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.     

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.     

Effects of ultraviolet radiation on protein content,respiratory electron transport system (ETS) activity and superoxide dismutase (SOD) activity of Antarctic plankton

Depletion of stratospheric ozone causes a significant increase in UV radiation in the Antarctic regions. Its effects include DNA damage, as well as impairment of photosynthesis, respiration, protein synthesis and other metabolic functions. Defence systems of cells are directed against free oxygen radicals liberated through UV radiation. One of their main components of defence systems are superoxide dismutases (SODs). The effects of ultraviolet radiation A and B (UVAR and UVBR) on protein synthesis, respiratory electron transfer (ETS) activity and superoxide dismutase (SOD) activity in Antarctic plankton were examined. Samples were taken in the Gerlache Strait (Antarctica). Three stations were situated in an area, which showed a Cryptomonas bloom. Two stations were located in areas having a bloom of green nanoflagellates. Samples were exposed for 3 h to photosynthetically active radiation (PAR), or to PAR + UVAR or to PAR + UVAR + UVBR, under fixed experimental irradiances. UVBR inhibited protein synthesis and ETS activity, and enhanced SOD activity. UVAR enhanced protein synthesis and ETS activity, and inhibited SOD activity. Samples, which had received more solar radiation prior to experiments showed less inhibition of protein synthesis by experimental UVBR, which suggests acclimation to ambient radiation. Cryptomonas-dominated stations showed less SOD activity than the green flagellate-dominated stations, which might be related to the protection conferred by their phycoerythrin.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Effects of UVB radiation on the carragenophyte <Emphasis Type="Italic">Kappaphycus alvarezii</Emphasis> (Rhodophyta,Gigartinales): changes in ultrastructure,growth, and photosynthetic pigments

Damage to the ozone layer has led to increased levels of ultraviolet radiation at the earth’s surface. Increased ultraviolet radiation can affect macroalgae in many important ways, including reduced growth rate, changes in cell biology and ultrastructure. Kappaphycus alvarezii is a red macroalga of economic interest due to its production of kappa carrageenan. In this study, we examined two strains of K. alvarezii (green and red) exposed to ultraviolet B radiation (UVBR) for 3 h per day during 28 days of cultivation in vitro. UVBR caused changes in the ultrastructure of cortical and subcortical cells, which included increased thickness of the cell wall and plastoglobuli, reduced intracellular spaces, changes in the cell contour, and destruction of chloroplast internal organization. While the green strain exposed to photosynthetically active radiation (PAR) showed growth rates of 6.75% day⁻¹, the red strain grew only 6.35% day⁻¹. Upon exposure to PAR + UV-B, a decreasing trend in growth rates was observed for both strains, with the green strain growing 3.0% day⁻¹ and the red strain growing 2.77% day⁻¹. Significant differences in growth rates between control and UV-B-exposed algae were also found in both strains. Furthermore, compared with control algae, phycobiliprotein contents (phycoerythrin, phycocyanin, and allophycocyanin) were observed to decrease in both strains after PAR + UV-B exposure. However, while the chlorophyll a levels increased in both strains, the green strain showed no significant differences in chlorophyll a levels. Taken together, these findings strongly suggested that UVBR negatively affects the ultrastructure, growth rates, and photosynthetic pigments of intertidal macroalgae and, in the long term, their economic viability.     

Effect of ultraviolet-B radiation on growth,photosynthetic pigments,and cell biology of <Emphasis Type="Italic">Kappaphycus alvarezii</Emphasis> (Rhodophyta,Gigartinales) macroalgae brown strain

