Characterization of enzymatic antioxidants in the lichen Ramalina lacera and their response to rehydration

Usnea aurantiaco-atra (Jacq) Bory: Metabolites and Biological Activities

BACKGROUND

Lichens are complex symbiotic associations between a fungus and an alga or cyanobacterium. Due to their great adaptability to the environment, they have managed to colonize many terrestrial habitats, presenting a worldwide distribution from the poles to the tropical regions and from the plains to the highest mountains. In the flora of the Antarctic region, lichens stand out due to their variety and development and are a potential source of new bioactive compounds.

METHODS

A phytochemical study of the Antarctic lichen Usnea aurantiaco-atra (Jacq) Bory was conducted with the intention of determining the most important metabolites. In addition, the cytotoxic and antioxidant activities of its extracts were determined.

RESULTS

Cytotoxicity studies revealed that the hexane extract contains usnic acid as a majority metabolite, in addition to linoleic acid, ergosterols and terpenes, and demonstrates cytotoxic activity against an A375 melanoma cell line. On the other hand, the presence of total phenols in the extracts did not influence their antioxidant activity.

CONCLUSIONS

U. aurantiaco-atra contains mainly usnic acid, although there are terpenes and ergosta compounds that could be responsible for its cytotoxic activity. The presence of phenols did not confer antioxidant properties.

Comparative Transcriptomic and Proteomic Analyses Provide New Insights into the Tolerance to Cyclic Dehydration in a Lichen Phycobiont

Desiccation tolerance (DT) is relatively frequent in non-vascular plants and green algae. However, it is poorly understood how successive dehydration/rehydration (D/R) cycles shape their transcriptomes and proteomes. Here, we report a comprehensive analysis of adjustments on both transcript and protein profiles in response to successive D/R cycles in Coccomyxa simplex (Csol), isolated from the lichen Solorina saccata. A total of 1833 transcripts and 2332 proteins were differentially abundant as a consequence of D/R; however, only 315 of these transcripts/proteins showed similar trends. Variations in both transcriptomes and proteomes along D/R cycles together with functional analyses revealed an extensive decrease in transcript and protein levels during dehydration, most of them involved in gene expression, metabolism, substance transport, signalling and folding catalysis, among other cellular functions. At the same time, a series of protective transcripts/proteins, such as those related to antioxidant defence, polyol metabolism and autophagy, was upregulated during dehydration. Overall, our results show a transient decrease in most cellular functions as a result of drying and a gradual reactivation of specific cell processes to accommodate the hydration status along successive D/R cycles. This study provides new insights into key mechanisms involved in the DT of Csol and probably other dehydration-tolerant microalgae. In addition, functionally characterising the high number of genes/proteins of unknown functions found in this study may lead to the discovery of new DT mechanisms.

Effects of dissolved organic matter from sediment and soil samples on the growth and physiology of four bloom-forming algal species

Algal blooms negatively impact the water quality of reservoirs; however, the role of dissolved organic matter (DOM) in bloom formation in reservoirs has not been investigated. Therefore, we assessed the compositions of sediment- and soil-derived DOM and their effects on the growth, physiology, and photosynthetic activity of Microcystis aeruginosa, Anabaena sp., Chlamydomonas sp., and Peridiniopsis sp. (bloom-forming species). Sediment DOM promoted the growth of all algal species, whereas soil DOM significantly promoted the growth of Chlamydomonas sp. and Peridiniopsis sp.; this effect was due to enhanced stress tolerance and photosynthetic efficiency exhibited by these algae under DOM treatment. However, soil DOM slightly inhibited the growth of Anabaena sp. by increasing reactive oxygen species levels and inactivating some photosystem II reaction centers. The tyrosine-like substance, humic acid-like substances, and unsaturated aliphatic compounds were the main DOM components that affected algal growth. The findings of this study will provide a theoretical foundation for the development of bloom-prevention strategies for river-type reservoirs.

Effects of dissolved organic matter derived from two herbs on the growth, physiology, and physico-chemical characteristics of four bloom-forming algae species

While algal blooms occur frequently in lakes and reservoirs worldwide, the effects of dissolved organic matter (DOM) from lakeside and riparian zones on bloom formation are not well understood. In this study, we characterized the molecular composition of DOM from Cynodon dactylon (L.) Pers. (CD-DOM) and Xanthium sibiricum Patrin ex Widder (XS-DOM) and assessed their effects on the growth, physiology, volatile organic compounds (VOCs), and stable carbon isotope in four bloom-forming algae species (Microcystis aeruginosa, Anabaena sp., Chlamydomonas sp., and Peridiniopsis sp.). Stable carbon isotope analysis showed that the four species were affected by DOM. Both DOM types increased the cell biomass, polysaccharide and protein contents, chlorophyll fluorescence parameter values, and VOCs release of Anabaena sp., Chlamydomonas sp. and Microcystis aeruginosa, suggesting that DOM stimulated algal growth by increasing nutrient sources, photosynthetic efficiency, and stress tolerance. And in general, these three strains exhibited better growth at higher DOM concentrations. However, DOM treatment inhibited the growth of Peridiniopsis sp., as indicated by the increases in reactive oxygen species, damage in photosystem II reaction centers, and blockage in electron transport. Fluorescence analysis showed that tryptophan-like compounds were the main DOM components that affected algal growth. Molecular-level analysis suggested that unsaturated aliphatic compounds may be the most important DOM components. The findings indicate that CD-DOM and XS-DOM promote the blue-green algal blooms formation and thus should be considered in the management of natural water quality.

Genotoxic potential of bisphenol A: A review

Bisphenol A (BPA), as a major component of some plastic products, is abundant environmental pollutant. Due to its ability to bind to several types of estrogen receptors, it can trigger multiple cellular responses, which can contribute to various manifestations at the organism level. The most studied effect of BPA is endocrine disruption, but recently its prooxidative potential has been confirmed. BPA ability to induce oxidative stress through increased ROS production, altered activity of antioxidant enzymes, or accumulation of oxidation products of biomacromolecules is observed in a wide range of organisms - estrogen receptor-positive and -negative. Subsequently, increased intracellular oxidation can lead to DNA damage induction, represented by oxidative damage, single- and double-strand DNA breaks. Importantly, BPA shows several mechanisms of action and can trigger adverse effects on all organisms inhabiting a wide variety of ecosystem types. Therefore, the main aim of this review is to summarize the genotoxic effects of BPA on organisms across all taxa.

