UV-induced changes in pigment content and light penetration in the fruticose lichen Cladonia arbuscula ssp. mitis

Molecular mechanism of salinity and waterlogging tolerance in mangrove Kandelia obovata

Mangrove forests are colloquially referred to as "Earth's kidneys" and serve many important ecological and commercial functions. Salinity and waterlogging stress are the most important abiotic stressors restricting the growth and development of mangroves. Kandelia obovata (K. obovata) is the greatest latitudinally-distributed salt mangrove species in China.Here, morphology and transcriptomics were used to study the response of K. obovata to salt and waterlogging stress. In addition, weighted gene co-expression network analysis of the combined gene expression and phenotypic datasets was used to identify core salinity- and waterlogging-responsive modules. In this study, we observed that both high salinity and waterlogging significantly inhibited growth and development in K. obovata. Notably, growth was negatively correlated with salt concentration and positively correlated with waterlogging duration, and high salinity was significantly more inhibitive than waterlogging. A total of 7, 591 salt-responsive and 228 waterlogging-responsive differentially expressed genes were identified by RNA sequencing. Long-term salt stress was highly correlated with the measured physiological parameters while long-term waterlogging was poorly correlated with these traits. At the same time, 45 salinity-responsive and 16 waterlogging-responsive core genes were identified. All 61 core genes were mainly involved in metabolic and biosynthesis of secondary metabolites pathways. This study provides valuable insight into the molecular mechanisms of salinity and waterlogging tolerance in K. obovata, as well as a useful genetic resource for the improvement of mangrove stress tolerance using molecular breeding techniques.

Melanisation in Boreal Lichens Is Accompanied by Variable Changes in Non-Photochemical Quenching

Lichens often grow in microhabitats where they absorb more light than they can use for fixing carbon, and this excess energy can cause the formation of harmful reactive oxygen species (ROS). Lichen mycobionts can reduce ROS formation by synthesizing light-screening pigments such as melanins in the upper cortex, while the photobionts can dissipate excess energy radiationlessly using non-photochemical quenching (NPQ). An inherent problem with using fluorimetry techniques to compare NPQ in pale and melanised thalli is that NPQ is normally measured through a variously pigmented upper cortex. Here we used a dissection technique to remove the lower cortices and medullas of Lobaria pulmonaria and Crocodia aurata and then measure NPQ from the underside of the thallus. Results confirmed that NPQ can be satisfactorily assessed with a standard fluorimeter by taking measurement from above using intact thalli. However, photobionts from the bottom of the photobiont layer tend to have slightly lower rates of PSII activity and lower NPQ than those at the top, i.e., display mild "shade" characteristics. Analysis of pale and melanised thalli of other species indicates that NPQ in melanised thalli can be higher, similar or lower than pale thalli, probably depending on the light history of the microhabitat and presence of other tolerance mechanisms.

Metabolism of Mycosporine-Glutamicol in the Lichen Cladonia arbuscula subsp. squarrosa under Seasonal Changes and Elevated Exposure to UV-B or PAR Irradiation

Cladonia arbuscula in its environmental niches is regularly affected by daily and annual variations in solar radiation. Mycosporine-glutamicol, Myc-Glu(OH), which it synthesizes, may act as a significant cellular UV-protector. Therefore, we studied this compound concentration in lichen thalli concerning seasonal changes and increased exposure to UV-B and photosynthetically active radiation (PAR) with/without simultaneous CO₂ deprivation. Myc-Glu(OH) occurred year-round and exhibited a strong seasonality. The most crucial role in the control of its synthesis played UV-B radiation, although its high concentration was also found after PAR irradiation at 1000 µmol m^-2 s^-1. As PAR intensity increased to 2000 µmol m^-2 s^-1, the rate of Myc-Glu(OH) synthesis slowed down. In turn, under dark/PAR irradiation with simultaneous deprivation of CO₂ in the atmosphere surrounding C. arbuscula and during darkness with continuous access to atmospheric CO₂, its production was insignificant. Obtained data confirmed that Myc-Glu(OH) plays an important role in protecting C. arbuscula from UV damage and favours its adaptation to environmental stress in its natural habitat. They also suggest that its synthesis is a synergism of multiple factors. Consequently, further studies should focus on their evaluation and the identification of a lichen partner actively involved in Myc-Glu(OH) biogenesis.

