UV RADIATION,PHOSPHORUS, AND THEIR COMBINED EFFECTS ON THE TAXONOMIC COMPOSITION OF PHYTOPLANKTON IN A BOREAL LAKE1

We examined how UV radiation and phosphorus (P) affect the taxonomic composition, abundance, and biomass of phytoplankton in an oligotrophic boreal lake. We exposed phytoplankton to three different solar radiation regimes (PAR + UV‐A radiation [UVAR]+ UV‐B radiation [UVBR], PAR + UVAR, and PAR only) and to five levels of P. The biomass of small chrysophytes was reduced by 350% after exposure to PAR + UVAR + UVBR compared with PAR only. No other taxa were found to be negatively affected by exposure to UVBR. Several taxa (e.g. Chry‐ sochromulina laurentiana Kling) were sensitive to UVAR, whereas others (e.g. Tabellaria flocculosa (Roth) Kutzing) were not affected by UV radiation exposure. Principal components analysis ordination separated phytoplankton that were negatively affected by UV radiation and/or positively affected by P treatments (e.g. small chrysophytes, Cryptomonas rostratiformis, T. flocculosa) from those that generally were unaffected by either treatment (e.g. desmids, some Cyanobacteria). Richness, Shannon‐Weaver diversity, and evenness were significantly higher in phytoplankton communities shielded from UVAR and UVBR. The relationship between diversity and richness was positive in all phytoplankton samples except in those exposed to UVBR. Thus, UVBR‐exposed phytoplankton communities were dominated by a few species even though the number of taxa remained relatively unchanged. Consequently, alterations in the UV environments of lakes resulting from climate warming (e.g. drought) and land‐use change (e.g. increased P export) will likely promote shifts in the community composition of lake phytoplankton.     

CHRONIC EFFECTS OF ULTRAVIOLET-B RADIATION ON GROWTH AND CELL VOLUME OF PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

Cultures of the temperate estuarine diatom, Phaeodactylum tricornutum Bohlin (NEPCC Clone 31), were grown under ambient intensities of ultraviolet-A radiation (UVAR), photosynthetically active radiation (PAR), and various intensities of ultraviolet-B radiation (UVBR; 290–320 nm). Growth rates and cell volumes were monitored for 36 d. UVBR decreased growth rates and increased cell volumes. Sensitivity of growth to UVBR increased with time. Growth rates of P. tricornutum decreased with increasing ratios of UVBR:UVAR + PAR.     

Habitat related variation in UV tolerance of tropical marine red macrophytes is not temperature dependent

Because tropical marine macrophytes experience high ultraviolet-B radiation (UVBR: 280–320 nm) it is assumed that they have high UV tolerance. This was investigated by examining the relative UV sensitivity of five Caribbean red macrophytes. Furthermore, the possibility of temperature dependence of UV effects was examined over a tropical temperature range. Algal fragments of intertidally occurring Gelidiopsis planicaulis (Taylor) Taylor, Wurdemannia miniata (Duby) Feldman and Hamel, and Hypnea spinella (Agardh) Kützing, and the subtidal species Bryothamnion triquetrum (Gmelin) Howe and Heterosiphonia gibbesii (Harvey) Falkenberg were repeatedly subjected to artificial UVBR and ultraviolet-A radiation (UVAR: 320–400 nm) at 22, 26 and 30°C, whereas exposure to photosynthetically active radiation (PAR) served as control. Growth rates, optimal quantum yield of PSII and accumulation of DNA damage were monitored for 10 days, whereas the relative abundance of the D1 reaction centre binding protein and the presence of UV absorbing compounds were investigated in some samples. UVAR and UVBR significantly depressed growth rates of all species. UVBR exposure caused accumulation of DNA damage and resulted in stronger growth reduction than UVAR. UVBR and UVAR caused a depression of optimal quantum yield and a lower D1 abundance. However, the former recovered fast and acclimated to the UV treatments. Some species produced UV absorbing compounds in response to UVAR. UV exposure caused less pronounced effects in intertidally occurring species than in subtidal species. UV effects on growth, the accumulation of DNA damage and UV induced depression of optimal quantum yield were independent of temperature in most species. We conclude that high UVBR in tropical regions may depress in situ growth rates of these intertidal and subtidal red macrophytes.     

