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In the wake of global climate change, phytoplankton productivity and species composition is expected to change due to altered external conditions such as temperature, nutrient accessibility, pH and exposure to solar visible (PAR) and ultraviolet radiation (UVR). The previous light history is also of importance for the performance of phytoplankton cells. In order to assess the combined impacts of UVR and temperature on the dinoflagellate Gymnodinium chlorophorum we analyzed the effective photochemical quantum yield (Y), relative electron transport rate vs. irradiance curves (rETR vs. I), percentage of motile cells and swimming velocity. Cells were grown at three different temperatures (15, 20 and 25 °C) and two PAR intensities: low light (LL, 100 μmol photons m(-2) s(-1)) and high light (HL, 250 μmol photons m(-2) s(-1)). Pre-acclimated cells were then exposed to either PAR only (P), PAR+UV-A (PA) or PAR+UV-A+UV-B (PAB) radiation at two different irradiances, followed by a recovery period in darkness. The Y decreased during exposure, being least inhibited in P and most in PAB treatments. Inhibition was higher and recovery slower in LL-grown cells than in HL-grown cells at 15° and 20 °C, but the opposite occurred at 25 °C, when exposed to high irradiances. Maximal values of rETR were determined at t0 as compared to the different (before and after exposure) radiation treatments. The effects of temperature and UVR on rETR were antagonistic in LL-grown cells (i.e., less UVR inhibition at higher temperature), while it was synergistic in HL cells. Swimming velocity and percentage of motile cells were not affected at all tested temperatures and exposure regimes, independent of the light history. Our results indicate that, depending on the previous light history, increased temperature and UVR as predicted under climate change conditions, can have different interactions thus conditioning the photosynthetic response of G. chlorophorum., (Copyright © 2014 Elsevier B.V. All rights reserved.)

The spectral conversion of incident sunlight by appropriate photoluminescent materials has been a widely studied issue for improving the efficiency of photovoltaic solar energy harvesting. By using phosphors with suitable excitation/emission properties, also the light conditions for plants can be adjusted to match the absorption spectra of chlorophyll dyes, in this way increasing the photosynthetic activity of the plant. Here, we report on the application of this principle to a high plant, Spinacia oleracea. We employ a calcium strontium sulfide phosphor doped with divalent europium (Ca0.4Sr0.6S:Eu(2+), CSSE) on a backlight conversion foil in photosynthesis experiments. We show that this phosphor can be used to effectively convert green to red light, centering at a wavelength of ~650 nm which overlaps the absorption peaks of chlorophyll a/b pigments. A measurement system was developed to monitor the photosynthetic activity, expressed as the CO2 assimilation rate of spinach leaves under various controlled light conditions. Results show that under identical external light supply which is rich in green photons, the CO2 assimilation rate can be enhanced by more than 25% when the actinic light is modified by the CSSE conversion foil as compared to a purely reflecting reference foil. These results show that the phosphor could be potentially applied to modify the solar spectrum by converting the green photons into photosynthetically active red photons for improved photosynthetic activity.

This article is a highlight of the paper by Li et al. in this issue of Photochemistry and Photobiology as well as a short summary of the research on the effects of solar UV-B radiation on primary production in the oceans. Laboratory experiments under controlled conditions using artificial light sources indicate species-specific damage of many phytoplankton groups. Mesocosm studies in enclosures of limited volume allow analyzing UV effects in multigeneration monitoring of natural assemblages. Field studies to determine the effects of short-wavelength solar radiation require sensitive instrumentation and measurements over extended areas of the open ocean to yield significant results. Results from a cruise described in the paper by Li et al. indicate clear effects of UV-B and UV-A on the photosynthetic carbon fixation of phytoplankton communities with spatial differences between coastal and open-ocean waters. Increasing temperatures and acidification in the ocean due to global climate change may exacerbate the detrimental effects of solar UV-B radiation., (© 2011 The Author. Photochemistry and Photobiology © 2011 The American Society of Photobiology.)

