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.     

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.     

Combined effects of solar ultraviolet radiation and nutrients addition on growth, biomass and taxonomic composition of coastal marine phytoplankton communities of Patagonia

Experiments (6-8 days) were conducted during late summer, late fall and late winter, 2003 with waters collected off Bahía Nueva, Chubut, Argentina (42.7 degrees S, 65 degrees W) to determine the combined effects of solar ultraviolet radiation (UVR, 280-400 nm) and nutrient addition on phytoplankton communities. Samples were put in UVR-transparent containers and incubated under two radiation treatments: (a) Samples exposed to full solar radiation (PAB treatment, 280-400 nm) and (b) Samples exposed only to PAR (PAR 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 set, nutrients remained at the concentration of the seawater. Chlorophyll a, biomass, UV-absorbing compounds and taxonomic composition were recorded throughout the experiments. N cultures always had significantly higher growth rates (P<0.05) than that in non-enriched cultures. At the beginning of experiments, phytoplankton communities were generally dominated by monads and flagellates but by the end, diatoms comprised the bulk of biomass, with only one to four taxa dominating, suggesting a selection towards more tolerant/less sensitive species. Over the experimental time frame, the observed taxonomic changes were mostly due to nutrient availability, and to a lesser extent to solar UVR exposure. Overall, the results indicate that environmental conditions (i.e., light history, nutrient concentration) together with the physiological status of the cells play a very important role at the time to assess the combined effect of nutrient addition and solar radiation on coastal phytoplankton assemblages from Patagonia.     

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.     

Vertical mixing and ecological effects of ultraviolet radiation in planktonic communities

We present a mathematical model for a phytoplankton-zooplankton system, based on a predator-prey scheme. The model considers the effects of sinking in the phytoplankton, vertical mixing and attenuation of photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) in the water column. In a first approach, the model was studied under conditions of average PAR irradiance and shows fluctuations and stable equilibrium points. Secondly, we introduced the effects of photoperiod and photoinhibition by UVR and vertical mixing. Under these conditions, the phytoplankton biomass oscillates depending on the combined effects of UVR and mixing. Higher inhibition by UVR and longer mixing periods can induce strong fluctuations in the system but can also produce higher plankton peaks.     

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.     

A Five‐year Study of Solar Ultraviolet Radiation in Southern Chile (39° S): Potential Impact on Physiology of Coastal Marine Algae?

This study reports 5 years of (1998-2003) data on continuous solar-irradiation measurements from a scanning spectroradiometer (SUV-100) in Valdivia, Chile (39 degrees S), accompanied by evaluation of the impact of ultraviolet radiation (UVR) on marine macroalgae of this site. UVR conditions showed a strong seasonal variation, which was less pronounced toward longer wavelengths. Daily maximum dose rates (clear days) averaged in winter-summer: UV-B(290-315 nm) 0.30-2.1, UV-B(290-320 nm) 0.70-3.7, UV-A(315-400 nm) 20.6-62.1, UV-A(320-400 nm) 20.2-60.5 W m(-2), and photosynthetically active radiation (PAR) 969-2423 micromol m(-2) s(-1). The corresponding daily doses (all the days) ranged: UV-B(290-315 nm) 2.6-40.7, UV-B(290-320 nm) 6.7-78.5, UV-A(315-400 nm) 228-1539, UV-A(320-400 nm) 224-1501, and PAR 2008-13308 kJ m(-2) d(-1). Taking into consideration action spectra of a biological interest, the risk of UV exposure could be up to 37 times higher in summer than in winter. The photosynthetic activity (as maximum quantum yield of chlorophyll fluorescence, F(v)/F(m)) of the brown alga Lessonia nigrescens from the infralittoral zone was markedly more sensitive to UVR than of the green alga Enteromorpha intestinalis from the upper midlittoral, and the UV-B wave band increased markedly photoinhibition. In L. nigrescens, maximal photoinhibition (40%) took place at weighted (the action spectrum for photoinhibition of photosynthesis) UVR doses of 800 kJ m(-2), irrespective of the season (corresponding midsummer daily dose in Valdivia is 480 kJ m(-2)). In winter, when this alga was at its most sensitive, the weighted UV dose causing 35-40% photoinhibition was around 200 kJ m(-2). In E. intestinalis, weighted doses of 800 kJ m(-2) resulted in low photoinhibition (<10 %) and no clear seasonal patterns could be inferred. These results confirm that midday summer levels of UV-B and their daily doses in southern Chile are high enough to produce stress to intertidal macroalgae.     

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.     

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.     