EFFECTS OF UV-B-INDUCED DNA DAMAGE AND PHOTOINHIBITION ON GROWTH OF TEMPERATE MARINE RED MACROPHYTES: HABITAT-RELATED DIFFERENCES IN UV-B TOLERANCE

The sensitivity to UV-B radiation (UVBR: 280–315 nm) was tested for littoral (Palmaria palmata[L.] O. Kuntze, Chondrus crispus Stackhouse) and sublittoral (Phyllophora pseudoceranoides S. G. Gmelin, Rhodymenia pseudopalmata[Lamouroux] Silva, Phycodrys rubens[L.] Batt, Polyneura hilliae[Greville] Kylin) red macrophytes from Brittany, France. Algal fragments were subjected to daily repeated exposures of artificial UVBR that were realistic for springtime solar UVBR at the water surface in Brittany. Growth, DNA damage, photoinhibition, and UV-absorbing compounds were monitored during 2 weeks of PAR + UV-A radiation (UVAR) + UVBR, whereas PAR + UVAR and PAR treatments were used as controls. The littoral species showed a higher UV tolerance than the sublittoral species. After 2 weeks, growth of P. palmata and C. crispus was not significantly affected by UVBR, and DNA damage, measured as the number of cyclobutane-pyrimidine dimers per 10⁶ nucleotides, was negligible. Photoinhibition, determined as the decline in optimal quantum yield, was low and decreased during the course of the experiment, coinciding with the production of UV-absorbing compounds in these species. In contrast, no UV-absorbing compounds were induced in the sublittoral species. Growth rates of P. pseudoceranoides and R. pseudopalmata were reduced by 40% compared with the PAR treatment. Additionally, constant levels of DNA damage and pronounced photoinhibition were observed after the UVBR treatments. Growth was completely halted for Phycodrys rubens and Polyneura hilliae, whereas DNA damage accumulated in the course of the experiment. Because Phycodrys rubens and Polyneura hilliae showed the same degree of photoinhibition as the other sublittoral species, it appears that the accumulation of DNA damage may have been responsible for the complete inhibition of growth. The results suggest an important role of DNA repair pathways in determining the UV sensitivity in red macrophytes.     

Sensitivity of Laminariales zoospores from Helgoland (North Sea) to ultraviolet and photosynthetically active radiation: implications for depth distribution and seasonal reproduction

Depth distribution of kelp species in Helgoland (North Sea) is characterized by occurrence of Laminaria digitata in the upper sublittoral, whereas L. saccharina and L. hyperborea dominate the mid and lower sublittoral region. Laminaria digitata is fertile in summer whereas both other species are fertile in autumn/winter. To determine the light sensitivity of the propagules, zoospores of L. digitata, L. saccharina and L. hyperborea were exposed in the laboratory to different exposure times of photosynthetically active radiation (PAR; 400–700 nm), PAR + UVA radiation (UVAR; 320–400 nm) and PAR + UVAR + UVB radiation (UVBR; 280–320 nm). Optimum quantum yield of PSII and DNA damage were measured after exposure. Subsequently, recovery of photosynthetic efficiency and DNA damage repair, as well as germination rate were measured after 2 and 3 d cultivation in dim white light. Photosynthetic efficiency of all species was photoinhibited already at 20 µmol photons m⁻² s⁻¹ PAR, whereas UV radiation (UVR) had a significant additional effect on photoinhibition. Recovery of the PSII function was observed in all species but not in spores exposed to irradiation longer than 4 h of PAR + UVA + UVB and 8 h of PAR + UVA. The amount of UVB-induced DNA damage measured as cyclobutane–pyrimidine dimers (CPDs) increased with exposure time and highest damage was detected in the spores of lower subtidal L. hyperborea relative to the other two species. Significant removal of CPDs indicating repair of DNA damage was observed in all species after 2 d in low white light especially in the spores of upper subtidal L. digitata. Therefore, efficient DNA damage repair and recovery of PSII damage contributed to the germination success but not in spores exposed to 16 h of UVBR. UV absorption of zoospore suspension in L. digitata is based both on the absorption by the zoospores itself as well as by exudates in the medium. In contrast, the absorption of the zoospore suspension in L. saccharina and L. hyperborea is based predominantly on the absorption by the exudates in the medium. This study indicates that UVR sensitivity of zoospores is related to the seasonal zoospore production as well as the vertical distribution pattern of the large sporophytes.     