Recently it was shown that the unicellular flagellate Euglena gracilis changes the sign of gravitaxis from negative to positive upon excessive radiation. This sign change persists in a cell culture for hours even if subsequently transferred to dim light. To test the ecological relevance of this behavior, a vertical column experiment was performed (max. depth 65 cm) to test distribution, photosynthetic efficiency and motility in different horizons of the column (surface, 20, 40 and 65 cm). One column was covered with a UV cut-off filter, which transmits photosynthetically active radiation (PAR) only, the other with a filter which transmits PAR and UV. The columns were irradiated with a solar simulator (PAR 162 W m(-2), UV-A 32.6 W m(-2), UV-B 1.9 W m(-2)). The experiment was conducted for 10 days, normally with a light/dim light cycle of 12 h:12 h, but in some cases the light regime was changed (dim light instead of full radiation). Under irradiation the largest fraction of cells was found at the bottom of the column. The cell density decreased toward the surface. Photosynthetic efficiency, determined with a pulse amplitude modulated fluorometer, was negligible at the surface and increased toward the bottom. While the cell suspension showed a positive gravitaxis at the bottom, the cells in the 40 cm horizon were bimodally oriented (about the same percentage of cells swimming upward and downward, respectively). At 20 cm and at the surface the cells showed negative gravitaxis. Positive gravitaxis was more pronounced in the UV + PAR samples. At the surface and in the 20 and 40 cm horizons photosynthetic efficiency was better in the PAR-only samples than in the PAR + UV samples. At the bottom photosynthetic efficiency was similar in both light treatments. The data suggest that high light reverses gravitaxis of the cells, so that they move downward in the water column. At the bottom the light intensity is lower (attenuation of the water column and self shading of the cells) and the cells recover. After recovery the cells swim upward again until the negative gravitaxis is reversed again.

The relative importance of photosynthetically active radiation and UV on photoinhibition has been studied in the cyanobacterium Anabaena sp. by measuring the effective quantum yield, growth, pigmentation and fluorescence emission.

The mechanism of photoinhibition was investigated in three representative macroalgal species growing on the coast of Patagonia: the chlorophyte Ulva rigida C. Agardh, the rhodophyte Porphyra columbina Montagne and the phaeophyte Dictyota dichotoma (Huds.) Lamour. Dark adapted specimens were exposed to 15 min unfiltered solar radiation to induce photoinhibition, and subsequently the recovery of the photosynthetic quantum yield was followed for up to 6 h. Photoinhibition is believed to be due to the damage and proteolysis of the D1 protein in the reaction center of Photosystem II. During recovery this protein is resynthesized. In order to prove this hypothesis, inhibitors of the chloroplast protein synthesis, streptomycin and chloramphenicol were applied. Both retarded the repair process indicating an inhibition of the D1 protein resynthesis during recovery after the damage they experienced during light exposure. Some algal groups use the xanthophyll cycle to ameliorate the inhibition by excessive light. Dithiothreitol, an inhibitor of violaxanthin de-epoxidase, was administered, to impair the xanthophyll cycle. It strongly affected both photoinhibition and recovery even in the red algal species, which do not have the xanthophyll cycle, indicating that this drug has significant side effects and should be used with caution for the study of the involvement of this protective cycle in algae. Pigmentation was followed in the three species using absorption spectroscopy of thallus extracts at 665 nm during continuous exposure to natural solar radiation or radiation deprived of the UV component during two days. The results indicated a pronounced variation in pigmentation over time due to bleaching and resynthesis. Solar radiation was monitored during the experiments in three channels (UV-B, UV-A and PAR) using an ELDONET instrument on site.

The productivity of aquatic primary producers depends on a number of biotic and abiotic factors, such as pH, CO2 concentration, temperature, nutrient availability, solar UV and PAR irradiances, mixing frequency as well as herbivore pressure and the presence of viruses, among others. The effects of these factors, within a climate change context, may be additive, synergistic or antagonistic. Since some of them, e.g. solar radiation and temperature, vary along a latitudinal gradient, this perspective about the effects of global climate change on primary producers will consider ecosystems individually, separated into polar (Arctic and Antarctic), temperate and tropical waters. As coastal waters are characterized by lower light penetration and higher DOM and nutrient concentrations, they are considered in a separate section. Freshwater systems are also governed by different conditions and therefore also treated in their own section. Overall, we show that although there are general common trends of changes in variables associated with global change (e.g. the impact of UVR on photosynthesis tends to decrease with increasing temperature and nutrient input), the responses of aquatic primary producers have great variability in the different ecosystems across latitudes. This is mainly due to direct or indirect effects associated with physico-chemical changes that occur within water bodies. Therefore we stress the need for regional predictions on the responses of primary producers to climate change as it is not warranted to extrapolate from one system to another. [ABSTRACT FROM AUTHOR]

