Willem H. van de Poll

NUTRIENT LIMITATION AND HIGH IRRADIANCE ACCLIMATION REDUCE PAR AND UV-INDUCED VIABILITY LOSS IN THE ANTARCTIC DIATOM CHAETOCEROS BREVIS (BACILLARIOPHYCEAE)1

The effects of high PAR (400–700 nm), UVA (315–400 nm), and UVB (280–315 nm) radiation on viability and photosynthesis were investigated for Chaetoceros brevis Schütt. This Antarctic marine diatom was cultivated under low, medium, and high irradiance and nitrate, phosphate, silicate, and iron limitation before exposure to a simulated surface irradiance (SSI) treatment, with and without UVB radiation. Light‐harvesting and protective pigment composition and PSII parameters were determined before SSI exposure, whereas viability was measured by flow cytometry in combination with a viability stain after the treatment. Recovery of PSII efficiency was measured after 20 h in dim light in a separate experiment. In addition, low and high irradiance acclimated cells were exposed outdoors for 4 h to assess the effects of natural PAR, UVA, and UVB on viability. Low irradiance acclimated cells were particularly sensitive to photo induced viability loss, whereas no viability loss was found after acclimation to high irradiance. Furthermore, nutrient limitation reduced sensitivity to photo induced viability loss, relative to nutrient replete conditions. No additional viability loss was found after UVB exposure. Sunlight exposed cells showed no additional UVB effect on viability, whereas UVA and PAR significantly reduced the viability of low irradiance acclimated cells. Recovery of PSII function was nearly complete in cultures that survived the light treatments. Increased resistance to high irradiance coincided with an increased ratio between protective‐ and light‐harvesting pigments before the SSI treatment, demonstrating the importance of nonphotochemical quenching by diatoxanthin for survival of near‐surface irradiance. We conclude that a sudden transfer to high irradiance can be fatal for low irradiance acclimated C. brevis.

The sensitivity of Emiliania huxleyi (Prymnesiophyceae) to ultraviolet‐b radiation

Emiliania huxleyi (Lohm.) Hay et Miller is an important component of the phytoplankton in open ocean waters. The sensitivity of this cosmopolitan alga to natural levels of UVB radiation has never been tested. Since DNA is believed to be a major target of natural UVB radiation (UVBR: 280–315 nm) in living cells, experiments with E. huxleyi were performed using growth rate reduction and DNA damage as indicators of UVBR stress. Specific growth rate, cell volume, pigment content, and CPD (cyclobutane pyrimidine dimer) formation (a measure for DNA damage) were followed during and after prolonged exposure of a series of cultures to a range of UVBR levels. E. huxleyi was found to be very sensitive to UVBR: at a daily weighted UVBR dose of only 400 J·m−2 ·d−1 (BEDDNA300nm), growth was halted. At this UVBR level, both cell volume and contents of the major photosynthetic and photoprotective pigments had increased. The UVBR vulnerability of E. huxleyi cannot be explained by a high potential for cyclobutane thymine dimer formation (the most abundant CPD type) due to a high T content of nuclear DNA: the CG content of this E. huxleyi strain is high (68%) compared with other species. The high UVBR sensitivity may be related to the stage of the cell cycle during UVBR exposure, in combination with low repair capacity. It is concluded that E. huxleyi may experience UVBR stress through the formation of cyclobutane pyrimidine dimers, with subsequent low repair capacity and thereby arrest of the cell cycle.

Ultraviolet-B-induced cyclobutane-pyrimidine dimer formation and repair in arctic marine macrophytes