Effect of UV-B radiation on the content of UV-B absorbing compounds and photosynthetic parameters in Parmotrema austrosinense from two contrasting habitats

We studied the resistance of Parmotrema austrosinense to UV-B stress. We focused on the effects of a high dose UV-B radiation on the content of chlorophylls, carotenoids and UV-B screening compounds. Photosynthetic parameters were measured by chlorophyll fluorescence (potential and effective quantum yields, photochemical and non-photochemical quenching) and evaluated in control and UV-B-treated lichens. Lichens from two different locations in Cordoba, Argentina, were selected: (i) high altitude and dry plots at (Los Gigantes) and (ii) lowland high salinity plots (Salinas Grandes). UV-B treatment led to a decrease in the content of photosynthetic pigments and UV-B screens (absorbance decrease in 220-350 nm) in the samples from Salinas Grandes, while in Los Gigantes samples, an increase in UV-B screen content was observed. Chlorophyll fluorescence parameters showed a UV-B-induced decline in F_V /F_M , Φ_PSII and qP indicating limitation of primary photosynthetic processes in photosystem II (PSII) of symbiotic alga, more pronounced in Salinas Grandes samples. Protective mechanism of PSII were activated by the UV-B treatment to a higher extent in samples from Salinas Grandes (NPQ 0.48) than in Los Gigantes samples (NPQ 0.26). We concluded that site-related characteristics, and in particular different UV-B radiation regimen, had a strong effect on resistance of the photosynthetic apparatus of P. austrosinense to UV-B radiation.

Living strategy of cold-adapted fungi with the reference to several representative species

Our planet is dominant with cold environments that harbour enormously diverse cold-adapted fungi comprising representatives of all phyla. Investigation based on culture-dependent and independent methods has demonstrated that cold-adapted fungi are cosmopolitan and occur in diverse habitants and substrates. They live as saprobes, symbionts, plant and animal parasites and pathogens to perform crucial functions in different ecosystems. Pseudogymnoascus destructans caused bat white-nose syndrome and Ophiocordyceps sinensis as Chinese medicine are the representative species that have significantly ecological and economic significance. Adaptation to cold niches has made this group of fungi a fascinating resource for the discovery of novel enzymes and secondary metabolites for biotechnological and pharmaceutical uses. This review provides the current understanding of living strategy and ecological functions of cold-adapted fungi, with particular emphasis on how those fungi overcome the extreme low temperature and perform their ecological function.

Raman spectroscopy detection of biomolecules in biocrusts from differing environmental conditions

Lichens and cyanobacteria colonize inhospitable places covering a wide climate range due to their different survival strategies, such as the synthesis of protective biomolecules. The effect of ecological factors on the synthesis of biomolecules has not been widely analysed. This study aimed to assess the effects of four factors (species, microclimate, seasonality and hydration state) and their interactions on the biomolecule frequency detected by Raman Spectroscopy. We included cyanobacterial biocrusts, and the lichens Diploschistes diacapsis, Squamarina lentigera, and Lepraria isidiata; two contrasted microclimates (typical and marginal), two contrasted seasons (hot and dry vs cool and wet) and two hydration states (dry and wet). "Species" was the most influential factor in the identity and frequency of the main biomolecules. Microclimatic differences in the range of the local specific habitats only influenced the biomolecules in cyanobacteria. There was a quadruple interaction among the factors, the effects being different mainly depending on the species. At D. diacapsis, the production of their main biomolecules depended on microclimate, although it also depended on seasonality. Nevertheless, in L. isidiata and S. lentigera microclimatic differences did not significantly affect the production of biomolecules. In the lichen species, the microhabitats exposed to relatively larger incident radiation did not show significantly larger relative frequency of photoprotective biomolecules. No clear connection between higher production of oxalates and drier microhabitats was found, suggesting that the synthesis of oxalates is not related to water reserve strategy. The pros and cons of monitor biomolecules in biocrust by Raman spectrometry were also discussed.