PHOTOSYNTHETIC INSENSITIVITY OF THE TERRESTRIAL CYANOBACTERIUM NOSTOC FLAGELLIFORME TO SOLAR UV RADIATION WHILE REHYDRATED OR DESICCATED1

Photosynthetic performance of the terrestrial cyanobacterium Nostoc flagelliforme (M. J. Berkeley et M. A. Curtis) Bornet et Flahault during rehydration and desiccation has been previously characterized, but little is known about the effects of solar UV radiation (280–400 nm) on this species. We investigated the photochemical activity during rehydration and subsequent desiccation while exposing the filamentous colonies to different solar radiation treatments. Photochemical activity could be reactivated by rehydration under full‐spectrum solar radiation, the species being insensitive to both ultraviolet‐A radiation (UVAR; 315–400 nm) and ultraviolet‐B radiation (UVBR). When the rehydrated colonies were exposed for desiccation, the effective PSII photochemical yield was inhibited by visible radiation (PAR) at the initial stage of water loss, then increased with further decrease in water content, and reached its highest value at the water content of 10%–30%. However, no significant difference was observed among the radiation treatments except for the moment when they were desiccated to critical water content of about 2%–3%. At such a critical water content, significant reduction by UVBR of the effective quantum yield was observed in the colonies that were previously rehydrated under indoor light [without ultraviolet radiation (UVR)], but not in those reactivated under scattered or direct solar radiation (with UVR), indicating that preexposure to UVR during rehydration led to higher resistance to UVR during desiccation. The photosynthetic CO₂ uptake by the desiccated colonies was enhanced by elevation of CO₂ but was not affected by both UVAR and UVBR. It increased with enhanced desiccation to reach the maximal values at water content of 40%–50%. The UV‐absorbing compounds and the colony sheath were suggested to play an important role in screening harmful UVR.     

Interaction of Photoprotective and Acclimation Mechanisms in Ulva rigida (Chlorophyta) in Response to Diurnal Changes in Solar Radiation in Southern Chile

Species of the genus Ulva (Chlorophyta) are regarded as opportunistic organisms, which efficiently adjust their metabolism to the prevailing environmental conditions. In this study, changes in chlorophyll‐a fluorescence‐based photoinhibition of photosynthesis, electron transport rates, photosynthetic pigments, lipid peroxidation, total phenolic compounds, and antioxidant metabolism were investigated during a diurnal cycle of natural solar radiation in summer (for 12 h) under two treatments: photosynthetically active radiation (PAR: 400–700 nm) and PAR+ ultraviolet (UV) radiation (280–700 nm). In the presence of PAR alone, Ulva rigida showed dynamic photoinhibition, and photosynthetic parameters and pigment concentrations decreased with the intensification of the radiation. On the other hand, under PAR+UV conditions a substantial decline up to 43% was detected and an incomplete fluorescence recovery, also, P‐I curve values remained low in relation to the initial condition. The phenolic compounds increased their concentration only in UV radiation treatments without showing a correlation with the antioxidant activity. The enzimatic activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX) increased over 2‐fold respect at initial values during the onset of light intensity. In contrast, catalase (CAT) increased its activity rapidly in response to the radiation stress to reach maxima at 10 a.m. and decreasing during solar. The present study suggests that U. rigida is capable of acclimating to natural radiation stress relies on a concerted action of various physiological mechanisms that act at different times of the day and under different levels of environmental stress.     