The significance of ultraviolet‐B radiation (UVBR: 280–315 nm)–induced DNA damage as a stress factor for Arctic marine macrophytes was examined in the Kongsfjord (Spitsbergen, 78°55.5′N, 11°56.0′E) in summer. UVBR penetration in the water column was monitored as accumulation of cyclobutane‐pyrimidine dimers (CPD) in bare DNA. This showed that UVBR transparency of the fjord was variable, with 1% depths ranging between 4 and 8 m. In addition, induction and repair kinetics of CPD were studied in several subtidal macrophytes obtained from the Kongsfjord (5–15 m). Surface exposure experiments demonstrated CPD accumulation in Palmaria palmata, Devaleraea ramentacea, Phycodrys rubens, Coccotylus truncatus and Odonthalia dentata. In artificial light, field collected material of P. palmata, D. ramentacea, P. rubens and Laminaria saccharina showed efficient CPD repair, with only 10% of the artificially induced CPD remaining after 5 h. No significant differences in repair rate were observed among these species. CPD repair was slower or absent in O. dentata, C. truncatus and Monostroma arcticum, indicating that fast repair mechanisms such as photolyase were not continuously expressed in these species. CPD repair rates were not directly related to the vertical distribution of algae in the water column and to the reported UV sensitivity of the examined species. Dosimeter incubations showed that maximal exposure to DNA damaging wavelengths was low for all examined species. Furthermore, most species collected below the 1% depth for DNA damage displayed efficient CPD repair, suggesting that UVBR‐induced CPD currently impose a minor threat for mature stages of these species growing in the Kongsfjord, Spitsbergen.

Temperature dependence of UV radiation effects in Arctic and temperate isolates of three red macrophytes

The temperature dependence of UV effects was studied for Arctic and temperate isolates of the red macrophytes Palmaria palmata, Coccotylus truncatus and Phycodrys rubens. The effects of daily repeated artificial ultraviolet B and A radiation (UVBR: 280–320 nm, UVAR: 320–400 nm) treatments were examined for all isolates at 6, 12 and 18 °C by measuring growth, optimal quantum yield of PSII (Fv/Fm) and cyclobutane-pyrimidine dimer (CPD) accumulation. Furthermore, possible ecotypic differences in UV sensitivity between Arctic and temperate isolates were evaluated. Large species-specific differences in UV sensitivity were observed for all parameters: the lower subtidal species C. truncatus and P. rubens were highly sensitive to the UV treatments, whereas P. palmata, which predominantly occurs in the upper subtidal zone, was not affected by these treatments. Only minor differences were found between Arctic and temperate isolates, suggesting that no differences in UV sensitivity have evolved in these species. Relative growth rates were temperature-dependent, whereas species-specific UV effects on growth rates were relatively independent of temperature. In contrast, the species-specific decrease in Fv/Fm and its subsequent recovery were temperature-dependent in all species. UV effects on Fv/Fm were lower at 12 and 18 °C compared with 6 °C. In addition, UV effects on Fv/Fm decreased in the course of the experiment at all temperatures, indicating acclimation to the UV treatments. CPDs accumulated during the experiment in both isolates of P. rubens, whereas CPD concentrations remained low for the other two species. CPD accumulation appeared to be independent of temperature. The results suggest that summer temperatures occurring in temperate regions facilitate repair of UV-induced damage and acclimation to UV radiation in these algae compared with Arctic temperatures. Because the differences in UV effects on Fv/Fm, growth and CPD accumulation were relatively small over a broad range of temperatures, it was concluded that the influence of temperature on UV effects is small in these species.

Does ultraviolet radiation affect the xanthophyll cycle in marine phytoplankton

This Perspective summarizes the state of knowledge of the impact of ultraviolet radiation on the photoprotective xanthophyll cycle in marine phytoplankton. Excess photosynthetically active radiation (PAR; 400-700 nm) and ultraviolet radiation (UVR; 280-400 nm) affect various cellular processes and can potentially lead to reduced growth or viability loss in situ. Algae deploy photoprotective mechanisms that limit the hazardous effects of excess light exposure. Xanthophyll cycle pigments play a crucial role in photoprotection via the development of non-photochemical fluorescence quenching (NPQ) during excess radiation exposure. Research on the interacting effects of excess PAR and UVR exposure on xanthophyll cycle pigment synthesis and xanthophyll cycle activity has produced contrasting views. The current contribution summarizes research on photoprotection via photoregulation (xanthophyll cycle activity) and photoacclimation (adjustment of the xanthophyll cycle pigment pool) for marine phytoplankton. Subsequently, UVR effects on the xanthophyll cycle and on xanthophyll cycle pigment pools are discussed and results of supporting experiments with the common diatom Thalassiosira weissflogii are presented. We show that UVR exposure may enhance xanthophyll cycle pigment synthesis. This suggests that UVR-induced reduction in de-epoxidation state does not necessarily imply reduced energy dissipating potential.