Application of Index of Atmospheric Purity (IAP) along elevation gradients in Gunung Jerai, Kedah, Malaysia

The utilisation of biological organisms, especially lichens in the environmental biomonitoring approach, has been proven to be an effective and low-cost technique suitable for developing countries like Malaysia. Index of Atmospheric Purity (IAP) tracked compositional changes in lichen communities which correlate with changes in levels of atmospheric pollution. Gunung Jerai was formed during the Cambrian Period; thus, it is a biodiversity hotspot ideal for a diverse range of lichens. In the present work, a total of 44 corticolous lichen species were sampled and identified to evaluate the pollution status of Gunung Jerai using IAP, starting from 80 to 1200 m with 300 m intervals. The samples were collected within 10 × 50 cm sampling grids attached to 60 trees, bringing a total of 120 000 cm² of the sampling area. The air quality of the sampling area was determined by IAP score, a low score indicated by high levels of pollution. Results showed that the lowest IAP score was recorded at 300 m; meanwhile, the highest IAP score was recorded at 900 m elevation. Elevational gradient and pollution have a significant effect on the IAP score of Gunung Jerai. On average, Gunung Jerai is indicated as having a low pollution status. However, several elevations of the rainforest showed high and moderate pollution status. The IAP method is best to assess environmental pollution and provide quicker results than chemical monitoring methods. Further research could be done to evaluate the other sampling sites adjacent to other areas of Gunung Jerai.

Antioxidant Response during the Kinetics of Anhydrobiosis in Two Eutardigrade Species

Anhydrobiosis, a peculiar adaptive strategy existing in nature, is a reversible capability of organisms to tolerate a severe loss of their body water when their surrounding habitat is drying out. In the anhydrobiotic state, an organism lacks all dynamic features of living beings since an ongoing metabolism is absent. The depletion of water in the anhydrobiotic state increases the ionic concentration and the production of reactive oxygen species (ROS). An imbalance between the increased production of ROS and the limited action of antioxidant defences is a source of biomolecular damage and can lead to oxidative stress. The deleterious effects of oxidative stress were demonstrated in anhydrobiotic unicellular and multicellular organisms, which counteract the effects using efficient antioxidant machinery, mainly represented by ROS scavenger enzymes. To gain insights into the dynamics of antioxidant patterns during the kinetics of the anhydrobiosis of two tardigrade species, Paramacrobiotus spatialis and Acutuncus antarcticus, we investigated the activity of enzymatic antioxidants (catalase, superoxide dismutase, glutathione peroxidase, and glutathione reductase) and the amount of non-enzymatic antioxidants (glutathione) in the course of rehydration. In P. spatialis, the activity of catalase increases during dehydration and decreases during rehydration, whereas in A. antarcticus, the activity of superoxide dismutase decreases during desiccation and increases during rehydration. Genomic varieties, different habitats and geographical regions, different diets, and diverse evolutionary lineages may have led to the specialization of antioxidant strategies in the two species.

Photoprotection and high-light acclimation in semi-arid grassland lichens – a cooperation between algal and fungal partners

In lichens, each symbiotic partner cooperates for the survival of the symbiotic association. The protection of the susceptible photosynthetic apparatus is essential for both participants. The mycobiont and photobiont contribute to the protection against the damaging effect of excess light by various mechanisms. The present study investigated the effect of seasonality and microhabitat exposure on photoprotection and photoacclimation in the photo- and the mycobiont of six lichen species with different thallus morphology in inland dune system in the Kiskunság region (Hungary) with shaded, more humid and exposed, drier dune sides. High-Performance Liquid Chromatography, spectrophotometry, chlorophyll a fluorescence kinetic technique were used, and micrometeorological data were collected. The four years data series revealed that the north-east-facing side was characterized by higher relative humidity and lower light intensities compared to the south-west-facing drier and more exposed sides. The south-west facing side was exposed to direct illumination 3–4 hours longer in winter and 1–2 hours shorter in summer than the north-east facing side of the dune, influencing the metabolism of sun and shade populations of various species. Because rapid desiccation caused short active periods of lichens during bright and drier seasons and on exposed microhabitats, the rapid, non-regulated non-photochemical quenching mechanisms in the photobiont had a significant role in protecting the photosynthetic system in the hydrated state. In dehydrated conditions, thalli were mainly defended by the solar screening metabolites produced by the mycobiont and curling during desiccation (also caused by the mycobiont). Furthermore, the efficacy of light use (higher chlorophyll and carotenoid concentration) increased because of short hydrated periods. Still, a lower level of received irradiation was appropriate for photosynthesis in dry seasons and on sun exposed habitats. In humid seasons and microhabitats, more extended active periods lead to increased photosynthesis and production of solar radiation protectant fungal metabolites, allowing a lower level of photoprotection in the form of regulated non-photochemical quenching by the photobiont. Interspecific differences were more pronounced than the intraspecific ones among seasons and microhabitat types.

Advances in Understanding of Desiccation Tolerance of Lichens and Lichen-Forming Algae

Lichens are symbiotic associations (holobionts) established between fungi (mycobionts) and certain groups of cyanobacteria or unicellular green algae (photobionts). This symbiotic association has been essential in the colonization of terrestrial dry habitats. Lichens possess key mechanisms involved in desiccation tolerance (DT) that are constitutively present such as high amounts of polyols, LEA proteins, HSPs, a powerful antioxidant system, thylakoidal oligogalactolipids, etc. This strategy allows them to be always ready to survive drastic changes in their water content. However, several studies indicate that at least some protective mechanisms require a minimal time to be induced, such as the induction of the antioxidant system, the activation of non-photochemical quenching including the de-epoxidation of violaxanthin to zeaxanthin, lipid membrane remodeling, changes in the proportions of polyols, ultrastructural changes, marked polysaccharide remodeling of the cell wall, etc. Although DT in lichens is achieved mainly through constitutive mechanisms, the induction of protection mechanisms might allow them to face desiccation stress in a better condition. The proportion and relevance of constitutive and inducible DT mechanisms seem to be related to the ecology at which lichens are adapted to.