The filamentous rhodophytes Callithamnion gaudichaudi Agardh and Ceramium sp. were utilized to study the effects of solar radiation (PAR, 400–700 nm, UV-B, 280–315 nm and UV-A, 315–400 nm) on the photosynthetic performance in situ in Patagonia waters (Argentina). A pulse amplitude modulated (PAM) fluorometer was used to determine the fluorescence parameters. The two species grew in different habitats in the eulittoral: Ceramium sp. was found only in rock pools while C. gaudichaudii grew on exposed rocks and fell dry during low tide. Both species differed in their fluorescence parameters and their sensitivity to solar radiation exposure. The photosynthetic quantum yield had its lowest values at noon, but it recovered in the afternoon/evening hours, when irradiances were lower. PAR (irradiance of about 400 W m−2 at noon) was responsible for most of the decrease in the yield on clear days, especially in Ceramium sp., but UVR (280–400 nm) also accounted for a significant decrease. Fluence rate response curves indicated that both species were adapted to low fluence rates and showed a pronounced non-photochemical quenching at intermediate and higher irradiances. Both species showed a rapid adaptation during measurement of fast induction kinetics but differed significantly in their fluorescence components. All photosynthetic pigments were bleached after 8 h exposure to solar radiation over a full day. Strong absorption in the UV-A range, most likely due to mycosporine-like amino acids, was detected in both strains. The pronounced sensitivity to solar radiation in situ and the recovery capacity of these two filamentous Rhodophyte species, as well as the presence of protective compounds, suggests that these algae have the ability to adapt to the relatively high radiation levels and changes in irradiance found in the Patagonia waters. [Copyright &y& Elsevier]

The chlorophyte Ulva is perceived as a simple and uniform algal form, with little functional differentiation within a thallus. We compared morphology, pigmentation, photosynthesis, growth, reproduction, and UV-B sensitivity between different thallus regions of Ulva pertusa Kjellman. Thallus thickness and cell size were significantly greater, whereas cell number was less in the basal region than in other regions. Photosynthetic pigment contents were lowest in the basal region and increased toward the marginal region. Photosynthetic capacity and photosynthetic efficiency normalized to fresh weight, area, volume, and cell number showed a progressive increase from the basal to marginal parts; however, on a chl basis those values were equal regardless of thallus part. Values of light saturation point were not statistically different between regions. Growth rates increased from marginal to basal and to middle parts of the thallus, whereas sporulation was highest in marginal (100%) followed by middle (30%) and basal parts (0%). Daily observation over 9 days showed that 56% of the basal cells divided once and did not produce spores, whereas every marginal cell went through its first division and 89% of the primary daughter cells also divided, resulting in 100% sporulation. A 7-day treatment with PAR and PAR + UV-A caused a significant decrease in the effective quantum yield of all thallus regions, followed by a recovery toward the initial values, whereas PAR + UV-A + UV-B irradiation led to greater photoinhibition and less recovery. Marked differences in the UV-B sensitivity were observed, with marginal parts being more sensitive and basal parts most resistant. [ABSTRACT FROM AUTHOR]

The chlorophyte Ulva is perceived as a simple and uniform algal form, with little functional differentiation within a thallus. We compared morphology, pigmentation, photosynthesis, growth, reproduction, and UV-B sensitivity between different thallus regions of Ulva pertusa Kjellman. Thallus thickness and cell size were significantly greater, whereas cell number was less in the basal region than in other regions. Photosynthetic pigment contents were lowest in the basal region and increased toward the marginal region. Photosynthetic capacity and photosynthetic efficiency normalized to fresh weight, area, volume, and cell number showed a progressive increase from the basal to marginal parts; however, on a chl basis those values were equal regardless of thallus part. Values of light saturation point were not statistically different between regions. Growth rates increased from marginal to basal and to middle parts of the thallus, whereas sporulation was highest in marginal (100%) followed by middle (30%) and basal parts (0%). Daily observation over 9 days showed that 56% of the basal cells divided once and did not produce spores, whereas every marginal cell went through its first division and 89% of the primary daughter cells also divided, resulting in 100% sporulation. A 7-day treatment with PAR and PAR + UV-A caused a significant decrease in the effective quantum yield of all thallus regions, followed by a recovery toward the initial values, whereas PAR + UV-A + UV-B irradiation led to greater photoinhibition and less recovery. Marked differences in the UV-B sensitivity were observed, with marginal parts being more sensitive and basal parts most resistant. [ABSTRACT FROM AUTHOR]