Interactive effects of ultraviolet radiation and nutrient addition on growth and photosynthesis performance of four species of marine phytoplankton

Experiments (6-8 days) were carried out during the austral summer of 2005 in Chubut, Argentina (43 degrees S, 65 degrees W) to determine the interactive effects of solar UVR (280-400 nm) and nutrient addition on growth and chlorophyll fluorescence of four species of marine phytoplankton--the diatoms Thalassiosira fluviatilis Hustedt and Chaetoceros gracilis Schütt, and the dinoflagellates Heterocapsa triquetra (Ehrenberg) Stein and Prorocentrum micans (Ehrenberg). Samples were incubated under three radiation treatments (two sets of each radiation treatment): (a) samples exposed to full solar radiation (PAR+UVR, PAB treatment, 280-700 nm); (b) samples exposed to PAR and UV-A (PA treatment, 320-700 nm) and (c) samples exposed only to PAR (P treatment, 400-700 nm). At the beginning of the experiments, nutrients (i.e., NaPO(4)H(2) and NaNO(3)) were added to one set of samples from each radiation treatment ("N" cultures) whereas in the other, the nutrients concentration was that of the culture medium. At all times, the lowest growth rates (mu) were determined in the PAB treatments, where enriched cultures had significantly higher mu (P<0.05) than non-enriched cultures. Daily cycles of photochemical quantum yield (Y) displayed a pattern of relatively high values early in the morning with a sharp decrease at noon; recovery was observed late in the afternoon. In general, higher Y values were determined in enriched cultures than in non-enriched cultures. As the experiments progressed, acclimation (estimated as the difference between Y at noon and that at time zero) was observed in all species although in variable degree. All species displayed some degree of UVR-induced decrease in the photochemical quantum yield, although it was variable among treatments and species. However, this effect decreased with time, and this pattern was more evident in the dinoflagellates, as the concentration of UV-absorbing compounds increased. Thus, under conditions of nutrient enrichment as may occur by river input or by re-suspension by mixing, dinoflagellates outcompete with diatoms because they may have a higher fitness under UVR stress.     

Impact of solar ultraviolet radiation on marine phytoplankton of Patagonia, Argentina

Patagonia area is located in close proximity to the Antarctic ozone "hole" and thus receives enhanced ultraviolet B (UV-B) radiation (280-315 nm) in addition to the normal levels of ultraviolet A (UV-A; 315-400 nm) and photosynthetically available radiation (PAR; 400-700 nm). In marine ecosystems of Patagonia, normal ultraviolet radiation (UVR) levels affect phytoplankton assemblages during the three phases of the annual succession: (1) prebloom season (late summer-fall), (2) bloom season (winter-early spring) and (3) postbloom season (late spring-summer). Small-size cells characterize the pre- and postbloom communities, which have a relatively high photosynthetic inhibition because of high UVR levels during those seasons. During the bloom, characterized by microplankton diatoms, photosynthetic inhibition is low because of the low UVR levels reaching the earth's surface during winter; this community, however, is more sensitive to UV-B when inhibition is normalized by irradiance (i.e. biological weighting functions). In situ studies have shown that UVR significantly affects not only photosynthesis but also the DNA molecule, but these negative effects are rapidly reduced in the water column because of the differential attenuation of solar radiation. UVR also affects photosynthesis versus irradiance (P vs E) parameters of some natural phytoplankton assemblages (i.e. during the pre- but not during the postbloom season). However, there is a significant temporal variability of P vs E parameters, which are influenced by the nutrient status of cells and taxonomic composition; taxonomic composition is in turn associated with the stratification conditions (e.g. wind speed and duration). In Patagonia, wind speed is one of the most important variables that conditions the development of the winter bloom by regulating the depth of the upper mixed layer (UML) and hence the mean irradiance received by cells. Studies on the interactive effects of UVR and mixing show that responses of phytoplankton vary according to the taxonomic composition and cell structure of assemblages; therefore cells use UVR if >90% of the euphotic zone is being mixed. In fact, cell size plays a very important role when estimating the impact of UVR on phytoplankton, with large cells being more sensitive when determining photosynthesis inhibition, whereas small cells are more sensitive to DNA damage. Finally, in long-term experiments, it was determined that UVR can shape the diatom community structure in some assemblages of coastal waters, but it is virtually unknown how these changes affect the trophodynamics of marine systems. Future studies should consider the combined effects of UVR on both phytoplankton and grazers to establish potential changes in biodiversity of the area.     