Tolerance to Cyclic Desiccation in Lichen Microalgae is Related to Habitat Preference and Involves Specific Priming of the Antioxidant System

Oxidative stress is a crucial challenge for lichens exposed to cyclic desiccation and rehydration (D/R). However, strategies to overcome this potential stress are still being unraveled. Therefore, the physiological performance and antioxidant mechanisms of two lichen microalgae, Trebouxia sp. (TR9) and Coccomyxa simplex (Csol), were analyzed. TR9 was isolated from Ramalina farinacea, a Mediterranean fruticose epiphytic lichen adapted to xeric habitats, while Csol is the phycobiont of Solorina saccata, a foliaceous lichen that grows on humid rock crevices. The tolerance to desiccation of both species was tested by subjecting them to different drying conditions and to four consecutive daily cycles of D/R. Our results show that a relative humidity close to that of their habitats was crucial to maintain the photosynthetic rates. Concerning antioxidant enzymes, in general, manganese superoxide dismutases (MnSODs) were induced after desiccation and decreased after rehydration. In TR9, catalase (CAT)-A increased, and its activity was maintained after four cycles of D/R. Ascorbate peroxidase activity was detected only in Csol, while glutathione reductase increased only in TR9. Transcript levels of antioxidant enzymes indicate that most isoforms of MnSOD and FeSOD were induced by desiccation and repressed after rehydration. CAT2 gene expression was also upregulated and maintained at higher levels even after four cycles of D/R in accordance with enzymatic activities. To our knowledge, this is the first study to include the complete set of the main antioxidant enzymes in desiccation-tolerant microalgae. The results highlight the species-specific induction of the antioxidant system during cyclic D/R, suggesting a priming of oxidative defence metabolism.

Interactions between Microcystis aeruginosa and coexisting bisphenol A at different nitrogen levels

Microcystis aeruginosa is known as a main contributor of cyanobacterial bloom. However, factors that drive its formation and dispersion remain poorly understood. The cellular-level responses to nutrient drivers of eutrophication were investigated. The results showed that growth rate of M. aeruginosa was significantly enhanced with the increasing bisphenol A (BPA) and nitrogen (N) level. Stress of BPA significantly inhibited cellular density, chlorophyll-a content across all the nutrient conditions, while Fv/Fm and rETRmax value were promoted by BPA. Responses of reactive oxygen species (ROS) value, superoxide dismutase (SOD) activity and malodialdehyde (MDA) content indicated that nitrogen deficiency and BPA caused oxidative stress to M. aeruginosa. Besides, nitrogen and BPA regulated the production and release of microcystins (MCs). M. aeruginosa exposed to BPA caused 95 up-regulated proteins, which was primarily associated with photosynthesis, nitrogen metabolism, glycolysis/glyconeogenesis and carbon fixation in photosynthetic organisms. The 91 down-regulated proteins were related to quorum sensing, longevity regulating and cell cycle-caulobacter, confirming that the driving force of regulating the change of cellular density and genes expression weakened. These findings provide important clues to elucidate the combined regulatory mechanisms of cyanobacterial blooms triggered by endocrine-disrupting compounds and environmental factors and help to effectively prevent and reduce cyanobacterial blooms.

Relation between water status and desiccation-affected genes in the lichen photobiont Trebouxia gelatinosa

The relation between water status and expression profiles of desiccation -related genes has been studied in the desiccation tolerant (DT) aeroterrestrial green microalga Trebouxia gelatinosa, a common lichen photobiont. Algal colonies were desiccated in controlled conditions and during desiccation water content (WC) and water potential (Ψ) were measured to find the turgor loss point (Ψ_tlp). Quantitative real-time PCR was performed to measure the expression of ten genes related to photosynthesis, antioxidant defense, expansins, heat shock proteins (HSPs), and desiccation related proteins in algal colonies collected during desiccation when still at full turgor (WC > 6 g H₂O g^-1 dry weight), immediately before and after Ψ_tlp (-4 MPa; WC ∼ 1 g H₂O g^-1 dry weight) and before and after complete desiccation (WC < 0.01 g H₂O g^-1 dry weight), quantifying the HSP70 protein levels by immunodetection. Our analysis showed that the expression of eight out of ten genes changed immediately before and after Ψ_tlp. Interestingly, the expression of five out of ten genes changed also before complete desiccation, i.e. between 0.2 and 0.01 g H₂O g^-1 dry weight. However, the HSP70 protein levels were not affected by changes in water status. The study provides new evidences of the link between the loss of turgor and the expression of genes related to the desiccation tolerance of T. gelatinosa, suggesting the former as a signal triggering inducible mechanisms.

The Effect of Bisphenol A on Growth, Morphology, Lipid Peroxidation, Antioxidant Enzyme Activity, and PS II in Cylindrospermopsis raciborskii and Scenedesmus quadricauda

To investigate the effect of bisphenol A (BPA) on Cylindrospermopsis raciborskii (Cyanobacteria) and Scenedesmus quadricauda (Chlorophyta), we grew the two species at BPA concentrations of 0, 0.1, 1, 2, 5, 10, and 20 mg/L and examined their growth, lipid peroxidation, antioxidant enzyme activity, and chlorophyll a fluorescence. The 96-h EC₅₀ values (effective concentration causing 50% growth inhibition) for BPA in C. raciborskii and S. quadricauda were 9.663 ± 0.047, and 13.233 ± 0.069 mg/L, respectively. A significant reduction in chlorophyll a concentration was found in C. raciborskii and S. quadricauda when BPA concentrations were greater than 1 and 2 mg/L, respectively. Furthermore, F _v/F _m, ΔF/F _m', and qP decreased significantly at 10 mg/L BPA in C. raciborskii but started to decrease at 10 mg/L in S. quadricauda. The changes in chlorophyll fluorescence parameters (α, rETR_max) that were obtained from the rapid light response curves of both algae species showed similar responses to F _v/F _m, ΔF/F _m', and qP under BPA-induced stress. Values for all of the chlorophyll fluorescence parameters in S. quadricauda were higher than in C. raciborskii; however, the nonphotochemical quenching measured in C. raciborskii was considerably higher than it was in S. quadricauda. In addition, lipid peroxidation (determined as MDA content) and antioxidant enzyme activities (SOD and CAT) increased in both species as the BPA concentration increased. These results suggest that C. raciborskii is more sensitive to the effects of BPA than S. quadricauda and that photosystem II might be a target for the activity of BPA in vivo.