Effect of ultraviolet-B radiation in laboratory on morphological and ultrastructural characteristics and physiological parameters of selected cultivar of Oryza sativa L.

Ultraviolet-B radiation (UVBR) affects plants in many important ways, including reduction of growth rate and primary productivity, and changes in ultrastructures. Rice (Oryza sativa) is one of the most cultivated cereals in the world, along with corn and wheat, representing over 50 % of agricultural production. In this study, we examined O. sativa plants exposed to ambient outdoor radiation and laboratory-controlled photosynthetically active radiation (PAR) and PAR + UVBR conditions for 2 h/day during 30 days of cultivation. The samples were studied for morphological and ultrastructural characteristics, and physiological parameters. PAR + UVBR caused changes in the ultrastructure of leaf of O. sativa and leaf morphology (leaf index, leaf area and specific leaf area, trichomes, and papillae), plant biomass (dry and fresh weight), photosynthetic pigments, phenolic compounds, and protein content. As a photoprotective acclimation strategy against PAR + UVBR damage, an increase of 66.24 % in phenolic compounds was observed. Furthermore, PAR + UVBR treatment altering the levels of chlorophylls a and b, and total chlorophyll. In addition, total carotenoid contents decreased after PAR + UVBR treatment. The results strongly suggested that PAR + UVBR negatively affects the ultrastructure, morphology, photosynthetic pigments, and growth rates of leaf of O. sativa and, in the long term, it could affect the viability of this economically important plant.     

Effects of UV radiation on five marine microphytobenthic Wadden sea diatoms isolated from the Solthörn tidal flat (Lower Saxony,southern North Sea) – Part I: growth and antioxidative defence strategies

The unconsolidated sediment of intertidal mudflats constitutes a highly unstable environment, due to continuously changing water levels and currents as well as temporary exposure to the air. Therefore, diatoms inhabiting marine intertidal areas are subjected to strongly changing surface light and UV intensities due to exposure at low tide. Five marine intertidal diatoms (Achnanthes exigua, Cocconeis peltoides, Diploneis littoralis, Navicula digitoradiata and Amphora exigua) were isolated from the Solthörn tidal flat (Lower Saxony, southern North Sea). Semi-continuous cultures were used to determine the effect of UV radiation (photosynthetically active radiation only [PAR], PAR+UV-B, PAR+UV-A, PAR+UV-B+UV-A) during short- and long-term exposure (6 h or 30 days). Growth rates, chlorophyll a (chl a), antioxidant capacities, accumulation of phenolic compounds (e.g. flavonoids) and DMSP, and activities of antioxidant enzymes (superoxide dismutase, ascorbate peroxidase, monodehydroascorbate reductase and glutathione reductase) were assessed. UV-A had only minor effects on cells, while growth rate, chl a content and protein content were significantly reduced after long-term UV-B exposure. Achnanthes exigua extracts showed the highest antioxidant capacity. The highest activity of SOD, APX and MDHAR was found under long-term combined UV exposure (PAR+UV-B+UV-A). Overall, the antioxidative defence of the five isolates was stimulated during exposure to UV radiation, as may be found during emersion. Emersion induces oxidative stress and, as a result, growth of the five diatom taxa was inhibited to suit changing environmental conditions. All five taxa tested in the present study showed species-specific acclimatization potentials, providing possible explanations for variability in population, species composition and ecosystem structures in the face of climatic variations.     

