The accumulation of Abscisic acid, sugars and sugar alcohols, phenolics, and pigment in the leaves suggests physiological adaptations. To cope with long-term drought, the data suggests that tissue water status and storage of reserves are important to support plant survival and regrowth. However, during short-term drought, osmoregulation and oxidative protection seems to be essential, probably to support the maintenance of active photosynthesis.

Enhanced culturing techniques for the mycobiont isolated from the lichen Xanthoria parietina

Lichens and their isolated symbionts are potentially valuable resources for biotechnological approaches. Especially mycobiont cultures that produce secondary lichen products are receiving increasing attention, but lichen mycobionts are notoriously slow-growing organisms. Sufficient biomass production often represents a limiting factor for scientific and biotechnological investigations, requiring improvement of existing culturing techniques as well as methods for non-invasive assessment of growth. Here, the effects of pH and the supplement of growth media with either D-glucose or three different sugar alcohols that commonly occur in lichens, D-arabitol, D-mannitol and ribitol, on the growth of the axenically cultured mycobiont isolated from the lichen Xanthoria parietina were tested. Either D-glucose or different sugar alcohols were offered to the fungus at different concentrations, and cumulative growth and growth rates were assessed using two-dimensional image analysis over a period of 8 weeks. The mycobiont grew at a pH range from 4.0 to 7.0, whereas no growth was observed at higher pH values. Varying the carbon source in Lilly-Barnett medium (LBM) by replacing 1% D-glucose used in the originally described LBM by either 1%, 2% or 3% of D-mannitol, or 3% of D-glucose increased fungal biomass production by up to 26%, with an exponential growth phase between 2 and 6 weeks after inoculation. In summary, we present protocols for enhanced culture conditions and non-invasive assessment of growth of axenically cultured lichen mycobionts using image analysis, which may be useful for scientific and biotechnological approaches requiring cultured lichen mycobionts.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11557-021-01707-7.

How dry is dry? Molecular mobility in relation to thallus water content in a lichen

Lichens can withstand extreme desiccation to water contents of ≤ 0.1 g H2O g-1 DW, and in the desiccated state are among the most extremotolerant organisms known. Desiccation-tolerant life-forms such as seeds, mosses and lichens survive 'vitrification', that is the transition of their cytoplasm to a 'glassy' state, which causes metabolism to cease. However, our understanding of the mechanisms of desiccation tolerance is hindered by poor knowledge of what reactions occur in the desiccated state. Using Flavoparmelia caperata as a model lichen, we determined at what water contents vitrification occurred upon desiccation. Molecular mobility was assessed by dynamic mechanical thermal analysis, and the de- and re-epoxidation of the xanthophyll cycle pigments (measured by HPLC) was used as a proxy to assess enzyme activity. At 20 °C vitrification occurred between 0.12-0.08 g H2O g-1 DW and enzymes were active in a 'rubbery' state (0.17 g H2O g-1 DW) but not in a glassy state (0.03 g H2O g-1 DW). Therefore, desiccated tissues may appear to be 'dry' in the conventional sense, but subtle differences in water content will have substantial consequences on the types of (bio)chemical reactions that can occur, with downstream effects on longevity in the desiccated state.

Adaptation to Aquatic and Terrestrial Environments in Chlorella vulgaris (Chlorophyta)

The globally distributed green microalga Chlorella vulgaris (Chlorophyta) colonizes aquatic and terrestrial habitats, but the molecular mechanisms underpinning survival in these two contrasting environments are far from understood. Here, we compared the authentic strain of C. vulgaris from an aquatic habitat with a strain from a terrestrial high alpine habitat previously determined as Chlorella mirabilis. Molecular phylogeny of SSU rDNA (823 bp) showed that the two strains differed by one nucleotide only. Sequencing of the ITS2 region confirmed that both strains belong to the same species, but to distinct ribotypes. Therefore, the terrestrial strain was re-assessed as C. vulgaris. To study the response to environmental conditions experienced on land, we assessed the effects of irradiance and temperature on growth, of temperature on photosynthesis and respiration, and of desiccation and rehydration on photosynthetic performance. In contrast to the aquatic strain, the terrestrial strain tolerated higher temperatures and light conditions, had a higher photosynthesis-to-respiration ratio at 25°C, still grew at 30°C and was able to fully recover photosynthetic performance after desiccation at 84% relative humidity. The two strains differed most in their response to the dehydration/rehydration treatment, which was further investigated by untargeted GC–MS-based metabolite profiling to gain insights into metabolic traits differentiating the two strains. The two strains differed in their allocation of carbon and nitrogen into their primary metabolites. Overall, the terrestrial strain had higher contents of readily available nitrogen-based metabolites, especially amino acids and the polyamine putrescine. Dehydration and rehydration led to differential regulation of the amino acid metabolism, the tricarboxylic acid cycle and sucrose metabolism. The data are discussed with a view to differences in phenotypic plasticity of the two strains, and we suggest that the two genetically almost identical C. vulgaris strains are attractive models to study mechanisms that protect from abiotic stress factors, which are more frequent in terrestrial than aquatic habitats, such as desiccation and irradiation.