Functional analysis of thioredoxin from the desert lichen-forming fungus, Endocarpon pusillum Hedwig, reveals its role in stress tolerance

Endocarpon pusillum is a lichen-forming fungus with an outstanding stress resistance property closely related to its antioxidant system. In this study, thioredoxin (Trx), one of the main components of antioxidant defense systems in E. pusillum (EpTrx), was characterized and analyzed both in transgenic yeasts and in vitro. Our analyses identified that the heterologous expression of EpTrx in the yeast Pichia pastoris significantly enhanced its resistance to osmotic and oxidative stresses. Assays in vitro showed EpTrx acted as a disulfide reductase as well as a molecular chaperone by assembling into various polymeric structures. Upon exposure to heat-shock stress, EpTrx exhibited weaker disulfide reductase activity but stronger chaperone activity, which coincided with the switching of the protein complexes from low molecular weight forms to high molecular weight complexes. Specifically, we found that Cys31 near but not at the active site was crucial in promoting the structural and functional transitions, most likely by accelerating the formation of intermolecular disulfide bond. Transgenic Saccharomyces cerevisiae harboring the native EpTrx exhibited stronger tolerance to oxidative, osmotic and high temperature stresses than the corresponding yeast strain containing the mutant EpTrx (C31S). Our results provide the first molecular evidence on how Trx influences stress response in lichen-forming fungi.

Comparative transcriptome analysis of the lichen-forming fungus Endocarpon pusillum elucidates its drought adaptation mechanisms

The lichen-forming fungus was isolated from the desert lichen Endocarpon pusillum that is extremely drought resistant. To understand the molecular mechanisms of drought resistance in the fungus, we employed RNA-seq and quantitative real-time PCR to compare and characterize the differentially expressed genes in pure culture at two different water levels and with that in desiccated lichen. The comparative transcriptome analysis indicated that a total of 1781 genes were differentially expressed between samples cultured under normal and PEG-induced drought stress conditions. Similar to those in drought resistance plants and non-lichenized fungi, the common drought-resistant mechanisms were differentially expressed in E. pusillum. However, the expression change of genes involved in osmotic regulation in E. pusillum is different, which might be the evidence for the feature of drought adaptation. Interestingly, different from other organisms, some genes involved in drought adaption mechanisms showed significantly different expression patterns between the presence and absence of drought stress in E. pusillum. The expression of 23 candidate stress responsive genes was further confirmed by quantitative real-time PCR using dehydrated E. pusillum lichen thalli. This study provides a valuable resource for future research on lichen-forming fungi and shall facilitate future functional studies of the specific genes related to drought resistance.

Transcriptomics of desiccation tolerance in the streptophyte green alga Klebsormidium reveal a land plant-like defense reaction

BACKGROUND

Water loss has significant effects on physiological performance and survival rates of algae. However, despite the prominent presence of aeroterrestrial algae in terrestrial habitats, hardly anything is known about the molecular events that allow aeroterrestrial algae to survive harsh environmental conditions. We analyzed the transcriptome and physiology of a strain of the alpine aeroterrestrial alga Klebsormidium crenulatum under control and strong desiccation-stress conditions.

PRINCIPAL FINDINGS

For comparison we first established a reference transcriptome. The high-coverage reference transcriptome includes about 24,183 sequences (1.5 million reads, 636 million bases). The reference transcriptome encodes for all major pathways (energy, carbohydrates, lipids, amino acids, sugars), nearly all deduced pathways are complete or missing only a few transcripts. Upon strong desiccation, more than 7000 transcripts showed changes in their expression levels. Most of the highest up-regulated transcripts do not show similarity to known viridiplant proteins, suggesting the existence of some genus- or species-specific responses to desiccation. In addition, we observed the up-regulation of many transcripts involved in desiccation tolerance in plants (e.g. proteins similar to those that are abundant in late embryogenesis (LEA), or proteins involved in early response to desiccation ERD), and enzymes involved in the biosynthesis of the raffinose family of oligosaccharides (RFO) known to act as osmolytes). Major physiological shifts are the up-regulation of transcripts for photosynthesis, energy production, and reactive oxygen species (ROS) metabolism, which is supported by elevated cellular glutathione content as revealed by immunoelectron microscopy as well as an increase in total antiradical power. However, the effective quantum yield of Photosystem II and CO2 fixation decreased sharply under the applied desiccation stress. In contrast, transcripts for cell integrative functions such as cell division, DNA replication, cofactor biosynthesis, and amino acid biosynthesis were down-regulated.

SIGNIFICANCE

This is the first study investigating the desiccation transcriptome of a streptophyte green alga. Our results indicate that the cellular response is similar to embryophytes, suggesting that embryophytes inherited a basic cellular desiccation tolerance from their streptophyte predecessors.

Ozone tolerance in lichens: a possible explanation from biochemical to physiological level using Flavoparmelia caperata as test organism

Lichens are among the best biomonitors of airborne pollutants, but surprisingly they reveal high tolerance to ozone (O3). It was recently suggested that this might be due to the high levels of natural defences against oxidative stress, related to their poikilohydric life strategy. The objective of this work is to give a thorough description of the biochemical and physiological mechanisms that are at the basis of the O3-tolerance of lichens. Chlorophyll a fluorescence (ChlaF) emission, histochemical ROS localization in the lichen thallus, and biochemical markers [enzymes and antioxidants involved in the ascorbate/glutathione (AsA/GSH) cycle; hydrogen peroxide (H2O2) and superoxide anion (O2(-))] were used to characterize the response of the epiphytic lichen Flavoparmelia caperata (L.) Hale exposed to O3 (250 ppb, 5 hd(-1), 2 weeks) at different watering regimes and air relative humidity (RH) in a fumigation chamber. After two-week exposure ChlaF was affected by the watering regime but not by O3. The watering regime influenced also the superoxide dismutase activity and the production of ROS. By contrast O3 strongly influenced the AsA/GSH biochemical pathway, decreasing the reduced ascorbate (AsA) content and increasing the enzymatic activity of ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) independently from the watering regime and the relative humidity applied. This study highlights that F. caperata can face the O3-induced oxidative stress thanks to high levels of constitutive enzymatic and non-enzymatic defences against ROS formed naturally during the dehydration-rehydration cycles to which lichens are frequently exposed.