Solar PAR and UV radiation affects the physiology and morphology of the cyanobacterium Anabaena sp. PCC 7120

Solar UV radiation (280-400 nm) may affect morphology of cyanobacteria, however, little has been evidenced on this aspect while their physiological responses were examined. We investigated the impacts of solar PAR and UVR on the growth, photosynthetic performance and morphology of the cyanobacterium Anabaena sp. PCC7120 while it was grown under three different solar radiation treatments: exposures to (a) constant low PAR (photosynthetic active radiation, 400-700 nm), (b) natural levels of solar radiation with and (c) without UV radiation (290-400 nm). When the cells were exposed to solar PAR or PAR+UVR, the photochemical efficiency was reduced by about 40% and 90%, respectively, on day one and recovered faster under the treatment without UVR over the following days. Solar UVR inhibited the growth up to 40%, reduced trichome length by up to 49% and depressed the differentiation of heterocysts. Negligible concentrations of UV-absorbing compounds were found even in the presence of UVR. During the first 2 d of exposure to natural levels of PAR, carotenoid concentrations increased but no prolonged increase was evident. Heterocyst formation was enhanced under elevated PAR levels that stimulated quantum yield and growth after an initial inhibition. Higher concentrations of carotenoids and a twofold increase in the carotenoid to chlorophyll a ratio provided protection from the high levels of solar PAR. Under radiation treatments with UVR the relatively greater decrease in chlorophyll a concentrations compared with the increase in carotenoids was responsible for the higher carotenoid: chlorophyll a ratio. Heterocyst formation was disrupted in the presence of solar UVR. However, the longer term impact of heterocyst disruption to the survival of Anabaena sp. requires further study.     

Variability of UVR effects on photosynthesis of summer phytoplankton assemblages from a tropical coastal area of the South China Sea

From June to September 2005, we carried out experiments to determine the ultraviolet radiation (UVR) -induced photoinhibition of summer phytoplankton assemblages from a coastal site of the South China Sea. Variability in taxonomic composition was determined throughout the summer, with a peak chlorophyll a (chl a approximately 20 microg chl a L(-1)) dominated by the diatom Skeletonema costatum that was detected early in the study period; the rest of the time samples were characterized by monads and flagellates, with low chl a values (1-5 chl a microg L(-1)). Surface water samples were placed in quartz tubes, inoculated with radiocarbon and exposed to solar radiation for 2-3 h to determine photosynthetic rates under three quality radiation treatments (i.e. PAB, 280-700 nm; PA, 320-700 nm and P, 400-700 nm) using different filters and under seven levels of ambient irradiance using neutral density screens (P vs E curves). UVR inhibition of samples exposed to maximum irradiance (i.e. at the surface) varied from -12.2% to 50%, while the daytime-integrated UVR-related photoinhibition in surface seawater varied from -62% to 7%. The effects of UVR on the photosynthetic parameters P(B)(max) and E(k) were also variable, but UV-B accounted for most of the observed variability. During sunny days, photosynthesis of microplankton (>20 microm) and piconanoplankton (<20 microm) were significantly inhibited by UVR (mostly by UV-B). However, during cloudy days, while piconanoplankton cells were still inhibited by UVR, microplankton cells used UVR (mostly UV-A) as the source of energy for photosynthesis, resulting in higher carbon fixation in samples exposed to UVR than the ones exposed only to photosynthetically active radiation (PAR). Our results indicate that size structure and cloudiness clearly condition the overall impact of UVR on phytoplankton photosynthesis in this tropical site of South China. In addition, model predictions for this area considering only PAR for primary production might have underestimated carbon fixation due to UVR contribution.     