Lichen secondary metabolites in Umbilicaria antarctica evaluated by acetone rinsing ( Short Communication )

Study of the extracts from an Antarctic lichen Umbilicaria antarctica was done using a spectrophotometric approach. Secondary compounds were extracted by acetone rinsing from dried thalli of U. antarctica. The extracts were dried out, and diluted in ethanol. Then, spectral absorbance of the extracts were measured within the wavelength of 190-700 nm. The spectra of the secondary compounds obtained by acetone rinsing (EAR – re-diluted (ethanol) extract gained during acetone rinsing) were compared with those from untreated thalli (control) and ethanol extract from the thalli of U. antarctica that passed acetone rinsing (ART). Spectral absorbance curves of the extracts gained by acetone rinsing were attributed to different prevailing secondary metabolites: usnic acid, lecanoric acid (U. antarctica). Spectral absorption curves of control thalli exbibited similar shape as ART spectral curves, however, the absorbance in the range of 230-310 nm reached higher values in control than in ART. Spectral absorbance curves from ART showed that a part of secondary metabolites still remained in the thalli. Photosynthetic pigments (carotenoids and chlorophylls) remained uneffected by acetone rinsing.

Production of UV-B screens and changes in photosynthetic efficiency in Antarctic Nostoc commune colonies and a lichen Xanthoria elegans depend on a dose and duration of UV-B stress

The survival of non-vascular autotrophs in the extreme polar conditions and the principles of their high tolerance to extreme physical factors have been intriguing scientists in last decades. Therefore, this study focuses on the capacity of production of UV-B screening pigments in two model Antarctic species, one algal lichen, and colony of a cyanobacterium. Dose-dependent activation of protective mechanisms of Antarctic cyanobacterium (Nostoc commune) and algal lichen (Xanthoria elegans), synthesis of UV-B screening compounds in particular, were studied together with the changes in photosynthetic efficiency induced by a background photosynthetically active radiation (PAR) supplemented with UV-B radiation. The samples were exposed to different doses of UV-B (280–320 nm), low (0.7 W m-2), medium (1.5 W m-2) and high (3.0 W m-2) for 5 days. Untreated samples (control) were shielded from UV-B radiation during experiment. Chlorophyll fluorescence parameters and secondary UV-B protective metabolites were analysed in the intervals of 24 h, 48 h and 120 h. Amount of UV-B screening pigments was measured spectrophotometrically using several specific wavelengths in UV-B absorption range. Results showed that if exposed to a low dose of UV-B radiation or a short-term treatment, both species exhibited an increase in UV-B screening pigments to protect the lichen photobiont against UV-B damage. However, if exposed to a high dose of UV-B radiation or a long-term treatment, a decrease of UV-B screening compounds occured. This implies that Antarctic lichen and cyanobacterium can protect themselves against an increase of stress factors ranging within physiological limits, like e.g. increased synthesis of UV-B screening compounds thanks to a thinning of the ozone layer and consequent increase in UV radiation doses incident on Antarctic terrestrial ecosystems. Nevertheless, the likely increased UV-B radiation due to more intense depletion of stratospheric ozone layer may lead to alterations in UV-B tolerance in Antarctic lichens in future.

Longitudinal photosynthetic gradient in crust lichens' thalli

In order to evaluate the self-shading protection for inner photobionts, the photosynthetic activities of three crust lichens were detected using Microscope-Imaging-PAM. The false color images showed that longitudinal photosynthetic gradient was found in both the green algal lichen Placidium sp. and the cyanolichen Peltula sp. In longitudinal direction, all the four chlorophyll fluorescence parameters Fv/Fm, Yield, qP, and rETR gradually decreased with depth in the thalli of both of these two lichens. In Placidium sp., qN values decreased with depth, whereas an opposite trend was found in Peltula sp. However, no such photosynthetic heterogeneity was found in the thalli of Collema sp. in longitudinal direction. Microscope observation showed that photobiont cells are compactly arranged in Placidium sp. and Peltula sp. while loosely distributed in Collema sp. It was considered that the longitudinal photosynthetic heterogeneity was ascribed to the result of gradual decrease of incidence caused by the compact arrangement of photobiont cells in the thalli. The results indicate a good protection from the self-shading for the inner photobionts against high radiation in crust lichens.

Intracellular urease activity in the lichen Cladonia verticillaris, and its implication for toxicity

Urea is currently used as a nitrogen fertilizer in many plant cultures, such as sugar cane. Several lichen species grow in the edges of the fields fertilized with urea. This implies that the hydrolysis of an excess of urea by soil bacteria or by the lichens themselves would increase the concentration of ammonia in the lichen thallus to a level that may be toxic to the photobiont. However, Cladonia verticillaris produces urease through positive feedback by urea supplied from the medium. This urease is partially secreted to the media or retained on the external surface of algal cells, as demonstrated herein by an adequate cytochemical reaction. This implies that ammonia produced by urea hydrolysis will be immediately dissolved in the water filling the intercellular spaces on the thallus. A possible protection mechanism against eventual ammonia toxicity, derived from the results described here, is also discussed.