Rehydration conditions for isolation of high quality RNA from the lichen Lobaria pulmonaria

Background

The poikilohydric nature of lichens enables them to survive repeated episodes of desiccation by utilizing water when it becomes available. During rehydration, RNA-degrading endonucleases may be released, reducing RNA quantity and quality. Re-generation of a steady-state condition where RNA quantity and quality no longer fluctuate establishes a framework for development of new hypotheses for future investigations. Using Lobaria pulmonaria as a model species, the objective of this study was to compare the effect of different rehydration conditions on the quantity and quality of RNA from the rehydrated thallus.

Findings

Spectrophotometric measurements of total RNA and cDNA were performed for samples prepared from dry lichen or lichen after rehydration (0.5 h, 1 h, 2 h, 4 h or 24 h), with limited light and dark conditions, and at three temperatures (15°C, 20°C or 32°C) for some of these conditions. The results showed that rehydration of the thallus for 4 h at 20°C in light yielded the highest concentration and quality of RNA. A higher RNA concentration was obtained in light than in dark conditions, but the RNA quality was unaffected.

Conclusions

This study suggests that allowance of 4 h for thallus rehydration should be adequate to ensure complete recovery of transcription. After 4 h at 20°C further studies can be carried out on the RNA in this model species.

Whole transcriptome characterization of the effects of dehydration and rehydration on Cladonia rangiferina, the grey reindeer lichen

BACKGROUND

Lichens are symbiotic organisms with a fungal and an algal or a cyanobacterial partner. Lichens inhabit some of the harshest climates on earth and most lichen species are desiccation-tolerant. Lichen desiccation-tolerance has been studied at the biochemical level and through proteomics, but the underlying molecular genetic mechanisms remain largely unexplored. The objective of our study was to examine the effects of dehydration and rehydration on the gene expression of Cladonia rangiferina.

RESULTS

Samples of C. rangiferina were collected at several time points during both the dehydration and rehydration process and the gene expression intensities were measured using a custom DNA microarray. Several genes, which were differentially expressed in one or more time points, were identified. The microarray results were validated using qRT-PCR analysis. Enrichment analysis of differentially expressed transcripts was also performed to identify the Gene Ontology terms most associated with the rehydration and dehydration process.

CONCLUSIONS

Our data identify differential expression patterns for hundreds of genes that are modulated during dehydration and rehydration in Cladonia rangiferina. These dehydration and rehydration events clearly differ from each other at the molecular level and the largest changes to gene expression are observed within minutes following rehydration. Distinct changes are observed during the earliest stage of rehydration and the mechanisms not appear to be shared with the later stages of wetting or with drying. Several of the most differentially expressed genes are similar to genes identified in previous studies that have investigated the molecular mechanisms of other desiccation-tolerant organisms. We present here the first microarray experiment for any lichen species and have for the first time studied the genetic mechanisms behind lichen desiccation-tolerance at the whole transcriptome level.

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.

The response of Asterochloris erici (Ahmadjian) Skaloud et Peksa to desiccation: a proteomic approach

The study of desiccation tolerance of lichens, and of their chlorobionts in particular, has frequently focused on the antioxidant system that protects the cell against photo-oxidative stress during dehydration/rehydration cycles. In this study, we used proteomic and transcript analyses to assess the changes associated with desiccation in the isolated phycobiont Asterochloris erici. Algae were dried either slowly (5-6 h) or rapidly (<60 min), and rehydrated after 24 h in the desiccated state. To identify proteins that accumulated during the drying or rehydration processes, we employed two-dimensional (2D) difference gel electrophoresis (DIGE) coupled with individual protein identification using trypsin digestion and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Proteomic analyses revealed that desiccation caused an increase in relative abundance of only 11-13 proteins, regardless of drying rate, involved in glycolysis, cellular protection, cytoskeleton, cell cycle, and targeting and degradation. Transcripts of five Hsp90 and two β-tubulin genes accumulated primarily at the end of the dehydration process. In addition, transmission electron microscopy (TEM) images indicate that ultrastructural cell injuries, perhaps resulting from physical or mechanical stress rather than metabolic damage, were more intense after rapid dehydration. This occurred with no major change in the proteome. These results suggest that desiccation tolerance of A. erici is achieved by constitutive mechanisms.

Prolonging the hydration and active metabolism from light periods into nights substantially enhances lichen growth

This study investigates how hydration during light and dark periods influences growth in two epiphytic old forest lichens, the green algal Lobaria pulmonaria and the cyanobacterial L. scrobiculata. The lichens were cultivated in growth chambers for 14 days (200 μmol m(-1) s(-2); 12 h photoperiod) at four temperature regimes (25/20 °C, 21/16 °C, 13/8 °C, and 6/1 °C; day/night temperatures) and two hydration regimes (12 h day-time hydration; 12 h day-time + 12 h night-time hydration). Growth was highly dynamic, showing that short-term growth experiments in growth cabinets have a high, but largely unexplored potential in functional lichen studies. The highest measured growth rates were not far from the maximal dry matter gain estimated from published net photosynthetic CO2 uptake data. For the entire data set, photobiont type, temperature, hydration regime and specific thallus mass accounted for 46.6 % of the variation in relative growth rate (RGR). Both species showed substantially higher relative growth rates based on both biomass (RGR) and thallus area (RTAGR) when they were hydrated day and night compared to hydration in light only. Chronic photoinhibition was substantial in thalli hydrated only during the day time and kept at the highest and lowest temperature regimes, resulting in exponential increases in RGR with increasing maximal PSII efficiency (F v/F m) in both species. However, the depression in F v/F m was stronger for the cyanolichen than for the cephalolichen at extreme temperatures. The growth-stimulating effect of night-time hydration suggests that nocturnal metabolic activity improves recovery of photoinhibition and/or enhances the conversion rate of photosynthates into thallus extension.