Differential responses of tetrasporophytes and gametophytes of Mazzaella laminarioides (Gigartinales,Rhodophyta) under solar UV radiation

The effects of solar UV radiation on mycosporine‐like amino acids (MAAs), growth, photosynthetic pigments (Chl a, phycobiliproteins), soluble proteins (SP), and C and N content of Mazzaella laminarioides tetrasporophytes and gametophytes were investigated. Apical segments of tetrasporophytes and gametophytes were exposed to solar radiation under three treatments (PAR [P], PAR+UVA [PA], and PAR+UVA+UVB [PAB]) during 18 d in spring 2009, Punta Arenas, Chile. Samples were taken after 2, 6, 12, and 18 d of solar radiation exposure. Most of the parameters assessed on M. laminarioides were significantly influenced by the radiation treatment, and both gametophytes and tetrasporophytes seemed to respond differently when exposed to high UV radiation. The two main effects promoted by UV radiation were: (i) higher synthesis of MAAs in gametophytes than tetrasporophytes at 2 d, and (ii) a decrease in phycoerythrin, phycocyanin, and SPs, but an increase in MAA content in tetrasporophytes at 6 and 12 d of culture. Despite some changes that were observed in biochemical parameters in both tetrasporophytes and gametophytes of M. laminarioides when exposed to UVB radiation, these changes did not promote deleterious effects that might interfere with the growth in the long term (18 d). The tolerance and resistance of M. laminarioides to higher UV irradiance were expected, as this intertidal species is exposed to variation in solar radiation, especially during low tide.     

DNA DAMAGE AND PHOTOSYNTHETIC PERFORMANCE IN THE ANTARCTIC TERRESTRIAL ALGA PRASIOLA CRISPA SSP. ANTARCTICA (CHLOROPHYTA) UNDER MANIPULATED UV-B RADIATION

The effect of reduced, natural ambient, and enhanced UV-B radiation (UVBR) on photosynthesis and DNA damage in the Antarctic terrestrial alga Prasiola crispa ssp. antarctica (Kützing) Knebel was investigated in two field experiments. Samples of P. crispa were collected underneath snow cover and exposed outside to reduced and natural UVBR in the austral spring. In a second experiment at the end of the austral summer, samples were exposed to ambient and enhanced UVBR. PSII efficiency, net photosynthetic rate (NP), dark respiration rate (DR), UV-absorbing pigments, and cyclobutyl pyrimidine dimer (CPD) formation were measured during the experiments. In October 1998, a spring midday maximum of 2.0 W·m ^{− 2} of UVBR did not significantly affect effective quantum yield (ΔF/F_m′), and a reduction in the ratio of variable to maximal fluorescence (F_v/F_m) in the late afternoon was transient. Exposure to natural ambient UVBR in October increased CPD values significantly. Midday maxima of UVBR during the experiments in October and January were comparable, but Setlow-DNA-weighted UVBR was more than 50% lower in January than in October. In January, 0.5 W·m ^{− 2} additional UVBR during 10 h did not have a negative effect on ΔF/F_m′. The reduction in F_v/F_m was not significant. NP and DR were not affected by supplementation of UVBR. Although photosynthetic activity remained largely unaffected by UVBR treatment, DNA damage was shown to be a sensitive parameter to monitor UVBR effects. Supplementation of additional UVBR did significantly enhance the amounts of CPD in exposed samples and repair took place overnight. It is concluded that PSII and whole-chain photosynthesis of P. crispa is well adapted to ambient and enhanced levels of UVBR but that CPD formation is more sensitive to UVBR than to photosynthesis.     