Kappaphycus alvarezii is a seaweed of great economic importance for the extraction of kappa carrageenan from its cell walls. The most common strains are dark red, brown, yellow, and different gradations of green. It is known that ultraviolet radiation (UVR) affects macroalgae in many important ways, including reduced growth rate, reduction of primary productivity, and changes in cell biology and ultrastructure. Therefore, we examined the brown strain of K. alvarezii exposed to ultraviolet-B radiaton (UVBR) for 3 h per day during 28 days of cultivation. The control plants showed growth rates of 7.27% d⁻¹, while plants exposed to UVBR grew only 4.0% d⁻¹. Significant differences in growth rates and in phycobiliproteins between control and exposed plants were also found. Compared with control plants, phycobiliprotein contents were observed to decrease after UV-B exposure. Furthermore, the chlorophyll a (Chl a) contents decreased and showed significant differences. UVBR also caused changes in the ultrastructure of cortical and subcortical cells, which included increased thickness of the cell wall and number of plastoglobuli, reduced intracellular spaces, changes in the cell contour, and destruction of chloroplast internal organization. Reaction with Toluidine Blue showed an increase in the thickness of the cell wall, and Periodic Acid-Schiff stain showed a decrease in the number of starch grains. By the significant changes in growth rates, photosynthetic contents and ultrastructual changes observed, it is clear that UVBR negatively affects intertidal macroalgae and, by extension, their economic viability.     

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.     

EFFECTS OF LIGHT INTENSITY AND TEMPERATURE INTERACTIONS ON GROWTH CHARACTERISTICS OF PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

Cell division rate, carbon fixation per cell, cell width and chloroplast length of Phaeodactylum tricornutum Bohlin were determined at 30 different combinations of light intensity and temperature. Division rate peaked at 23° C or less depending on light intensity. For each light intensity studied, carbon fixation increased directly with growth temperature from 14 to 25° C. The slope of this relationship was modified by light intensity. Cells grown at 23–25° C tended to be larger than those grown at lower temperatures, possibly due to increased carbon fixation per cell coupled with lower division rates. Chloroplasts were largest at a combination of temperatures above 21° C and low light intensities. This effect could cause cells to sink at a higher than normal rate due to reduced vacuole size and is presented as a possible mechanism affecting the distribution of P. tricornutum.     

Climate change enhances primary production in the western Antarctic Peninsula

Intense regional warming was observed in the western Antarctic Peninsula (WAP) over the last 50 years. Here, we investigate the impact of climate change on primary production (PP) in this highly productive region. This study is based on temporal data series of ozone thickness (1972–2010), sea ice concentration (1978–2010), sea‐surface temperature (1990–2010), incident irradiance (1988–2010) and satellite‐derived chlorophyll a concentration (Chl‐a, 1997–2010) for the coastal WAP. In addition, we apply a photosynthesis/photoinhibition spectral model to satellite‐derived data (1997–2010) to compute PP and examine the separate impacts of environmental forcings. Since 1978, sea ice retreat has been occurring earlier in the season (in March in 1978 and in late October during the 2000s) while the ozone hole is present in early spring (i.e. August to November) since the early 1990s, increasing the intensity of ultraviolet‐B radiation (UVBR, 280–320 nm). The WAP waters have also warmed over 1990–2010. The modelled PP rates are in the lower range of previously reported PP rates in the WAP. The annual open water PP in the study area increased from 1997 to 2010 (from 0.73 to 1.03 Tg C yr^?1) concomitantly with the increase in the production season length. The coincidence between the earlier sea ice retreat and the presence of the ozone hole increased the exposure to incoming radiation (UVBR, UVAR and PAR) and, thus, increased photoinhibition during austral spring (September to November) in the study area (from 0.014 to 0.025 Tg C yr^?1). This increase in photoinhibition was minor compared to the overall increase in PP, however. Climate change hence had an overall positive impact on PP in the WAP waters.     

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.     