Photoinhibition of photosynthesis in Antarctic lichen Usnea antarctica. I. Light intensity- and light duration-dependent changes in functioning of photosystem II

The paper deals with the differences in sensitivity of Antarctic lichen to photoinhibition. Thalli of Usnea antarctica were collected at the James Ross Island, Antarctica (57°52´57´´W, 63°48´02´´S) and transferred in dry state to the Czech Republic. After rewetting in a laboratory, they were exposed to 2 high light treatments: short-term (30 min), and long-term (6 h). In short-term treatment, the sample were exposed to 1000 and 2000 µmol m-2 s-1 of photosynthetically active radiation (PAR). In long-term experiment, PAR of 300, 600, and 1000 µmol m-2 s-1 were used. Photosynthetic efficiency of U. antarctica thalli was monitored by chlorophyll fluorescence parameters, potential (FV/FM) and actual (FPSII) quantum yield of photochemical processes in photosystem II in particular. In short-term treatments, the F0, FV and FM signals, as well as the values of FV/FM, and FPSII showed light-induced decrease, however substantial recovery after consequent 30 min. in dark. Longer exposition (60 min) to high light led to more pronounced decrease in chlorophyll fluorescence than after 30 min treatment, however dark recovery was faster in the thalli treated before for longer time (60 min). Long-term treatment by high light caused gradual decrease in FV/FM and FPSII with the time of exposition. The extent of the decrease was found light dose-dependent. The time course was biphasic for FV/FM but not for FPSII. The study showed that wet thalli of Usnea antarctica had high capacity of photoprotective mechanisms to cope well either with short- or long-term high light stress. This might be of particular importance in the field at the James Ross Island, particularly at the begining of growing season when melting water is available and, simultaneously, high light stress may happen on fully sunny days.

Light might regulate divergently depside and depsidone accumulation in the lichen Parmotrema hypotropum by affecting thallus temperature and water potential

Depsides and depsidones are the most common secondary products uniquely produced in lichens by the fungal symbiont, and they accumulate on the outer surface of its hyphae. Their biological roles are subject to debate. Quantitatively the compounds typical of a given lichen can vary dramatically from thallus to thallus. Several studies have addressed whether this variability is correlated with the light reaching different thalli, but the conclusions are contradictory. We addressed the question with the lichen Parmotrema hypotropum growing on unshaded, vertical tree trunks, a controlled natural environment where the light absorbed by each thallus over its lifetime is the only major position-dependent variable. The exact north-east-south-west orientation of each thallus was used to calculate its yearly light exposure based on astronomical and meteorological considerations. The calculated irradiation around the trunk, distributed over a continuous 40-fold intensity range, then was compared with the amount of compound per unit thallus weight, determined by quantitative thin layer chromatography. P. hypotropum accumulates the depside atranorin in the cortex and the depsidone norstictic acid in the medulla and around the algae. A direct correlation was observed between the yearly amount of light reaching the lichen and the amount of atranorin. In contrast, the amount of norstictic acid decreased with increasing light. Although we did not measure thallus temperature and water potential, a unifying interpretation of these and other published data is that depside/depsidone accumulation in lichens is mediated by localized changes in temperature and water potential produced by light absorption within each thallus. This suggests water relations-based functions for depsides and depsidones.

Mutants and isolates of Metarhizium anisopliae are diverse in their relationships between conidial pigmentation and stress tolerance