Symbiosis at its limits: ecophysiological consequences of lichenization in the genus Prasiola in Antarctica

BACKGROUND AND AIMS

Lichens represent a symbiotic relationship between at least one fungal and one photosynthetic partner. The association between the lichen-forming fungus Mastodia tessellata (Verrucariaceae) and different species of Prasiola (Trebouxiophyceae) has an amphipolar distribution and represents a unique case study for the understanding of lichen symbiosis because of the macroalgal nature of the photobiont, the flexibility of the symbiotic interaction and the co-existence of free-living and lichenized forms in the same microenvironment. In this context, we aimed to (1) characterize the photosynthetic performance of co-occurring populations of free-living and lichenized Prasiola and (2) assess the effect of the symbiosis on water relations in Prasiola, including its tolerance of desiccation and its survival and performance under sub-zero temperatures.

METHODS

Photochemical responses to irradiance, desiccation and freezing temperature and pressure-volume curves of co-existing free-living and lichenized Prasiola thalli were measured in situ in Livingston Island (Maritime Antarctica). Analyses of photosynthetic pigment, glass transition and ice nucleation temperatures, surface hydrophobicity extent and molecular analyses were conducted in the laboratory.

KEY RESULTS

Free-living and lichenized forms of Prasiola were identified as two different species: P. crispa and Prasiola sp., respectively. While lichenization appears to have no effect on the photochemical performance of the alga or its tolerance of desiccation (in the short term), the symbiotic lifestyle involves (1) changes in water relations, (2) a considerable decrease in the net carbon balance and (3) enhanced freezing tolerance.

CONCLUSIONS

Our results support improved tolerance of sub-zero temperature as the main benefit of lichenization for the photobiont, but highlight that lichenization represents a delicate equilibrium between a mutualistic and a less reciprocal relationship. In a warmer climate scenario, the spread of the free-living Prasiola to the detriment of the lichen form would be likely, with unknown consequences for Maritime Antarctic ecosystems.

Photobiont-dependent humidity threshold for chlorolichen photosystem II activation

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.

Characterization of D-Arabitol as Newly Discovered Carbon Source of Bacillus methanolicus

Bacillus methanolicus is a Gram-positive, thermophilic, methanol-utilizing bacterium. As a facultative methylotroph, B. methanolicus is also known to utilize D-mannitol, D-glucose and, as recently discovered, sugar alcohol D-arabitol. While metabolic pathways for utilization of methanol, mannitol and glucose are known, catabolism of arabitol has not yet been characterized in B. methanolicus. In this work we present the elucidation of this hitherto uncharted pathway. In order to confirm our predictions regarding genes coding for arabitol utilization, we performed differential gene expression analysis of B. methanolicus MGA3 cells grown on arabitol as compared to mannitol via transcriptome sequencing (RNA-seq). We identified a gene cluster comprising eight genes that was up-regulated during growth with arabitol as a sole carbon source. The RNA-seq results were subsequently confirmed via qRT-PCR experiments. The transcriptional organization of the gene cluster identified via RNA-seq was analyzed and it was shown that the arabitol utilization genes are co-transcribed in an operon that spans from BMMGA3_RS07325 to BMMGA3_RS07365. Since gene deletion studies are currently not possible in B. methanolicus, two complementation experiments were performed in an arabitol negative Corynebacterium glutamicum strain using the four genes discovered via RNA-seq analysis as coding for a putative PTS for arabitol uptake (BMMGA3_RS07330, BMMGA3_RS07335, and BMMGA3_RS07340 renamed to atlABC) and a putative arabitol phosphate dehydrogenase (BMMGA3_RS07345 renamed to atlD). C. glutamicum is a natural D-arabitol utilizer that requires arabitol dehydrogenase MtlD for arabitol catabolism. The C. glutamicum mtlD deletion mutant was chosen for complementation experiments. Heterologous expression of atlABCD as well as the arabitol phosphate dehydrogenase gene atlD from B. methanolicus alone restored growth of the C. glutamicum ΔmtlD mutant with arabitol. Furthermore, D-arabitol phosphate dehydrogenase activities could be detected in crude extracts of B. methanolicus and these were higher in arabitol-grown cells than in methanol- or mannitol-grown cells. Thus, B. methanolicus possesses an arabitol inducible operon encoding, amongst others, a putative PTS system and an arabitol phosphate dehydrogenase for uptake and activation of arabitol as growth substrate.