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.

Characterization of a transcriptome from a non-model organism, Cladonia rangiferina, the grey reindeer lichen, using high-throughput next generation sequencing and EST sequence data

BACKGROUND

Lichens are symbiotic organisms that have a remarkable ability to survive in some of the most extreme terrestrial climates on earth. Lichens can endure frequent desiccation and wetting cycles and are able to survive in a dehydrated molecular dormant state for decades at a time. Genetic resources have been established in lichen species for the study of molecular systematics and their taxonomic classification. No lichen species have been characterised yet using genomics and the molecular mechanisms underlying the lichen symbiosis and the fundamentals of desiccation tolerance remain undescribed. We report the characterisation of a transcriptome of the grey reindeer lichen, Cladonia rangiferina, using high-throughput next-generation transcriptome sequencing and traditional Sanger EST sequencing data.

RESULTS

Altogether 243,729 high quality sequence reads were de novo assembled into 16,204 contigs and 49,587 singletons. The genome of origin for the sequences produced was predicted using Eclat with sequences derived from the axenically grown symbiotic partners used as training sequences for the classification model. 62.8% of the sequences were classified as being of fungal origin while the remaining 37.2% were predicted as being of algal origin. The assembled sequences were annotated by BLASTX comparison against a non-redundant protein sequence database with 34.4% of the sequences having a BLAST match. 29.3% of the sequences had a Gene Ontology term match and 27.9% of the sequences had a domain or structural match following an InterPro search. 60 KEGG pathways with more than 10 associated sequences were identified.

CONCLUSIONS

Our results present a first transcriptome sequencing and de novo assembly for a lichen species and describe the ongoing molecular processes and the most active pathways in C. rangiferina. This brings a meaningful contribution to publicly available lichen sequence information. These data provide a first glimpse into the molecular nature of the lichen symbiosis and characterise the transcriptional space of this remarkable organism. These data will also enable further studies aimed at deciphering the genetic mechanisms behind lichen desiccation tolerance.

Antioxidant, antimicrobial and antiproliferative activities of five lichen species

The antioxidative, antimicrobial and antiproliferative potentials of the methanol extracts of the lichen species Parmelia sulcata, Flavoparmelia caperata, Evernia prunastri, Hypogymnia physodes and Cladonia foliacea were evaluated. The total phenolic content of the tested extracts varied from 78.12 to 141.59 mg of gallic acid equivalent (GA)/g of extract and the total flavonoid content from 20.14 to 44.43 mg of rutin equivalent (Ru)/g of extract. The antioxidant capacities of the lichen extracts were determined by 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals scavenging. Hypogymnia physodes with the highest phenolic content showed the strongest DPPH radical scavenging effect. Further, the antimicrobial potential of the lichen extracts was determined by a microdilution method on 29 microorganisms, including 15 strains of bacteria, 10 species of filamentous fungi and 4 yeast species. A high antimicrobial activity of all the tested extracts was observed with more potent inhibitory effects on the growth of Gram (+) bacteria. The highest antimicrobial activity among lichens was demonstrated by Hypogymnia physodes and Cladonia foliacea. Finally, the antiproliferative activity of the lichen extracts was explored on the colon cancer adenocarcinoma cell line HCT-116 by MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) viability assay and acridine orange/ethidium bromide staining. The methanol extracts of Hypogymnia physodes and Cladonia foliacea showed a better cytotoxic activity than the other extracts. All lichen species showed the ability to induce apoptosis of HCT-116 cells.

Effects of urban air pollutants on elemental accumulation and identification of oxidative stress biomarkers in the transplanted lichen Pseudovernia furfuracea

Determining the origins of heavy metals, their accumulation, and their detoxification mechanisms constitutes a major problem in understanding environmental pollution in urban areas. The objective of this study was to detect the relative air quality in Eskisehir city center (Turkey) through the transplanted epiphytic lichen Pseudovernia furfuracea as a biomonitor of the concentrations of some heavy metals accumulated and to describe their toxic effects on lichen physiology during the study period of one year. The influence of heavy metal accumulations on macroelement levels were also examined. In addition to analysis of B, K, Ca, P, S, Al, Fe, Mg, Mn, Ni, Zn, Pb, and Cu, total soluble proteins and oxidative stress parameters through glutathione (GSH) contents and glutathione S-transferase (GST) activities were also determined. Results indicate that, although the heavy metal concentrations were found to be gradually increased from the unpolluted control zone to the city center, the concentrations of macroelements S and Ca were higher in the polluted area. However, B, K, Mg, and P elements were found to be higher in the unpolluted area than in the polluted zones. It seems that heavy metals induce oxidative stress in lichens; we found enhancement of GSH concentrations and GST activity. Most probably, the expected destruction in this organism was reduced by the intervention of antioxidant capacity. Therefore, this report confirms the fact that the epiphytic lichen P. furfuracea is a good model for biomonitoring atmospheric quality for a long-term transplantation study, and oxidative stress parameters stand out as a tool for an early environment assessment of other physiological parameters.

Degradation of the disease-associated prion protein by a serine protease from lichens

The disease-associated prion protein (PrP^TSE), the probable etiological agent of the transmissible spongiform encephalopathies (TSEs), is resistant to degradation and can persist in the environment. Lichens, mutualistic symbioses containing fungi, algae, bacteria and occasionally cyanobacteria, are ubiquitous in the environment and have evolved unique biological activities allowing their survival in challenging ecological niches. We investigated PrP^TSE inactivation by lichens and found acetone extracts of three lichen species (Parmelia sulcata, Cladonia rangiferina and Lobaria pulmonaria) have the ability to degrade prion protein (PrP) from TSE-infected hamsters, mice and deer. Immunoblots measuring PrP levels and protein misfolding cyclic amplification indicated at least two logs of reductions in PrP^TSE. Degradative activity was not found in closely related lichen species or in algae or a cyanobacterium that inhabit lichens. Degradation was blocked by Pefabloc SC, a serine protease inhibitor, but not inhibitors of other proteases or enzymes. Additionally, we found that PrP levels in PrP^TSE-enriched preps or infected brain homogenates are also reduced following exposure to freshly-collected P. sulcata or an aqueous extract of the lichen. Our findings indicate that these lichen extracts efficiently degrade PrP^TSE and suggest that some lichens could have potential to inactivate TSE infectivity on the landscape or be a source for agents to degrade prions. Further work to clone and characterize the protease, assess its effect on TSE infectivity and determine which organism or organisms present in lichens produce or influence the protease activity is warranted.