Vertical migration and motility responses in three marine phytoplankton species exposed to solar radiation

Diurnal vertical migration in the water column and the impact of solar radiation on motility were investigated in three marine phytoplankton species: Tetraselmis suecica, Dunaliella salina and Gymnodinium chlorophorum. Cells were exposed to solar radiation either in ultraviolet radiation (UVR, 280-400 nm) transparent Plexiglas tubes (45 cm length, 10 cm diameter) or in quartz tubes under three radiation treatments: PAB (280-700 nm), PA (320-700 nm) and P (400-700 nm). The three species displayed different behavior after exposure to solar radiation. Tetraselmis suecica was insensitive to UVR and under high solar radiation levels, cells accumulated preferentially near the surface. Exposure experiments did not indicate any significant changes in swimming speed nor in the percentage of motile cells after 5 h of exposure. On the other hand, D. salina was sensitive to UV-B displaying a significant decrease in swimming speed and percentage of motile cells after 2-3 h of exposure. Moreover, D. salina cells migrated deep in the water column when irradiance was high. The response of G. chlorophorum was in between that of the other two species tested, with a slight (but significant) decrease in swimming speed and percentage of motile cells in all radiation treatments after 5 h of exposure. While G. chlorophorum cells were more or less homogenously distributed in the water column, a slight (but significant) avoidance response to high radiation was observed at local noon, with cells migrating deep in the water column. Our data clearly indicate that these sub-lethal effects of solar radiation are species-specific and they might have important implications for the aquatic ecosystem.     

Solar UV Irradiances Modulate Effects of Ocean Acidification on the Coccolithophorid Emiliania huxleyi

Emiliania huxleyi, the most abundant coccolithophorid in the oceans, is naturally exposed to solar UV radiation (UVR, 280–400 nm) in addition to photosynthetically active radiation (PAR). We investigated the physiological responses of E. huxleyi to the present day and elevated CO₂ (390 vs 1000 μatm; with pH_NBS 8.20 vs 7.86) under indoor constant PAR and fluctuating solar radiation with or without UVR. Enrichment of CO₂ stimulated the production rate of particulate organic carbon (POC) under constant PAR, but led to unchanged POC production under incident fluctuating solar radiation. The production rates of particulate inorganic carbon (PIC) as well as PIC/POC ratios were reduced under the elevated CO₂, ocean acidification (OA) condition, regardless of PAR levels, and the presence of UVR. However, moderate levels of UVR increased PIC production rates and PIC/POC ratios. OA treatment interacted with UVR to influence the alga's physiological performance, leading to reduced specific growth rate in the presence of UVA (315–400 nm) and decreased quantum yield, along with enhanced nonphotochemical quenching, with addition of UVB (280–315 nm). The results clearly indicate that UV radiation needs to be invoked as a key stressor when considering the impacts of ocean acidification on E. huxleyi.     

Effects of solar ultraviolet radiation on photosynthesis of the marine red tide alga Heterosigma akashiwo (Raphidophyceae)