A R2R3-MYB transcription factor gene, FtMYB13, from Tartary buckwheat improves salt/drought tolerance in Arabidopsis

Abiotic stress causes various negative impacts on plants, such as water loss, reactive oxygen species (ROS) accumulation and decreased photosynthesis. R2R3-MYB transcription factors (TFs) play crucial roles in the response of plants to abiotic stress. However, their functions in Tartary buckwheat, a strongly abiotic and resistant coarse cereal, haven't been fully investigated. In this paper, we report that a R2R3-MYB from Tartary buckwheat, FtMYB13, is not an activator of transcriptional activity but is located in the nucleus. Moreover, compared to the wild type (WT), transgenic Arabidopsis overexpressing FtMYB13 had a lower sensitivity to ABA and caused improved drought/salt tolerance, which was attributed to the higher proline content, greater photosynthetic efficiency, higher transcript abundance of some stress-related genes and the smaller amount of reactive oxygen species (ROS) and malondialdehyde (MDA) in the transgenic lines compared to WT. Consequently, our work indicates that FtMYB13 is involved in mediating plant responses to ABA, as well as salt and drought.

Survey of the occurrence of desiccation-induced quenching of basal fluorescence in 28 species of green microalgae

Desiccation-induced chlorophyll fluorescence quenching seems to be an indispensable part of desiccation resistance in the surveyed 28 green microalgal species. Lichens are desiccation tolerant meta-organisms. In the desiccated state photosynthesis is inhibited rendering the photobionts potentially sensitive to photoinhibition. As a photoprotective mechanism, strong non-radiative dissipation of absorbed light leading to quenching of chlorophyll fluorescence has been proposed. Desiccation-induced quenching affects not only variable fluorescence, but also the so-called basal fluorescence, F₀. This phenomenon is well-known for intact lichens and some free living aero-terrestrial algae, but it was often absent in isolated lichen algae. Therefore, a thorough screening for the appearance of desiccation-induced quenching was undertaken with 13 different aero-terrestrial microalgal species and lichen photobionts. They were compared with 15 aquatic green microalgal species, among them also three marine species. We asked the following questions: Do isolated lichen algae show desiccation-induced quenching? Are aero-terrestrial algae different in this respect to aquatic algae and is the potential for desiccation-induced quenching coupled to desiccation tolerance? How variable is desiccation-induced quenching among species? Most of the aero-terrestrial algae, including all lichen photobionts, showed desiccation-induced quenching, although highly variable in extent, whereas most of the aquatic algae did not. All algae displaying quenching were also desiccation tolerant, whereas all algae unable to perform desiccation-induced quenching were desiccation intolerant. Desiccation-induced fluorescence quenching seems to be an indispensable part of desiccation resistance in the investigated species.

Mechanisms of drought-induced dissipation of excitation energy in sun- and shade-adapted drought-tolerant mosses studied by fluorescence yield change and global and target analysis of fluorescence decay kinetics

Some mosses stay green and survive long even under desiccation. Dissipation mechanisms of excess excitation energy were studied in two drought-tolerant moss species adapted to contrasting niches: shade-adapted Rhytidiadelphus squarrosus and sun-adapted Rhytidium rugosum in the same family. (1) Under wet conditions, a light-induced nonphotochemical quenching (NPQ) mechanism decreased the yield of photosystem II (PSII) fluorescence in both species. The NPQ extent saturated at a lower illumination intensity in R. squarrosus, suggesting a larger PSII antenna size. (2) Desiccation reduced the fluorescence intensities giving significantly lower F ₀ levels and shortened the overall fluorescence lifetimes in both R. squarrosus and R. rugosum, at room temperature. (3) At 77 K, desiccation strongly reduced the PSII fluorescence intensity. This reduction was smaller in R. squarrosus than in R. rugosum. (4) Global and target analysis indicated two different mechanisms of energy dissipation in PSII under desiccation: the energy dissipation to a desiccation-formed strong fluorescence quencher in the PSII core in sun-adapted R. rugosum (type-A quenching) and (5) the moderate energy dissipation in the light-harvesting complex/PSII in shade-adapted R. squarrosus (type-B quenching). The two mechanisms are consistent with the different ecological niches of the two mosses.