Response of the agarophyte Gelidium floridanum after in vitro exposure to ultraviolet radiation B: changes in ultrastructure, pigments, and antioxidant systems

As a source of agar, the red macroalga Gelidium floridanum is a seaweed of great economic importance. However, it grows in a region exposed to high ultraviolet B radiation (UVBR). Therefore, to study the in vitro effect of UVBR on this plant, apical segments of G. floridanum were cultivated and exposed to photosynthetically active radiation (PAR) at 80?μmol photons m^?2?s^?1 and PAR + UVBR at 1.6?W?m^?2 at 3?h per day for 7?days. The samples were processed for electron microscopy, and agar yield, growth rates, mitochondrial activity, protein levels, chlorophyll a, phycobiliproteins, carotenoids and phenolic compounds, and photosynthetic performance were examined. After 7?days of exposure to PAR + UVBR, G. floridanum experienced ultrastructural damage that was primarily observed in the internal organization of chloroplasts, increased cell wall thickness, as well as increased volume of plastoglobuli and free ribosomes. Moreover, this exposure might have caused photodamage and photoinhibition of photosynthetic pigments (chlorophyll a and phycobiliproteins), leading to a decrease in photosynthetic efficiency, relative electron transport rate, and maximum photosynthetic rate. These phenomena were matched with a corresponding decrease in growth rates and depigmentation, combined with partial necrosis of the apical segments exposed to PAR + UVBR. Additionally, the UVBR-induced damage elicited a marked cellular antioxidant response, possibly as a consequence of free radical generation.     

The Effect of Elevated UV Radiation on Fucus spp. (Fucales,Phaeophyta) Zygote and Embryo Development

Abstract: This study has shown that in Fucus serratus and Fucus distichus, young zygotes and embryos are highly susceptible to elevated levels of both UVA (UVAR) and UVB radiation (UVBR). Zygotes treated with UVAR are able to polarise and germinate, but are very slow to divide; if they do, they often have skewed division planes or deformed rhizoids. Those treated with UVAR and UVBR remain spherical, they do not polarise, germinate to form rhizoids or undergo cell division. We suggest that the UVR may be affecting the cytoskeleton. Conversely, zygotes and embryos of Fucus spiralis are able to withstand these same UVR levels and, at the light microscope level, appear to develop normally. When the brown algal phenolic compound phloroglucinol was placed in a filter covering the developing embryos, normal development was seen under all treatments. Phenolic compounds protect the developing fucoids from UVR. In comparison with the other two species, Fucus spiralis grows high up on the shore and is exposed for much longer periods of time and, presumably, to higher levels of natural UVR. The failure of the juvenile stages of F. serratus and F. distichus to withstand UVR stress may have implications for the continued survival of these species in the intertidal, and may prove detrimental to the population as a whole if UVR levels increase.     

The effects of ultraviolet radiation-B response on the morphology,ultrastructure, and photosynthetic pigments of Laurencia catarinensis and Palisada flagellifera (Ceramiales,Rhodophyta): a comparative study

The present work aims to compare the effects of ultraviolet radiation on the morphology, ultrastructure, and photosynthetic pigments of two species of Ceramiales: Laurencia catarinensis and Palisada flagellifera. To accomplish this, plants were cultivated and exposed to photosynthetically active radiation (PAR) at 60 μmol photons m^?2 s^?1 and PAR + ultraviolet B radiation (UVBR) at 0.35 W m^?2 at 3 h per day for 7 days. Subsequently, the apical segments of L. catarinensis and P. flagellifera were analyzed under light and transmission microscopy, and both growth rates and photosynthetic pigments were studied. After exposure to PAR + UVBR, L. catarinensis exhibited a high reduction in growth rate and loss of biomass, in addition to a reduction in the concentration of chlorophyll a when compared with treated plants of P. flagellifera. However, after 7 days of exposure to PAR + UVBR, both plants showed a reduction in phycobiliprotein content. Toluidine blue reaction did not show cell wall changes in treated species with PAR + UVBR. Staining of L. catarinensis and P. flagellifera with periodic acid–Schiff (PAS) after UVBR exposure showed an increase in the number of starch grains in both species. When observed by transmission electron microscopy, treated samples of both species also showed disrupted thylakoids of the chloroplasts and an increased number of plastoglobuli. Based on this line of evidence, the present study demonstrates that ultraviolet radiation negatively affects the intertidal macroalgae L. catarinensis and P. flagellifera.     