In order to assess the short- and long-term impacts of UV radiation (UVR, 280-400nm) on the red tide alga, Heterosigma akashiwo, we exposed the cells to three different solar radiation treatments (PAB: 280-700nm, PA: 320-700nm, P: 400-700nm) under both solar and artificial radiation. A significant decrease in the effective quantum yield (Y) during high irradiance periods (i.e., local noon) was observed, but the cells partially recovered during the evening hours. Exposure to high irradiances for 15, 30, and 60min under a solar simulator followed by the recovery (8h) under dark, 9 and 100micromolphotonsm(-2)s(-1) of PAR, highlighted the importance of the irradiance level during the recovery period. Regardless the radiation treatments, the highest recovery (both in rate and total Y) was found at a PAR irradiance of 9micromolphotonsm(-2)s(-1), while the lowest was observed at 100micromolphotonsm(-2)s(-1). In all experiments, PAR was responsible for most of the observed inhibition; nevertheless, the cells exposed only to PAR had the highest recovery in any condition, as compared to the other radiation treatments. In long-term experiments (10 days) using semi-continuous cultures, there was a significant increase of UV-absorbing compounds (UV(abc)) per cell from 1.2 to >4x10(-6)microgUV(abc)cell(-1) during the first 3-5 days of exposure to solar radiation. The highest concentration of UV(abc) was found in samples exposed in the PAB as compared to PA and P treatments. Growth rates (mu) mimic the behavior of UV-absorbing compounds, and during the first 5 days mu increased from <0.2 to ca. 0.8, and stayed relatively constant at this value during the rest of the experiment. The inhibition of the Y decreased with increasing acclimation of cells. All our data indicates that H. akashiwo is a sensitive species, but was able acclimate relatively fast (3-5 days) synthesizing UV-absorbing compounds and thus reducing any impact either on photosystem II or on growth.     

Differential impacts of solar UV radiation on photosynthetic carbon fixation from the coastal to offshore surface waters in the South China Sea

We carried out experiments during an expedition (14 August to 14 September, 2007) that covered up to 250,000 km(2) to investigate the effects of solar UV radiation (UVR, 280-400 nm) on the photosynthetic carbon fixation of tropical phytoplankton assemblages in surface seawater of the South China Sea. From coastal to pelagic surface seawaters, UV-B (280-315 nm) caused similar inhibition, while UV-A (315-400 nm) induced photosynthetic inhibition increased from coastal to offshore waters. UV-B resulted in an inhibition by up to 27% and UV-A by up to 29%. Under reduced levels of solar radiation with heavy overcast, UV-A resulted in enhanced photosynthetic carbon fixation by up to 25% in coastal waters where microplankton was abundant. However, such a positive impact was not observed in the offshore waters where piconanoplankton was more abundant. The daily integrated inhibition of UV-A reached 4.3% and 13.2%, and that of UV-B reached 16.5% and 13.5%, in the coastal and offshore waters, respectively.     

Variable fluorescence parameters in the filamentous Patagonian rhodophytes, Callithamnion gaudichaudii and Ceramium sp. under solar radiation

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.     

Antioxidative Responses of Two Marine Microalgae During Acclimation to Static and Fluctuating Natural UV Radiation

Photoacclimation properties were investigated in two marine microalgae exposed to four ambient irradiance conditions: static photosynthetically active radiation (PAR: 400–700 nm), static PAR + UVR (280–700 nm), dynamic PAR and dynamic PAR + UVR. High light acclimated cultures of Thalassiosira weissflogii and Dunaliella tertiolecta were exposed outdoors for a maximum of 7 days. Dynamic irradiance was established by computer controlled vertical movement of 2 L bottles in a water filled basin. Immediate (<24 h), short-term (1–3 days) and long-term (4–7 days) photoacclimation was followed for antioxidants (superoxide dismutase, ascorbate peroxidase and glutathione cycling), growth and pigment pools. Changes in UVR sensitivity during photoacclimation were monitored by measuring UVR-induced inhibition of carbon assimilation under standardized UV conditions using an indoor solar simulator. Both species showed immediate antioxidant responses due to their transfer to the outdoor conditions. Furthermore, upon outdoor exposure, carbon assimilation and growth rates were reduced in both species compared with initial conditions; however, these effects were most pronounced in D. tertiolecta . Outdoor UV exposure did not alter antioxidant levels when compared with PAR-only controls in both species. In contrast, growth was significantly affected in the static UVR cultures, concurrent with significantly enhanced UVR resistance. We conclude that antioxidants play a minor role in the reinforcement of natural UVR resistance in T. weissflogii and D. tertiolecta .     

Patterns of DNA damage and photoinhibition in temperate South-Atlantic picophytoplankton exposed to solar ultraviolet radiation.