Unraveling the Photoprotective Response of Lichenized and Free-Living Green Algae (Trebouxiophyceae, Chlorophyta) to Photochilling Stress

Lichens and free-living terrestrial algae are widespread across many habitats and develop successfully in ecosystems where a cold winter limits survival. With the goal of comparing photoprotective responses in free-living and lichenized algae, the physiological responses to chilling and photochilling conditions were studied in three lichens and their isolated algal photobionts together as well as in a fourth free-living algal species. We specifically addressed the following questions: (i) Are there general patterns of acclimation in green algae under chilling and photochilling stresses? (ii) Do free-living algae exhibit a similar pattern of responses as their lichenized counterparts? (iii) Are these responses influenced by the selection pressure of environmental conditions or by the phylogenetic position of each species? To answer these questions, photosynthetic fluorescence measurements as well as pigment and low molecular weight carbohydrate pool analyses were performed under controlled laboratory conditions. In general, photochemical efficiency in all free-living algae decreased with increasing duration of the stress, while the majority of lichens maintained an unchanged photochemical activity. Nevertheless, these patterns cannot be generalized because the alga Trebouxia arboricola and the lichen Ramalina pollinaria (associated with Trebouxia photobionts) both showed a similar decrease in photochemical efficiency. In contrast, in the couple Elliptochloris bilobata-Baeomyces rufus, only the algal partner exhibited a broad physiological performance under stress. This study also highlights the importance of the xanthophyll cycle in response to the studied lichens and algae to photochilling stress, while the accumulation of sugars was not related to cold acclimation, except in the alga E. bilobata. The differences in response patterns detected among species can be mainly explained by their geographic origin, although the phylogenetic position should also be considered, especially in some species.

Challenges facing an understanding of the nature of low-energy excited states in photosynthesis

While the majority of the photochemical states and pathways related to the biological capture of solar energy are now well understood and provide paradigms for artificial device design, additional low-energy states have been discovered in many systems with obscure origins and significance. However, as low-energy states are naively expected to be critical to function, these observations pose important challenges. A review of known properties of low energy states covering eight photochemical systems, and options for their interpretation, are presented. A concerted experimental and theoretical research strategy is suggested and outlined, this being aimed at providing a fully comprehensive understanding.

Desiccation tolerance and lichenization: a case study with the aeroterrestrial microalga Trebouxia sp. (Chlorophyta)

A comparative study of isolated vs. lichenized Trebouxia sp. showed that lichenization does not influence the survival capability of the alga to the photo-oxidative stress derived from prolonged desiccation. Coccoid algae in the Trebouxia genus are the most common photobionts of chlorolichens but are only sporadically found in soil or bark outside of a lichen. They all appear to be desiccation tolerant, i.e. they can survive drying to water contents of below 10%. However, little is known about their longevity in the dry state and to which extent lichenization can influence it. Here, we studied the longevity in the dry state of the lichenized alga (LT) Trebouxia sp. in the lichen Parmotrema perlatum, in comparison with axenically grown cultures (CT) isolated from the same lichen. We report on chlorophyll fluorescence emission and reactive oxygen species (ROS) production before desiccation, after 15-45 days in the dry state under different combinations of light and air humidity and after recovery for 1 or 3 days in fully hydrated conditions. Both the CT and the LT were able to withstand desiccation under high light (120 µmol photons m(-2) s(-1) for 14 h per day), but upon recovery after 45 days in the dry state the performance of the CT was better than that of the LT. By contrast, the quenching of excess light energy was more efficient in the LT, at high relative humidities especially. ROS production in the LT was influenced mostly by light exposure, whereas the CT showed an oxidative burst independent of the light conditions. Although lichenization provides benefits that are essential for the survival of the photobiont in high-light habitats, Trebouxia sp. can withstand protracted periods of photo-oxidative stress even outside of a lichen thallus.

Chemistry and biological activity of ramalina lichenized fungi

Lichens are a form of symbiont between a fungus and an alga or cyanobacterium, which contains a wide variety of organic compounds with certain secondary metabolite classes typical of these organisms. The Ramalina genus has approximately 246 species distributed around the World, of which in this review approximately 118 species with published chemical or biological activity studies of extracts or isolated compounds were cited. From the 153 mentioned compounds, only 27 passed were tested for biological activity, being usnic acid the most studied compound and the one showing the best results in almost all in vitro tests performed, although other compounds also presented excellent results as antimicrobial, antitumor and anti-inflammatory agents, among others. Extracts of several species also presented significant results in performed biological tests, demonstrating the potential that these organisms have, in particular, the gender Ramalina, to produce bioactive molecules that can be used as a model for the production of pharmaceuticals.