Alterations in architecture and metabolism induced by ultraviolet radiation-B in the carragenophyte <Emphasis Type="Italic">Chondracanthus teedei</Emphasis> (Rhodophyta,Gigartinales)

The in vivo effect of ultraviolet radiation-B (UVBR) in apical segments of Chondracanthus teedei was examined. Over a period of 7 days, the segments were cultivated and exposed to photosynthetically active radiation (PAR) at 80 μmol photons m⁻² s⁻¹ and PAR + UVBR at 1.6 W m⁻² for 3 h per day. The samples were processed for electron microscopy and histochemistry; also was analyzed growth rates, mitochondrial activity, protein levels, content of photosynthetic pigments and photosynthetic performance. UVBR elicited increased cell wall thickness and accumulation of plastoglobuli, changes in mitochondrial organization and destruction of chloroplast internal organization. Compared to controls, algae exposed to PAR + UVBR showed a growth rate reduction of 55%. The content of photosynthetic pigments, including chlorophyll a and phycobiliproteins, decreased after exposure to PAR + UVBR. This result agrees with the decreased photosynthetic performance observed after exposing algae to PAR + UVBR. Irradiation also elicited increased activity of the antioxidant enzyme glutathione peroxidase and decreased mitochondrial NADH dehydrogenase activity, which correlated with the decreased protein content in plants exposed to PAR + UVBR. Taken together, these findings strongly indicate that UVBR negatively affects the architecture and metabolism of the carragenophyte C. teedei.     

Effects of temperature on the photoreactivation of ultraviolet-b–induced dna damage in Palmaria palmata (Rhodophyta)

The accumulation of DNA damage (thymine dimers and 6-4 photoproducts) induced by ultraviolet-B radiation was studied in Palmaria palmata (L.) O. Kuntze under different light and temperature conditions, using specific monoclonal antibodies and subsequent chemiluminescent detection. Both types of damage were repaired much faster under ultraviolet-A radiation (UVAR) plus photosynthetically active radiation (PAR) than in darkness, which indicates photoreactivating activity. At 12° C, all thymine dimers were repaired after 2 h irradiation with UVAR plus PAR, whereas 6-4 photoproducts were almost completely repaired after 4 h. After 19 h of darkness, almost complete repair of 6-4 photoproducts was found, and 67% of the thymine dimers were repaired. In a second set of experiments, repair of DNA damage under UVAR plus PAR was compared at three different temperatures (0, 12, and 25° C). Again, thymine dimers were repaired faster than 6-4 photoproducts at all three temperatures. At 0° C, significant repair of thymine dimers was found but not of 6-4 photoproducts. Significant repair of both thymine dimers and 6-4 photoproducts occurred at 12 and 25° C. Optimal repair efficiency was found at 25° C for thymine dimers but at 12° C for 6-4 photoproducts, which suggests that the two photorepair processes have different temperature characteristics.     

Responses of Antarctic Iridaea cordata (Rhodophyta) tetraspores exposed to ultraviolet radiation

In Antarctica ozone depletion is highest during spring, coinciding with the reproduction of many seaweed species. Propagules are the life-stage of an alga most susceptible to environmental perturbations. Therefore, fertile thalli of Iridaea cordata (Turner) Bory (Rhodophyta) were collected in the eulittoral of King George Island (Antarctica) to examine spore susceptibility to ultraviolet radiation (UVR). In the laboratory, freshly released tetraspores were exposed to photosynthetically active radiation (PAR) (400–700 nm), PAR+UV-A (320–700 nm) or PAR+UV-A+UV-B (280–700 nm). Photosynthetic efficiency was measured during 1–8 h of exposure and after 48 h of recovery. Additionally, mycosporine-like amino acids (MAAs) and DNA damage were determined. Saturating irradiance of photosynthesis of freshly released tetraspores was 57 µmol photons m⁻² s⁻¹. Exposure to increasing fluence of PAR reduced photosynthetic efficiency. UVR further decreased the photosynthetic efficiencies of the tetraspores but spores were able to recover completely after UVR exposure and 2 days post-cultivation under low PAR. DNA damage was minimal and lesions were effectively repaired under photoreactivating light. Concentrations of the MAAs shinorine and palythine were higher in tetraspores treated with UVR than in spores only exposed to PAR. Generally, the tetraspores show a good UV tolerance. This flexible response of the tetraspores of this species to changing radiation conditions enables the alga to grow along a considerable depth gradient from the sublittoral to the eulittoral where they can be exposed to enhanced UVBR under conditions of stratospheric ozone depletion.     