Natural marine phytoplankton assemblages from Bahía Bustamante (Chubut, Argentina, 45 degrees S, 66.5 degrees W), mainly consisting of cells in the picoplankton size range (0.2-2 microm), were exposed to various UVBR (280-315 nm) and UVAR (315-400 nm) regimes in order to follow wavelength-dependent patterns of cyclobutane pyrimidine dimer (CPD) induction and repair. Simultaneously, UVR induced photosynthetic inhibition was studied in radiocarbon incorporation experiments. Biological weighting functions (BWFs) for photoinhibition and for CPD induction, the latter measured in bare calf thymus DNA, differed in the UVAR region: carbon incorporation was reduced markedly due to UVAR, whereas no measurable UVAR effect was found on CPD formation. In contrast, BWFs for inhibition of photosynthesis and CPD accumulation were fairly similar in the UVBR region, especially above 300 nm. Incubation of phytoplankton under full solar radiation caused rapid CPD accumulation over the day, giving maximum damage levels exceeding 500 CPD MB(-1) at the end of the afternoon. A clear daily pattern of CPD accumulation was found, in keeping with the DNA effective dose measured by a DNA dosimeter. In contrast, UVBR induced photosynthetic inhibition was not dose related and remained nearly constant during the day. Screening of UVBR or UVR did not cause significant CPD removal, indicating that photoreactivation either by PAR or UVAR was of minor importance in these organisms. High CPD levels were found in situ early in the morning, which remained unaffected notwithstanding treatments favoring photorepair. These results imply that a proportion of cells had been killed by UVBR exposure prior to the treatments. Our data suggest that the limited potential for photoreactivation in picophytoplankton assemblages from the southern Atlantic Ocean causes high CPD accumulation as a result of UVBR exposure.     

Influence of temperature and UVR on photosynthesis and morphology of four species of cyanobacteria

During the late austral spring of 2009 we carried out experiments (4days of duration) with four cyanobacteria species, Anabaena sp., Nostoc sp., Arthrospira platensis and Microcystis sp., to assess the combined effects of temperature and solar radiation on photosynthesis performance and morphology. Two experimental temperatures (18°C and 23°C, simulating a 5°C increase under a scenario of climate change) and three radiation treatments (by using different filters/materials) were implemented: (i) P (PAR, 400-700nm), (ii) PA (PAR+UV-A, 320-700nm) and, (iii) PAB (PAR+UV-A+UV-B, 280-700nm). In general, samples under the P treatment had less decrease/higher recovery rates of effective photochemical quantum yield (Y) than those receiving UV-A or UV-A+UV-B. The effects of increased temperature were species-specific: At the end of the experiments, it was seen that increased temperature benefited photosynthetic performance of Anabaena sp. and Nostoc sp. but not of Microcystis sp. and A. platensis. Higher temperature was also associated to an increase in the chain area of Anabaena sp., and to bigger trichomes in A. platensis; however, no morphological effects were observed in Microcystis sp. In addition, in Nostoc sp. the increase in temperature counteracted the UVR impact on the reduction of the chain area. How these effects and mechanisms will affect the trophodynamics and production of aquatic ecosystems is still uncertain, but the specificity of the responses suggests that not all cyanobacteria would be equally benefited by temperature increases therefore affecting the balance and interaction among species in the water column.     

Hydrogen peroxide formation and decay in iron-rich geothermal waters: the relative roles of abiotic and biotic mechanisms