Conidial pigmentation is involved in protection against heat and UV radiation in several fungal species. In this study, we compare the tolerance of 17 color mutants of wild-type ARSEF 23 plus 13 color mutants of wild-type ARSEF 2575 of Metarhizium anisopliae var. anisopliae to wet-heat and UV-B or simulated-solar radiation. The stress tolerance of each mutant was compared with that of its wild-type parent, and with the most thermo- and UV-tolerant wild-type Metarhizium we have tested to date, M. anisopliae var. acridum (ARSEF 324). The color of each isolate or mutant was identified with the PANTONE Color Standard book [Eiseman, L., Herbert, L., 1990. The PANTONE((R)) Book of Color: over 1000 color standards: color basics and guidelines for design, fashion, furnishing... and more. Harry N. Abrams, Inc., Publishers, New York]. In addition, the pigments of each mutant or wild-type were extracted and the UV absorbances of the extracts compared to the stress tolerance of those isolates; but no relationships were detected. Color mutants of ARSEF 23, in general, were less UV tolerant than their parent wild-type. With ARSEF 23 and its mutants, conidial pigmentation was important to conidial tolerance to UV-B and simulated-solar radiation; but color had less impact on ARSEF 2575 and its mutants. The ARSEF 2575 color mutants were less variable in UV tolerance than those of ARSEF 23, even though very similar colors occurred in the two groups of mutants. When color mutants of ARSEF 23 reverted to wild-type color they recovered wild-type levels of UV tolerance. Results of UV-B and UV-A exposures of wild-types ARSEF 23 and ARSEF 2575 conidia indicated that they are equally tolerant of UV-A, but differ in UV-B-response. For thermotolerance, several mutants were more heat tolerant than their wild-type parents. Accordingly, darker pigmentation of wild-type isolates was not important to protection against heat.

The lichens Xanthoria elegans and Cetraria islandica maintain a high protection against UV-B radiation in Arctic habitats

This study reports UV screening pigments in the upper cortices of two widespread lichens collected in three sun-exposed locations along a latitudinal gradient from the Arctic lowland to alpine sites of the Central European Alps. Populations from the Alps receive 3-5 times higher UV-B irradiance than their Arctic counterparts from Svalbard because of latitudinal and altitudinal gradients in UV-B irradiance. In Cetraria islandica, the screening capacity of melanin in the upper cortices was assessed by direct measurements of cortical transmittance (250-1,000 nm). A comparison of cortical transmittances in brown sun-exposed and pale shade-adapted forest C. islandica thalli showed that fungal melanins strongly absorb both UV-B and photosynthetically active radiation (PAR). For Xanthoria elegans cortical UV-B absorbing pigments, mainly the orange parietin, were extracted and quantified. Field experiments with extracted, parietin-deficient X. elegans thalli cultivated under various filters showed that UV-B was essential for the induction of parietin synthesis. The parietin resynthesis in these parietin-deficient samples increased with decreasing latitude of their location in which they had been sampled, which may imply that the synthesis of pigments is habitat specific. However, no latitudinal gradient in cortical screening capacity was detected for any of the two species investigated in the field. This implies that Arctic populations maintain a high level of screening pigments in spite of low ambient UV-B, and that the studied lichen species presumably may tolerate an increase in UV-B radiation due to the predicted thinning of the ozone layer over polar areas.

Long-term impacts of simulated climatic change on secondary metabolism, thallus structure and nitrogen fixation activity in two cyanolichens from the Arctic

• Although the most pronounced effects of stratospheric ozone depletion and climate warming probably will occur in polar regions, arctic lichens have not been much studied in relation to climate change. • Samples of two arctic cyanolichens of the genus Peltigera, exposed in situ to ambient and enhanced UV-B radiation and ambient and increased temperatures, were collected in 2001, 5 yr after the establishment of the experimental set-up. Thallus dimensions and size, coverage of soralia, nitrogen fixation activity and levels of UV-C-absorbing substances were measured. • Warming had pronounced positive effects on the tridepsides methyl gyrophorate and gyrophoric acid, and unidentified trace substances. However, the combination of enhanced UV-B and increased temperatures did not lead to higher than control levels. Warming reduced coverage of soralia. There were no significant treatment effects on thallus size, dimensions and nitrogen fixation activity. • UV-B radiation did not to have any adverse effects. The accumulation of tridepsides with warming may be related to increased activity of pathogenic microorganisms or insect herbivores.

Is parietin a UV-B or a blue-light screening pigment in the lichen Xanthoria parietina?