Stratospheric ozone depletion: High arctic tundra plant growth on Svalbard is not affected by enhanced UV-B after 7 years of UV-B supplementation in the field

The response of tundra plants to enhanced UV-B radiation simulating 15 and 30% ozone depletion was studied at two high arctic sites (Isdammen and Adventdalen, 78° N, Svalbard).The set-up of the UV-B supplementation systems is described, consisting of large and small UV lamp arrays, installed in 1996 and 2002. After 7 years of exposure to enhanced UV-B radiation, plant cover, density, morphological (leaf fresh and dry weight, leaf thickness, leaf area, reproductive and ecophysiological parameters leaf UV-B absorbance, leaf phenolic content, leaf water content) were not affected by enhanced UV-B radiation. DNA damage in the leaves was not increased with enhanced UV-B in Salix polaris and Cassiope tetragona. DNA damage in Salix polaris leaves was higher than in leaves of C. tetragona. The length of male gametophyte moss plants of Polytrichum hyperboreum was reduced with elevated UV-B as well as the number of Pedicularis hirsuta plants per plot, but the inflorescence length of Bistorta vivipara was not significantly affected. We discuss the possible causes of tolerance of tundra plants to UV-B (absence of response to enhanced UV-B) in terms of methodology (supplementation versus exclusion), ecophysiological adaptations to UV-B and the biogeographical history of polar plants

OXIDATIVE STRESS RESPONSES IN THE MARINE ANTARCTIC DIATOM CHAETOCEROS BREVIS (BACILLARIOPHYCEAE) DURING PHOTOACCLIMATION(1).

The enzyme superoxide dismutase (SOD) holds a key position in the microalgal antioxidant network. The present research focused on oxidative stress responses in the Antarctic diatom Chaetoceros brevis F. Schütt during transition to excess (including ultraviolet radiation [UVR]) and limiting irradiance conditions. Over a 4 d period, cellular responses of thiobarbituric acid reactive substances (TBARS, a general oxidative stress indicator), SOD activity, photosynthetic and xanthophyll cycle pigments, PSII efficiency, and growth were determined. In addition, oxidative responses were measured during a daily cycle. Changing irradiance conditions significantly affected growth rates of C. brevis. PSII efficiency decreased significantly during periodic excess irradiance and increased under low irradiance conditions. Transition to excess irradiance increased the ratio of xanthophyll to light‐harvesting pigments, whereas the opposite was observed for cultures transferred to low irradiance. This acclimation process was completed after 2 d in the new irradiance environment. SOD activity increased significantly after the first day regardless of the new irradiance environment but returned to preexposure values on the fourth day. We hypothesize that SOD activity may be temporarily elevated in C. brevis after irradiance shifts, thereby reducing oxidative stress when photoacclimation is in progress.

Excessive irradiance and antioxidant responses of an Antarctic marine diatom exposed to iron limitation and to dynamic irradiance

The synergistic effects of iron limitation and irradiance dynamics on growth, photosynthesis, antioxidant activity and excessive PAR (400-700 nm) and UV (280-400 nm) sensitivity were investigated for the Antarctic marine diatom Chaetoceros brevis. Iron-limited and iron-replete cultures were exposed to identical daily irradiance levels, supplied as dynamic (20-1350 micromol m(-2) s(-1)) and constant (260 micromol m(-2) s(-1)) irradiance. After acclimation, growth, maximal quantum yield of PSII (F(v)/F(m)), pigment composition, and the activities of the antioxidant enzymes superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR) were determined. Then, excessive irradiance sensitivity was assessed by monitoring pigment composition, F(v)/F(m) and viability loss during a single excessive PAR and UV treatment. Iron limitation reduced growth rates, F(v)/F(m) dynamics, and cellular pigments pools. Cellular pigment concentrations were higher under dynamic irradiance than under constant irradiance but this difference was less pronounced under iron limitation compared to iron-replete conditions. SOD and APX activities increased during dynamic irradiance under iron limitation, suggesting increased radical formation around PSII. Despite these physiological differences, no effects on growth were observed between constant and dynamic irradiance cultivation in iron-limited and iron-replete cells. The applied culturing conditions did not affect glutathione reductase activity in C. brevis. F(v)/F(m) and xanthophyll de-epoxidation dynamics during excessive irradiance were not different for iron-limited and replete cells and viability loss was not found during excessive irradiance. This study revealed photoacclimation differences between iron-limited and iron-replete C. brevis cultures that did not affect growth rates and excessive irradiance sensitivity after acclimation to constant and dynamic irradiance.