Hydrogen peroxide (H2O2) is widely distributed in surface waters where the primary photochemical formation pathway involves the interaction between dissolved organic carbon (DOC) and ultraviolet radiation (UVR). In laboratory studies using iron-rich water from Yellow-stone's Chocolate Pots spring, H2O2 formation depended on sample treatment (unfiltered, < 0.2 micron filtered, autoclaved) prior to irradiation, suggesting several formation pathways. Similar H2O2 formation in filtered and unfiltered water indicates that it is primarily soluble material that is responsible for H2O2 formation. H2O2 formation with soluble material probably includes only photochemical reactions with DOC and/or metals. Greater H2O2 formation in unfiltered and filtered water than in autoclaved water suggests that the agent(s) involved in H2O2 formation is (are) not stable at high temperatures and pressures and degrade to nonphotoreactive species. Such unstable agents may include DOC and/or dissolved complexes of iron or other metals. UVR absorbance occurs across the UV spectrum and, though slightly greater in the UVA range (320-400 nm), is similar to that of other surface waters. Increased UVR absorbance after autoclaving suggested degradation or alteration of some components, which in turn affected H2O2 formation. The spectral region used for irradiation affected net formation and yield. H2O2 formation in water irradiated with UVA radiation was 2.5-3 times that formed in water irradiated with UVB radiation (280-320 nm) in experiments using artificial light sources. Apparent quantum yields comparable to those reported by others could not be calculated because the instrumental designs are not the same. However, approximate quantum yields were calculated for these experiments but should be viewed with caution. Quantum yields were higher in these experiments (0.0040 mol H2O2 per mol photon at 310 nm and 0.0012 mol H2O2 per mol photon at 350 nm) than values reported by other researchers (< 0.0007 mol H2O2 per mol photon at 300 nm and < 0.0005 H2O2 per mol photon at 340 nm; [Scully, N. M., D. R. S. Lean, D. J. McQueen and W. J. Cooper (1996) Limnol. Oceanogr. 41, 540-548]). In natural solar source experiments, H2O2 formation was greater in experiments with UVA and photosynthetically active radiation (PAR; 400-700 nm) than with PAR alone or with UVB, UVA and PAR. However, H2O2 capacity (nM H2O2 W-1 h-1 m2) was greatest with UVB radiation and lowest with PAR radiation. Source regions could not be studied separately. Dark decay of H2O2 occurred via two mechanisms. The main mechanism responsible for H2O2 decay involved particulate matter (probably microorganisms), whereas a secondary mechanism involved soluble matter (i.e. DOC, metal ions and other dissolved species involved in Fenton reactions).     

Seasonal Impacts of Solar UV Radiation on Photosynthesis of Phytoplankton Assemblages in the Coastal Waters of the South China Sea

We carried out experiments to evaluate seasonal changes in the impacts of UV radiation (UVR, 280–400 nm) on photosynthetic carbon fixation of phytoplankton assemblages. Surface water samples were obtained in the coastal area of the South China Sea, where chlorophyll a ranged 0.72–3.82 μg L⁻¹. Assimilation numbers (photosynthetic carbon fixation rate per chl a) were significantly higher during summer 2005 than those in spring and winter 2004. The mean values obtained under photosynthetically active radiation (PAR) were 2.83 (spring 2004), 4.35 (winter 2004) and 7.29 μg C (μg chl a)⁻¹ h⁻¹ (summer 2005), respectively. The assimilation numbers under PAR + UVR were 1.58, 2.71 and 5.28 μg C (μg chl a)⁻¹ h⁻¹, for spring, winter and summer, respectively. UVR induced less inhibition of photosynthesis during summer 2005 than during the other seasons, in spite of the higher UVR during summer. The seasonal differences in the productivity and photosynthetic response to UV were mainly due to changes in water temperature, while irradiance and vertical mixing explained >80% of the observed variability. Our data suggest that previous studies in the SCS using UV-opaque vessels might have overestimated the phytoplankton production by about 80% in spring, 61% in winter and 38% in summer.     

Physiological responses of Acropora cervicornis to increased solar irradiance

The effects of increased UV radiation (UV-B [280-320 nm] + UV-A [320-400 nm]; hereafter UVR) on the growth, production of photosynthetic pigments and photoprotective mycosporine-like amino acids (MAAs) were studied in the threatened Caribbean coral Acropora cervicornis transplanted from 20 to 1 m depth in La Parguera, Puerto Rico. The UVR exposure by the transplanted colonies was significantly higher than that at 20 m, while photosynthetically active radiation (PAR) only increased by 9%. Photosynthetic pigments, quantified with HPLC, as well as linear extension rates and skeletal densities, were significantly reduced 1 month after transplantation to 1 m depth, while MAAs increased significantly despite immediate paling experienced by transplanted colonies. While these colonies showed a significant reduction in photosynthetic pigments, there were no significant reductions in zooxanthellae densities suggesting photoacclimation of the coral's symbionts to the new radiation conditions. The results suggest that while corals might be able to survive sudden increases in UVR and PAR, their skeletal structure can be greatly debilitated due to a reduction in the photosynthetic capacity of their symbionts and a possible relocation of resources.     