DEPTH DISTRIBUTIONS OF DNA DAMAGE IN ANTARCTIC MARINE PHYTO- AND BACTERIOPLANKTON EXPOSED TO SUMMERTIME UV RADIATION

During a survey from January to March 1998, the occurrence of UV-B radiation (UVBR)- induced DNA damage in Antarctic marine phytoplankton and bacterioplankton was investigated. Sampling was done in Ryder Bay, off the British base Rothera Station, 67°S, 68°W (British Antarctic Survey). Samples were taken regularly during the survey period at fixed depths, after which DNA damage was measured in various plankton size fractions (>10, 2–10, and 0.2–2 μm). Incident solar radiation was measured using spectroradiometry, whereas attenuation of biologically effective UVBR was studied using a DNA dosimeter. A diatom bloom was found in the bay during the research period, judging from microscopic observations and HPLC analyses of taxon-specific pigments. The high phytoplankton biomass likely caused strong attenuation of DNA effective UVBR (K_bd-eff). K_bd-eff values ranged from 0.83·m ^{− 1} at the peak of the bloom to 0.47·m ^{− 1} at the end of the season. UVBR-mediated DNA damage, as measured by cyclobutane pyrimidine dimer (CPD) abundance, was detected in all plankton size fractions. Highest levels were found in the smallest size fraction, mainly consisting of heterotrophic bacteria. Clear CPD depth profiles were found during mid-summer (January, beginning of February) with surface levels exceeding 100 CPDs per million nucleotides in the bacterioplankton fraction. At that time, melting of the continuously present shelf ice caused strong salinity gradients in the upper meters, thereby stimulating water column stabilization. At the end of February and beginning of March, this phenomenon was less pronounced or absent. At that time, DNA damage was homogeneously distributed over the first 10 m, ranging between 20 and 30 CPDs per million nucleotides for the smallest size fraction.     

Occurrence and induction of a ultraviolet‐absorbing substance in the cyanobacterium Fischerella muscicola TISTR8215

The filamentous cyanobacterium Fischerella muscicola TISTR8215 was tested for the presence of ultraviolet (UV)‐absorbing mycosporine‐like amino acids (MAAs) and their induction by UV radiation. Reverse‐phase high performance liquid chromatographic coupled with photodiode‐array detection studies revealed the presence of a MAA having an absorption maximum at 332 nm and a retention time of around 16.1 min. Based on absorption maximum, the compound was designated as M‐332. This is the first report for the occurrence of a MAA and its inducibility as influenced by UV radiation in Fischerella strains studied so far. Photosynthetically active radiation (PAR) had no significant impact on MAA induction. PAR + UV‐A radiation significantly induced the synthesis of M‐332; however, PAR + UV‐A + UV‐B radiation conferred highest impact on MAA synthesis. The cultures exposed to alternate light and dark conditions showed the induction of M‐332 synthesis mostly during the light period in contrast to the decreased levels of M‐322 during the dark period suggesting a circadian induction of its synthesis. Overall results indicate that F. muscicola may protect itself from deleterious short wavelength UV radiation by synthesizing the photoprotective compounds particularly during summer time in its natural brightly‐lit habitats.