Xanthoria parietina is a widespread lichen coloured by the orange cortical pigment parietin (= physcion). We studied the pigment content in 60 thalli sampled in 4 habitats along a sun-shade gradient from evergreen boreal forests through open deciduous stands to sea cliffs. The significant positive regression between contents of parietin per unit area and site factors (reflecting the openness of the canopy relative to an open sky) across sampled habitats suggested a photoprotective role of parietin at UV-B and/or blue wavelengths, the two absorbance maxima of parietin. UV-B susceptibility of X. parietina, measured as permanent reductions in photosystem II, decreased highly significantly with increasing parietin content per thallus area. However, as much as three-fold greater UV-B irradiances than ambient daily summer maxima, maintained continuosly for 240 h were required to cause UV-B damage even in thalli of shaded habitats. Since a previous study has documented a high PAR susceptibility of parietin-deficient X. parietina in the absence of UV-B, there are reasons to believe that the blue light screening of parietin is functionally more important than the UV-B screening. A strong positive relationship between parietin content per unit area and reflectance at 500 nm allows the parietin content in X. parietina thalli to be assessed non-destructively by reflectance measurements.

Protection against UVB irradiation by natural filters extracted from lichens

Natural substances extracted from lichens and boldo tree were tested in vivo and in vitro as possible UV-light filters. The protection factors were compared with that found for the references: Nivea sun Spray LSF 5, octylmethoxycinnamate (OMC) and 4-tert.-butyl-4'-methoxy dibenzoylmethane (BM-DBM). The stability of the single compounds was studied through UV-Vis spectroscopy. Usnic acid resulted to be the best UVB filter, with an in vivo protection factor similar to Nivea sun Spray LSF 5. Most of the single compounds studied in vitro resulted to have higher or similar filtering power than octylmethoxycinnamate. The protection factors as well as the good UV-light absorption of their photo-products suggest that these natural substances may be useful as new filters in sun-screen preparations.

Effects of ultraviolet radiation and PAR on the content of usnic and divaricatic acids in two arctic-alpine lichens

The fruticose lichen Flavocetraria nivalis and the crustose lichen Ophioparma ventosa, both common in light-exposed arctic-alpine environments, were exposed to ultraviolet radiation (UVR) in growth chambers for 30 days. Treatment with visible light (PAR) served as control. Both species accumulate the UV-absorbing phenolic compound usnic acid in the upper cortex. The latter species also synthesises several UV-absorbing medullary compounds, among them divaricatic acid. The effects of treatment with UVR on the synthesis of these two compounds were investigated by analysing the compounds quantitatively by RP-HPLC. UV-exposed thallus tips of F. nivalis contained higher concentrations of usnic acid than those not grown under UVR. Both treatments had a positive effect on the synthesis of usnic acid in O. ventosa. An additional experiment with O. ventosa was performed by first storing samples in a low-light habitat for 1 year to obtain near-zero levels of phenolics, and thereby exposing the samples to UVR and PAR for 90 days. A rapid resynthesis of usnic acid was observed for both treatments. The amounts of divaricatic acid were highly variable in all groups, and were not correlated with usnic acid concentrations or treatments. A comparison of O. ventosa from three different habitat types showed that the highest usnic acid amounts were found in the habitat with the highest levels of solar radiation. Results indicate that the induction of usnic acid production by UVR depends on the species studied, and on how well acclimatised the lichen samples are to solar radiation before they are exposed to supplementary UVR. In lichens with an already well-developed internal screening capacity, like the population of F. nivalis, enhanced UVR need not induce further accumulation of usnic acid, but removal of UVR may induce a biodegradation of usnic acid. Results also indicate that PAR is just as important as UVR for triggering the resynthesis of usnic acid in shade-adapted lichens. Divaricatic acid seems to be of low importance for the UV-screening properties of O. ventosa.

The role of UV-B radiation in aquatic and terrestrial ecosystems--an experimental and functional analysis of the evolution of UV-absorbing compounds