Short-term antioxidative responses of 15 microalgae exposed to excessive irradiance including ultraviolet radiation

Short-term photosensitivity and oxidative stress responses were compared for three groups of marine microalgae: Antarctic microalgae, temperate diatoms and temperate flagellates. In total, 15 low-light-acclimated species were exposed to simulated surface irradiance including ultraviolet radiation (SSI). Photosensitivity was assessed as the rate of recovery of Fv/Fm in the hours following SSI treatment. Before, during and after the SSI treatment, oxidative stress responses were assessed by following xanthophyll content and cycling, and activities of superoxide dismutase, ascorbate peroxidase and glutathione reductase, and glutathione redox status. When acclimated to low irradiance, antioxidant levels were not group specific. Superoxide dismutase activity was positively correlated with cell size, whereas in general, ascorbate peroxidase activity appeared to be lower and glutathione redox status appeared to be higher in the Antarctic than in the temperate species. After SSI exposure, the strong inhibition of PSII was followed by variable rates of recovery, although four species remained photosynthetically inactive. SSI tolerance appeared unrelated to geographic or taxonomic background, or to cell size. PSII recovery was enhanced in species with decreasing superoxide dismutase activity, glutathione redox status and increased xanthophyll cycle activity. We conclude that antioxidant responses are highly species specific and not related to the geographic or taxonomic background. Furthermore, xanthophyll cycling seems more important than antioxidants. Finally, it can be hypothesized that glutathione could function as a stress sensor and response regulator.

Xanthophyll cycle activity and photosynthesis of Dunaliella tertiolecta (Chlorophyceae) and Thalassiosira weissflogii (Bacillariophyceae) during fluctuating solar radiation

van de Poll W.H., Buma A.G.J., Visser R.J.W., Janknegt P.J., Villafañ V.E. and Helbling E.W. 2010. Xanthophyll cycle activity and photosynthesis of Dunaliella tertiolecta (Chlorophyceae) and Thalassiosira weissflogii (Bacillariophyceae) during fluctuating solar radiation. Phycologia 49: 249–259. DOI: 10.2216/08-83.1 Short-term ultraviolet (UV) radiation (280–400 nm) effects on xanthophyll cycle activity and photosynthesis were assessed during fluctuating irradiance (60- and 10-min cycles – saturating irradiance to near-zero irradiance) for the marine algae Thalassiosira weissflogii (Bacillariophyceae) and Dunaliella tertiolecta (Chlorophyceae). Laboratory cultures were cycled, as above, up and down the water column for 8 h under solar radiation, during which photosystem II (PSII) quantum yield in the light [(Fm′ − Ft)/Fm′] was monitored over 1-min intervals. In addition, pigment composition, xanthophyll de-epoxidation state and carbon assimilation were assessed during the fluctuating irradiance cycles. Although PSII quantum yield in the light of both species mirrored irradiance, the PSII response to irradiance fluctuations changed over time as PSII quantum yield was downregulated at midday. This coincided with maximal xanthophyll de-epoxidation that developed during the course of the day for both species. In contrast to the de-epoxidation levels, nonphotochemical quenching (NPQ) and PSII quantum yield in the light fluctuated with the irradiance dynamics at noon in both species. Maximal xanthophyll de-epoxidation and NPQ at noon was lower under photosynthetically active radiation (PAR) + UV than under PAR exposure for T. weissflogii during the 10-min cycle, whereas this was not found for the 60-min cycle and in D. tertiolecta. Synthesis of xanthophyll cycle pigments occurred in both species, and was faster for D. tertiolecta during PAR + UV than during PAR exposure. Carbon incorporation and on most occasions PSII quantum yield in the light were lower during UV exposure for both species, regardless of xanthophyll de-epoxidation state. UV effects on carbon assimilation were higher during 10-min than during 60-min irradiance fluctuation cycles. However, the 10-min irradiance fluctuation cycle appeared to enhance overall carbon assimilation in D. tertiolecta but depressed productivity of T. weissflogii, as compared with the 60-min cycles.