Dynamics of potentially protective compounds in Rhodophyta species from Patagonia (Argentina) exposed to solar radiation

The impact of solar radiation upon potentially protective compounds (i.e., UV-absorbing compounds and carotenoids) was assessed in four Rhodophyte species from Patagonia (i.e., Ceramium sp. Lyngbye, Corallina officinalis Linnaeus, Callithamnion gaudichaudii Agardh and Porphyra columbina Montagne) during short-term (i.e., 46 h) experiments. Algae were exposed to solar radiation under two treatments (PAR only: 400-700 nm, and PAR+UVR: 280-700 nm) and sub-samples were taken every 3 h (or longer periods at night) to determine the spectral absorption characteristics and concentration of UV-absorbing compounds, carotenoids and photosynthetic pigments. Except for C. gaudichaudii which displayed a decrease in chl-a concentration throughout the experiment, photosynthetic pigments had small variations in all species. UV-absorbing compounds concentration had species-specific responses: Ceramium sp. was the only species in which UV-absorbing compounds concentration varied as a function of solar irradiance, with maximum values around local noon. In C. officinalis and P. columbina UV-absorbing compounds concentration increased as compared to that of chl-a; in Ceramium sp. and C. gaudichaudii, however, there was no relationship between UV-absorbing compounds content and chl-a concentration. Carotenoids, on the other hand, did co-vary with chl-a in all species. Our data suggest that, with the exception of C. gaudichaudii, the differential responses of UV-absorbing compounds concentrations are more associated to the previous light history of the algae (i.e., in turn due to their position in the intertidal zone) rather than to the radiation treatment imposed to the samples. Based on our results, the variable impact of solar radiation upon productivity (and eventually biodiversity) of macroalgae from Patagonia might consequently differentially affect higher trophic levels of the aquatic food web.     

Photomovement of the swarmers of the brown algae Scytosiphon lomentaria and Petalonia fascia: effect of photon irradiance, spectral composition and UV dose

Photomovement measurements were carried out with swarmers of the brown algae Scytosiphon lomentaria (Lyngb.) Link and Petalonia fascia (O. F. Müll.) as a function of irradiance direction, photon irradiance, spectral composition and ultraviolet radiation (UVR, lambda=280-400 nm) dose. Swarmers from both species showed similar photomovement patterns: negative phototaxis occurred under photon irradiances of 10-90 micromol photons m(-2) s(-1), and no movement was observed at 190 micromol photons m(-2) s(-1). The translocational velocity measured between 10 and 90 micromol m(-2) s(-1) ranged from 100 to 200 microm s(-1). The accumulation of swarmers presented a peak at 450 nm (waveband of 50 nm), and smaller peaks at 400 and 500 nm; no effect was observed at wavelengths of 550 nm and above. The decline in phototactic index (an estimator of photomovement response) of swarmers was linearly correlated with the logarithm of UVR doses. These data were correlated with levels of natural solar radiation in the field. It is hypothesized that motility of swarmers could be a critical factor in the survival of these species under a scenario of increased UVR.     