Effects of UV-B irradiated algae on zooplankton grazing

We tested the effects of UV-B stressed algae on grazing rates of zooplankton. Four algal species (Chlamydomonas reinhardtii, Cryptomonas sp., Scenedesmus obliquus and Microcystis aeruginosa) were used as food and fed to three zooplankton species (Daphnia galeata, Bosmina longirostris and Brachionus calyciflorus), representing different taxonomic groups. The phytoplankton species were cultured under PAR conditions, and under PAR supplemented with UV-B radiation at two intensities (0.3 W m^–2 and 0.7 W m^–2, 6 hours per day). Ingestion and incorporation experiments were performed at two food levels (0.1 and 1.0 mg C l^–1) using radiotracer techniques. The effect of food concentration on ingestion and incorporation rate was significant for all three zooplankton species, but the effect of UV-B radiation was more complex. The reactions of the zooplankton species to UV-B stressed algae were different. UV-B stressed algae did not affect Daphnia grazing rates. For Bosmina the rates increased when feeding on UV-B stressed Microcystis and decreased when feeding on UV-B stressed Chlamydomonas, compared with non-stressed algae. Brachionus grazing rates were increased when feeding on UV-B stressed Cryptomonas and UV-B stressed Scenedesmus, and decreased when feeding on UV-B stressed Microcystis, compared with non-stressed algae. These results suggest that on a short time scale UV-B radiation may result in increased grazing rates of zooplankton, but also in decreased grazing rates. Long term effects of UV-B radiation on phytoplankton and zooplankton communities are therefore difficult to predict.     

UV-A Mediated Modulation of Photosynthetic Efficiency,Xanthophyll Cycle and Fatty Acid Production of <Emphasis Type="Italic">Nannochloropsis</Emphasis>

Nannochloropsis, a green microalga, is a source for commercially valuable compounds as extensively described and, in particular, is recognised as a good potential source of eicosapentaenoic acid (20:5ϖ3), an important polyunsaturated fatty acid for human consumption for prevention of several diseases. Climate change might include variation in the ultraviolet (UV) levels as one of the consequences derived from the anthropogenic activity. This paper shows the response of Nannochloropsis cultures exposed for 7 days to UV-A (320–400 nm) added to photosynthetically active radiation (PAR; 400–700 nm). Growth rates and photosynthetic activity were assessed to determine the impact of UV-A increased levels on the cell growth and basic metabolism activity. Xanthophyll pigments (zeaxanthin and violaxanthin), carotenoids (canthaxanthin and β-carotene) and polyunsaturated fatty acids (myristic, palmitic, palmitoleic, arachidonic and eicosapentaenoic acids) were measured for assessing the antioxidant response of the microalgae to added UV-A radiation to PAR. The results show that the modulated use of UV-A radiations can lead to increased growth rates, which are sustained in time by an increased light transduction activity. The expected antioxidant response to the incident UV-A radiation consisted of increases in zeaxanthin and β-carotene contents—synthesis of antioxidant carotenoids—and increases in the saturated fatty acids to polyunsaturated fatty acids ratio. The results suggest that modulated UV-A radiation can be used as a tool to stimulate value molecules accumulation in microalgae through an enhanced both light transduction process and antioxidant response, while sustaining cell growth.     

ADAPTATION OF CERATIUM FURCA AND GONYAULAX POLYEDRA (DINOPHYCEACE) TO DIFFERENT TEMPERATURES AND IRRADIANCES: GROWTH RATES AND CELL VOLUMES1

Growth rates and cell volumes of Ceratium furca Ehrenberg and Gonyaulax polyedra Stein were determined during the log phase of growth in cultures which had been extensively adapted to one of three temperatures and five irradiances. At each temperature, curves for the growth rate vs. irradiance for both species had light-limited and light-saturated regions. Three properties of these curves characterized the response of each species to temperature: the light-saturated growth rate, the irradiance at which growth became light-saturated and the compensation irradiance for growth. For both species, the first two properties generally decreased with declining growth temperature, while the compensation irradiance declined for Ceratium but had a V-shaped response pattern for Gonyaulax. The light-saturated growth rates were generally higher for Ceratium than for Gonyaulax, while the irradiance at which growth became saturated and the compensation irradiance were lower for Ceratium. The changes in cell volume associated with the irradiance and temperature of growth were very different for Ceratium and Gonyaulax. The cell size of Gonyaulax increased as irradiance and temperature decreased, while cell volumes of Ceratium did not change with temperature but were smallest at the highest and lowest growth irradiances. In general, the growth rate patterns were similar for Ceratium and Gonyaulax, while those for cell size were different. The maximum growth rate, the irradiance at which growth became saturated, the compensation irradiance, and the cell volume all showed that Ceratium grew at the same rate or faster than Gonyaulax over the entire range of irradiances and temperatures examined.