Ideal osmotic spaces for chlorobionts or cyanobionts are differentially realized by lichenized fungi

Lichens result from symbioses between a fungus and either a green alga or a cyanobacterium. They are known to exhibit extreme desiccation tolerance. We investigated the mechanism that makes photobionts biologically active under severe desiccation using green algal lichens (chlorolichens), cyanobacterial lichens (cyanolichens), a cephalodia-possessing lichen composed of green algal and cyanobacterial parts within the same thallus, a green algal photobiont, an aerial green alga, and a terrestrial cyanobacterium. The photosynthetic response to dehydration by the cyanolichen was almost the same as that of the terrestrial cyanobacterium but was more sensitive than that of the chlorolichen or the chlorobiont. Different responses to dehydration were closely related to cellular osmolarity; osmolarity was comparable between the cyanolichen and a cyanobacterium as well as between a chlorolichen and a green alga. In the cephalodium-possessing lichen, osmolarity and the effect of dehydration on cephalodia were similar to those exhibited by cyanolichens. The green algal part response was similar to those exhibited by chlorolichens. Through the analysis of cellular osmolarity, it was clearly shown that photobionts retain their original properties as free-living organisms even after lichenization.

Desiccation stress and tolerance in green algae: consequences for ultrastructure, physiological and molecular mechanisms

Although most green algae typically occur in aquatic ecosystems, many species also live partly or permanently under aeroterrestrial conditions, where the cells are exposed to the atmosphere and hence regularly experience dehydration. The ability of algal cells to survive in an air-dried state is termed desiccation tolerance. The mechanisms involved in desiccation tolerance of green algae are still poorly understood, and hence the aim of this review is to summarize recent findings on the effects of desiccation and osmotic water loss. Starting from structural changes, physiological, and biochemical consequences of desiccation will be addressed in different green-algal lineages. The available data clearly indicate a range of strategies, which are rather different in streptophycean and non-streptophycean green algae. While members of the Trebouxiophyceae exhibit effective water loss-prevention mechanisms based on the biosynthesis and accumulation of particular organic osmolytes such as polyols, these compounds are so far not reported in representatives of the Streptophyta. In members of the Streptophyta such as Klebsormidium, the most striking observation is the appearance of cross-walls in desiccated samples, which are strongly undulating, suggesting a high degree of mechanical flexibility. This aids in maintaining structural integrity in the dried state and allows the cell to maintain turgor pressure for a prolonged period of time during the dehydration process. Physiological strategies in aeroterrestrial green algae generally include a rapid reduction of photosynthesis during desiccation, but also a rather quick recovery after rewetting, whereas aquatic species are sensitive to drying. The underlying mechanisms such as the affected molecular components of the photosynthetic machinery are poorly understood in green algae. Therefore, modern approaches based on transcriptomics, proteomics, and/or metabolomics are urgently needed to better understand the molecular mechanisms involved in desiccation-stress physiology of these organisms. The very limited existing information is described in the present review.

Desiccation stress and tolerance in green algae: consequences for ultrastructure, physiological and molecular mechanisms

Although most green algae typically occur in aquatic ecosystems, many species also live partly or permanently under aeroterrestrial conditions, where the cells are exposed to the atmosphere and hence regularly experience dehydration. The ability of algal cells to survive in an air-dried state is termed desiccation tolerance. The mechanisms involved in desiccation tolerance of green algae are still poorly understood, and hence the aim of this review is to summarize recent findings on the effects of desiccation and osmotic water loss. Starting from structural changes, physiological, and biochemical consequences of desiccation will be addressed in different green-algal lineages. The available data clearly indicate a range of strategies, which are rather different in streptophycean and non-streptophycean green algae. While members of the Trebouxiophyceae exhibit effective water loss-prevention mechanisms based on the biosynthesis and accumulation of particular organic osmolytes such as polyols, these compounds are so far not reported in representatives of the Streptophyta. In members of the Streptophyta such as Klebsormidium, the most striking observation is the appearance of cross-walls in desiccated samples, which are strongly undulating, suggesting a high degree of mechanical flexibility. This aids in maintaining structural integrity in the dried state and allows the cell to maintain turgor pressure for a prolonged period of time during the dehydration process. Physiological strategies in aeroterrestrial green algae generally include a rapid reduction of photosynthesis during desiccation, but also a rather quick recovery after rewetting, whereas aquatic species are sensitive to drying. The underlying mechanisms such as the affected molecular components of the photosynthetic machinery are poorly understood in green algae. Therefore, modern approaches based on transcriptomics, proteomics, and/or metabolomics are urgently needed to better understand the molecular mechanisms involved in desiccation-stress physiology of these organisms. The very limited existing information is described in the present review.