Light histories influence the impacts of solar ultraviolet radiation on photosynthesis and growth in a marine diatom, Skeletonema costatum

Phytoplanktonic species acclimated to high light are known to show less photoinhibition. However, little has been documented on how cells grown under indoor conditions for decades without exposure to UV radiation (UVR, 280-400 nm) would respond differently to solar UVR compared to those in situ grown under natural solar radiation. Here, we have shown the comparative photosynthetic and growth responses to solar UVR in an indoor- (IS) and a naturally grown (WS) Skeletonema costatum type. In short-term experiment (<1 day), Phi(PSII) and photosynthetic carbon fixation rate were more inhibited by UVR in the IS than in the WS cells. The rate of UVR-induced damages of PSII was faster and their repair was significantly slower in IS than in WS. Even under changing solar radiation simulated for vertical mixing, solar UVR-induced higher inhibition of photosynthetic rate in IS than in WS cells. During long-term (10 days) exposures to solar radiation, the specific growth rate was much lower in IS than WS at the beginning, then increased 3 days later to reach an equivalent level as that of WS. UVR-induced inhibition of photosynthetic carbon fixation in the IS was identical with that of WS at the end of the long-term exposure. The photosynthetic acclimation was not accompanied with increased contents of UV-absorbing compounds, indicating that repair processes for UVR-induced damages must have been accelerated or upgraded.     

Spectrum-specific Damage and Solar Ultraviolet Radiation Avoidance in the Two-spotted Spider Mite

The spatial distribution of the two-spotted spider mite Tetranychus urticae Koch is biased toward the lower surfaces of leaves as compared with the upper leaf surfaces on their host plants. Because of the deleterious effects of solar ultraviolet (UV) irradiation, we hypothesized T. urticae remains on lower leaf surfaces as an adaptation to avoid solar UV radiation (UVR). We examined the effects of solar UVR components on females and tested whether spatial distribution was associated with solar UVR avoidance. Attenuation of solar UVR using UV opaque film increased fecundity and reduced the movement of females from the upper to the lower leaf surfaces. In contrast, diverting solar UVR to the lower leaf surface using a light reflection sheet caused the mites to move from the lower to the upper leaf surfaces; however, attenuated UV reflection did not, suggesting that they occupy the lower leaf surface to avoid solar UVR. In monochromatic UVR tests, no eggs hatched when placed under 280–300 nm radiation, whereas almost all eggs hatched at 320–360 nm. Adult females, however, did not avoid wavelengths of 280 and 300 nm, but avoided 320–340 nm. We conclude that T. urticae exploit UVA information to avoid ambient UVB radiation.     

Interaction of UV radiation and inorganic carbon supply in the inhibition of photosynthesis: spectral and temporal responses of two marine picoplankters

The effect of ultraviolet radiation (UVR) on inhibition of photosynthesis was studied in two species of marine picoplankton with different carbon concentration mechanisms: Nannochloropsis gaditana Lubian possesses a bicarbonate uptake system and Nannochloris atomus Butcher a CO2 active transport system. Biological weighting functions (BWFs) for inhibition of photosynthesis by UVR and photosynthesis vs irradiance (PI) curves for photosynthetically active radiation (PAR) were estimated for both species grown with an enriched CO2 supply (high dissolved inorganic carbon [DIC]: 1% CO2 in air) and in atmospheric CO2 levels (low DIC: 0.03% CO2). The response to UVR and PAR exposures was different in each species depending on the DIC treatment. Under PAR exposure, rates of maximum photosynthesis were similar between treatments in N. gaditana. However, the cultures growing in high DIC had lower sensitivity to UVR than the low DIC cultures. In contrast, N. atomus had higher rates of photosynthesis under PAR exposure with high DIC, but the BWFs were not significantly different between treatments. The results suggest that one or more processes in N. gaditana associated with HCO3- transport are target(s) for UV photodamage because there was relatively less UV inhibition of the high DIC-grown cultures in which inorganic carbon fixation is supplied by passive CO2 diffusion. Time courses of photochemical efficiency in PAR, during UV exposure and during subsequent recovery in PAR, were determined using a pulse amplitude modulated fluorometer. The results were consistent with the BWFs. In all time courses, a steady state was obtained after an initial decrease, consistent with a dynamic balance between damage and repair as found for other phytoplankton. However, the relationship of response to exposure showed a steep decline in activity that is consistent with a constant rate of repair. A novel feature of a model developed from a constant repair rate is an explicit threshold for photosynthetic response to UV.