Oxidative stress induces distinct physiological responses in the two Trebouxia phycobionts of the lichen Ramalina farinacea

BACKGROUND AND AIMS

Most lichens form associations with Trebouxia phycobionts and some of them simultaneously include genetically different algal lineages. In other symbiotic systems involving algae (e.g. reef corals), the relative abundances of different endosymbiotic algal clades may change over time. This process seems to provide a mechanism allowing the organism to respond to environmental stress. A similar mechanism may operate in lichens with more than one algal lineage, likewise protecting them against environmental stresses. Here, the physiological responses to oxidative stress of two distinct Trebouxia phycobionts (provisionally named TR1 and TR9) that coexist within the lichen Ramalina farinacea were analysed.

METHODS

Isolated phycobionts were exposed to oxidative stress through the reactive oxygen species propagator cumene hydroperoxide (CuHP). Photosynthetic pigments and proteins, photosynthesis (through modulated chlorophyll fluorescence), the antioxidant enzymes superoxide dismutase (SOD) and glutathione reductase (GR), and the stress-related protein HSP70 were analysed.

KEY RESULTS

Photosynthetic performance was severely impaired by CuHP in phycobionts, as indicated by decreases in the maximal PSII photochemical efficiency (F(v)/F(m)), the quantum efficiency of PSII (Φ(PSII)) and the non-photochemical dissipation of energy (NPQ). However, the CuHP-dependent decay in photosynthesis was significantly more severe in TR1, which also showed a lower NPQ and a reduced ability to preserve chlorophyll a, carotenoids and D1 protein. Additionally, differences were observed in the capacities of the two phycobionts to modulate antioxidant activities and HPS70 levels when exposed to oxidative stress. In TR1, CuHP significantly diminished HSP70 and GR but did not change SOD activities. In contrast, in TR9 the levels of both antioxidant enzymes and those of HSP70 increased in response to CuHP.

CONCLUSIONS

The better physiological performance of TR9 under oxidative conditions may reflect its greater capacity to undertake key metabolic adjustments, including increased non-photochemical quenching, higher antioxidant protection and the induction of repair mechanisms.

Antioxidant properties of some lichen species

Antioxidant activity of the acetone, methanol and aqueous extracts of the lichens Cladonia furcata, Hypogymnia physodes, Lasallia pustulata, Parmelia caperata and Parmelia sulcata has been screened in vitro by using different methods (DPPH radical scavenging, superoxide anion radical scavenging, reducing power, determination of total phenolic compounds and determination of total flavonoid content). Of the lichens tested, Lasallia pustulata had powerful antioxidant activities. Acetone, methanol and aqueous extracts of this lichen showed 90.93, 69.87 and 65.08% DPPH radical scavenging activities. Moreover, the tested extracts had effective reducing power and superoxide anion radical scavenging. Those various antioxidant activities were compared to standard antioxidants such as ascorbic acid, butylated hydroxyanisole (BHA) and α-tocopherol. In addition, total content of phenol and flavonoid in extracts were determined as pyrocatechol equivalent, and as rutin equivalent, respectively. The strong relationships between total phenolic and flavonoid contents and the antioxidative activities of tested extracts suggest that these compounds play important role in antioxidant activity. The present stady shows that tested lichen species demonstrated a strong antioxidant activity and can be considered as good sources of natural antioxidants.

Fungal-associated NO is involved in the regulation of oxidative stress during rehydration in lichen symbiosis

BACKGROUND

Reactive oxygen species (ROS) are normally produced in respiratory and photosynthetic electron chains and their production is enhanced during desiccation/rehydration. Nitric oxide (NO) is a ubiquitous and multifaceted molecule involved in cell signaling and abiotic stress. Lichens are poikilohydrous organisms that can survive continuous cycles of desiccation and rehydration. Although the production of ROS and NO was recently demonstrated during lichen rehydration, the functions of these compounds are unknown. The aim of this study was to analyze the role of NO during rehydration of the lichen Ramalina farinacea (L.) Ach., its isolated photobiont partner Trebouxia sp. and Asterochloris erici (Ahmadjian) Skaloud et Peksa (SAG 32.85 = UTEX 911).

RESULTS

Rehydration of R. farinacea caused the release of ROS and NO evidenced by the fluorescent probes DCFH₂-DA and DAN respectively. However, a minimum in lipid peroxidation (MDA) was observed 2 h post-rehydration. The inhibition of NO in lichen thalli with c-PTIO resulted in increases in both ROS production and lipid peroxidation, which now peaked at 3 h, together with decreases in chlorophyll autofluorescence and algal photobleaching upon confocal laser incidence. Trebouxia sp. photobionts generate peaks of NO-endproducts in suspension and show high rates of photobleaching and ROS production under NO inhibition which also caused a significant decrease in photosynthetic activity of A. erici axenic cultures, probably due to the higher levels of photo-oxidative stress.

CONCLUSIONS

Mycobiont derived NO has an important role in the regulation of oxidative stress and in the photo-oxidative protection of photobionts in lichen thalli. The results point to the importance of NO in the early stages of lichen rehydration.

Is plant endophyte-mediated defensive mutualism the result of oxidative stress protection?

In this review, we discuss the biology and beneficial effects of plant endophytes on host plants. The current explanation of endophyte protection (defensive mutualism) of host plants is based on the secondary metabolites (alkaloids) with antiherbivore properties produced by the symbiotic association between host plant and endophytes. We propose an alternative explanation of the mechanism of host protection through enhanced stress tolerance to oxidative stress. Several studies have demonstrated the production of different compounds (phenolics) with antioxidant capacity in endophyte-infected plants. Endophytes may also produce mannitol, other carbohydrates and small molecules (proline) with antioxidant capacity. We suggest that enhanced antioxidant production by symbiotic plants may be the result of the production of reactive oxygen species (ROS) by endophytes. In turn, symbiotic plants are protected from oxidative stress produced by plant diseases, droughts, heavy metals and other oxidative stressors by the production of antioxidants. We also discuss the lichen symbiosis and evaluate whether management of ROS also plays a role in this defensive mutualism. Future experiments are needed to evaluate the hypothesis that antioxidants are responsible for enhanced stress tolerance in endophyte-infected plants.