We analysed and compared the functioning of UV-B screening pigments in plants from marine, fresh water and terrestrial ecosystems, along the evolutionary line of cyanobacteria, unicellular algae, primitive multicellular algae, charophycean algae, lichens, mosses and higher plants, including amphibious macrophytes. Lichens were also included in the study. We were interested in the following key aspects: (a) does the water column function effectively as an 'external UV-B filter'?; (b) do aquatic plants need less 'internal UV-B screening' than terrestrial plants?; (c) what role does UV screening play in protecting the various plant groups from UV-B damage, such as the formation of thymine dimers?; and (d) since early land 'plants' (such as the predecessors of present-day cyanobacteria, lichens and mosses) experienced higher UV-B fluxes than higher plants, which evolved later, are primitive aquatic and land organisms (cyanobacteria, algae, lichens, mosses) better adapted to present-day levels of UV-B than higher plants? Furthermore, polychromatic action spectra for the induction of UV screening pigments of aquatic organisms have been determined. This is relevant for translating 'physical' radiation measurements of solar UV-B into 'biological' and 'ecological' effects. From the action spectra, radiation amplification factors (RAFs) have been calculated. These action spectra allow us to determine any mitigating or antagonistic effects in the ecosystems and therefore qualify the damage prediction for the ecosystems under study. We summarize and discuss the main results based on three years of research of four European research groups. The central theme of the work was the investigation of the effectiveness of the various screening compounds from the different species studied in order to gain some perspective of the evolutionary adaptations from lower to higher plant forms. The induction of mycosporine-like amino acids (MAAs) was studied in the marine dinoflagellate Gyrodinium dorsum, the green algal species Prasiola stipitata and in the cyanobacterium Anabaena sp. While visible (400-700 nm) and long wavelength UV-A (315-400 nm) showed only a slight effect, MAAs were effectively induced by UV-B (280-315 nm). The growth of the lower land organisms studied, i.e. the lichens Cladina portentosa, Cladina foliacaea and Cladonia arbuscula, and the club moss Lycopodiumannotinum, was not significantly reduced when grown under elevated UV-B radiation (simulating 15% ozone depletion). The growth in length of the moss Tortula ruralis was reduced under elevated UV-B. Of the aquatic plants investigated the charophytes Chara aspera showed decreased longitudinal growth under elevated UV-B. In the 'aquatic higher plants' studied, Ceratophyllum demersum, Batrachium trichophyllum and Potamogeton alpinus, there was no such depressed growth with enhanced UV-B. In Chara aspera, neither MAAs nor flavonoids could be detected. Of the terrestrial higher plants studied, Fagopyrum esculentum, Deschampsia antarctica, Vicia faba, Calamagrostis epigejos and Carex arenaria, the growth of the first species was depressed with enhanced UV-B, in the second species length growth was decreased, but the shoot number was increased, and in the latter two species of a dune grassland there was no reduced growth with enhanced UV-B. In the dune grassland species studied outdoors, at least five different flavonoids appeared in shoot tissue. Some of the flavonoids in the monocot species, which were identified and quantified with HPLC, included orientin, luteolin, tricin and apigenin. A greenhouse study with Vicia faba showed that two flavonoids (aglycones) respond particularly to enhanced UV-B. Of these, quercetin is UV-B inducible and mainly located in epidermal cells, while kaempferol occurs constitutively. In addition to its UV-screening function, quercetin may also act as an antioxidant. Polychromatic action spectra were determined for induction of the UV-absorbing pigments in three photosynthetic organisms, representing very different taxonomic groups and different habitats. In ultraviolet photobiology, action spectra mainly serve two purposes: (1) identification of the molecular species involved in light absorption; and (2) calculation of radiation amplification factors for assessing the effect of ozone depletion. Radiation amplification factors (RAFs) were calculated from the action spectra. In a somewhat simplified way, RAF can be defined as the percent increase of radiation damage for a 1% depletion of the ozone layer. Central European summer conditions were used in the calculations, but it has been shown that RAF values are not critically dependent on latitude or season. If only the ultraviolet spectral region is considered, the RAF values obtained are 0.7 for the green alga Prasiola stipitata, 0.4 for the dinoflagellate Gyrodinium dorsum, and 1.0 for the cyanobacterium Anabaena sp. In the case of P. stipitata, however, the effect of visible light (PAR, photosynthetically active radiation, 400-700 nm) is sufficient to lower the RAF to about 0.4, while the PAR effect for G. dorsum is negligible. RAFs for some damage processes, such as for DNA damage (RAF=2.1 if protective effects or photorepair are not considered [1]), are higher than those above. Our interpretation of this is that if the ozone layer is depleted, increased damaging radiation could overrule increased synthesis of protective pigments. In addition to investigating the functional effectiveness of the different screening compounds, direct UV effects on a number of key processes were also studied in order to gain further insight into the ability of the organisms to withstand enhanced UV-B radiation. To this end, the temperature-dependent repair of cyclobutane dimers (CPD) and (6-4) photoproducts induced by enhanced UV-B was studied in Nicotiana tabacum, and the UV-B induction of CPD was studied in the lichen Cladonia arbuscula. Also, photosynthesis and motility were monitored and the response related to the potential function of the screening compounds of the specific organism.