Dehydration rate and time of desiccation affect recovery of the lichen alga [corrected] Trebouxia erici: alternative and classical protective mechanisms

The mechanisms involved in desiccation tolerance of lichens and their photobionts are still poorly understood. To better understand these mechanisms we have studied dehydration rate and desiccation time in Trebouxia, the most abundant chlorophytic photobiont in lichen. Our findings indicate that the drying rate has a profound effect on the recovery of photosynthetic activity of algae after rehydration, greater than the effects of desiccation duration. The basal fluorescence (F'(o)) values in desiccated algae were significantly higher after rapid dehydration, than after slow dehydration, suggesting higher levels of light energy dissipation in slow-dried algae. Higher values of PSII electron transport were recovered after rehydration of slow-dried Trebouxia erici compared to rapid-dried algae. The main component of non-photochemical quenching after slow dehydration was energy dependent (q (E)), whereas after fast dehydration it was photoinhibition (q (I)). Although q (E) seems to play a role during desiccation recovery, no significant variations were detected in the xanthophyll cycle components. Desiccation did not affect PSI functionality. Classical antioxidant activities like superoxide dismutase or peroxidase decreased during desiccation and early recovery. Dehydrins were detected in the lichen-forming algae T. erici and were constitutively expressed. There is probably a minimal period required to develop strategies which will facilitate transition to the desiccated state in this algae. In this process, the xanthophyll cycle and classical antioxidant mechanisms play a very limited role, if any. However, our results indicate that there is an alternative mechanism of light energy dissipation during desiccation, where activation is dependent on a sufficiently slow dehydration rate.

Metabolic processes sustaining the reviviscence of lichen Xanthoria elegans (Link) in high mountain environments

To survive in high mountain environments lichens must adapt themselves to alternating periods of desiccation and hydration. Respiration and photosynthesis of the foliaceous lichen, Xanthoria elegans, in the dehydrated state were below the threshold of CO2-detection by infrared gas analysis. Following hydration, respiration totally recovered within seconds and photosynthesis within minutes. In order to identify metabolic processes that may contribute to the quick and efficient reactivation of lichen physiological processes, we analysed the metabolite profile of lichen thalli step by step during hydration/dehydration cycles, using 31P- and 13C-NMR. It appeared that the recovery of respiration was prepared during dehydration by the accumulation of a reserve of gluconate 6-P (glcn-6-P) and by the preservation of nucleotide pools, whereas glycolytic and photosynthetic intermediates like glucose 6-P and ribulose 1,5-diphosphate were absent. The large pools of polyols present in both X. elegans photo- and mycobiont are likely to contribute to the protection of cell constituents like nucleotides, proteins, and membrane lipids, and to preserve the integrity of intracellular structures during desiccation. Our data indicate that glcn-6-P accumulated due to activation of the oxidative pentose phosphate pathway, in response to a need for reducing power (NADPH) during the dehydration-triggered down-regulation of cell metabolism. On the contrary, glcn-6-P was metabolised immediately after hydration, supplying respiration with substrates during the replenishment of pools of glycolytic and photosynthetic intermediates. Finally, the high net photosynthetic activity of wet X. elegans thalli at low temperature may help this alpine lichen to take advantage of brief hydration opportunities such as ice melting, thus favouring its growth in harsh high mountain climates.

Photosynthetic recovery following desiccation of desert green algae (Chlorophyta) and their aquatic relatives

Recent molecular data suggest that desert green algae have evolved from freshwater ancestors at least 14 times in three major classes (Chlorophyceae, Trebouxiophyceae and Charophyceae), offering a unique opportunity to study the adaptation of photosynthetic organisms to life on land in a comparative phylogenetic framework. We examined the photorecovery of phylogenetically matched desert and aquatic algae after desiccation in darkness and under illumination. Desert algae survived desiccation for at least 4 weeks when dried in darkness, and recovered high levels of photosynthetic quantum yield within 1 h of rehydration in darkness. However, when 4 weeks of desiccation was accompanied by illumination, three of six desert taxa lost their ability to recover quantum yield during rehydration in the dark. Aquatic algae, in contrast, recovered very little during dark rehydration following even just 24 h of desiccation. Re-illuminating rehydrated algae produced a nearly complete recovery of quantum yield in all desert and two of five aquatic taxa. These contrasts provide physiological evidence that desert green algae possess mechanisms for photosynthetic recovery after desiccation distinct from those in aquatic relatives, corroborating molecular evidence that they are not happenstance, short-term visitors from aquatic environments. Photosensitivity during desiccation among desert algae further suggests that they may reside in protected microsites within crusts, and species specificity of photosensitivity suggests that disturbances physically disrupting crusts could lead to shifts or losses of taxonomic diversity within these habitats.

Responses of antioxidants in the lichen Ramalina lacera may serve as an early-warning bioindicator system for the detection of air pollution stress

The aim of this study was to identify, in the lichen Ramalina lacera, antioxidants that could provide indications of air pollution stress, and respond earlier than traditionally used structural/physiological parameters. The pollution-sensitive lichen R. lacera was transplanted from its relatively unpolluted natural habitat to two air-polluted sites for a period of up to 6 months. The superoxide dismutase and catalase activities, total water- and lipid-soluble low-molecular-weight antioxidant capacities and chlorophyll b/chlorophyll a ratios were assessed every 6 weeks. The earliest signs of oxidative stress were detected in the activities of fungal copper/zinc-superoxide dismutase, algal iron-superoxide dismutase and water-soluble low-molecular-weight antioxidants, which increased significantly as early as 42 days after exposure to pollution. Catalase activity increased in lichens transplanted to the polluted sites after 90 days. All activities decreased towards the end of the experiment. The impact of air pollution on R. lacera, using the traditionally employed parameter of chlorophyll b/chlorophyll a ratio, was only detected after 6 months of exposure to air pollution. Our results indicate that antioxidant parameters may serve as improved early-warning indicators of air pollution stress in lichens.