Responses of a marine red tide alga Skeletonema costatum (Bacillariophyceae) to long-term UV radiation exposures

UV radiation (280–400 nm) is known to affect phytoplankton in negative, neutral and positive ways depending on the species or levels of irradiation energy. However, little has been documented on how photosynthetic physiology and growth of red tide alga respond to UVR in a long-term period. We exposed the cells of the marine red tide diatom Skeletonema costatum for 6 days to simulated solar radiations with UV-A (320–400 nm) or UV-A + UV-B (295–400 nm) and examined their changes in photosynthesis and growth. Presence of UV-B continuously reduced the effective photosynthetic quantum yield of PSII, and resulted in complete growth inhibition and death of cells. When UV-B or UV-B + UV-A was screened off, the growth rate decreased initially but regained thereafter. UV-absorbing compounds and carotenoids increased in response to the exposures with UVR. However, mechanisms for photoprotection associated with the increased carotenoids or UV-absorbing compounds were not adequate under the continuous exposure to a constant level of UV-B (0.09 W m^?2, DNA-weighted). In contrast, under solar radiation screened off UV-B, the photoprotection was first accomplished by an initial increase of carotenoids and a later increase in UV-absorbing compounds. The overall response of this red tide alga to prolonged UV exposures indicates that S. costatum is a UV-B-sensitive species and increased UV-B irradiance would influence the formation of its blooms.     

DEVELOPMENT OF AN in vitro SYSTEM FOR THE ANALYSIS OF ULTRAVIOLET RADIATION-INDUCED SUPPRESSION OF NATURAL KILLER CELL ACTIVITY

Previous studies have shown that natural killer (NK) cell activity was suppressed in volunteer subjects exposed to ultraviolet radiation (UVR) from solarium lamps. The present studies were carried out to determine that spectrum of UVR responsible for suppression of NK activity and to develop in vitro methods to analyze the effectivenes of sunscreen agents in prevention of UVR-mediated suppression of NK activity and other aspects of immune function. UVR from a xenon are lamp source was used to irradiate peripheral blood lymphocytes (PBL) in wells of tissue culture flasks, and transmission interference filters were used to eliminate UVR of particular wavelengths. The results indicated that UVR from this source inhibited NK activity of PBL in a dose-dependent manner with a 50% inhibitory dose of 5.5 mJ/cm² when unfiltered and 29.6 mJ/cm² when diluted through cellulose acetate, which gave a UV spectrum similar to that in solar radiation. Equivalent suppression of NK activity was mediated by UV-A (UVR > 315 nm) at dose levels of 4.2 J/cm², which was approximately 140 times greater than the amount of UV-B (UVR > 315 nm) needed to suppress NK activity. Similar dose-response curves were seen for inhibition of mitogenic responses to phytohemagglutinin except that the latter appeared less sensitive than NK to inhibition by UV-A. These studies suggest that whe the greater proportion of UV-A in solar radiation adn its greater penetration into skin is taken into account, UV-A may have equivalent or greater direct immunosuppressive effects than UV-B. The mechanisms of their immunosuppressive effects may, however, differ. The in vitro system described here would appear to provide a simple test system for further analysis of UVR-indued imunosuppression.     

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.     

Photoacclimation to long-term ultraviolet radiation exposure of natural sub-Antarctic phytoplankton communities: Fixed surface incubations versus mixed mesocosms

Solar UVB radiation (280-320 nm) is known to have detrimental effects on marine phytoplankton. Associated with the seasonal ozone hole in Antarctica, stratospheric ozone depletion occasionally influences the sub-Antarctic (Beagle Channel, Argentina) region, enhancing levels of UVB. The primary objective of this work was to study the effects of several (i.e. 6-10) days of exposure to UVB on the taxonomic composition and photosynthetic inhibition of local phytoplankton communities. For different light treatments, fixed-depth incubations placed in an outdoors water tank were compared with incubations in 1900 L mesocosms, where vertical mixing was present. Phytoplankton growth was inhibited by UV radiation (UVR) in fixed-depth experiments but not in the mixed mesocosms. Under fixed and mixed conditions alike, photosynthesis was significantly inhibited by UVB at the beginning of the experiment but no longer after several days of exposure, suggesting that cells had acclimated to radiation conditions. There was a change in species composition in response to UVR exposure in both experiments, which likely explained acclimation. In the community exposed to fixed conditions this change was from a phytoflagellate-dominated assemblage to a community with high relative abundance of diatoms after 6 days of exposure. UVA was responsible for most of the observed growth inhibition; however, the reduction in photosynthesis was produced by UVB. The reasons behind this variability in responses to UVR are associated with species-specific sensitivity and acclimation, and the previous light history of cells. In the community exposed in mesocosms, an assemblage codominated by phytoflagellates and diatoms was observed at the beginning of the experiments. After 10 days of exposure, green algae (Eutreptiella sp.) had increased, and phytoflagellates were the dominant group. The synthesis of mycosporine-like amino acids (MAAs), antioxidant enzymes and photosynthetic antenna pigments, in relation to repair and protection processes, may explain the reduced inhibition of both growth and photosynthesis that was observed in the phytoplankton community after several days of exposure. For environments such as the Beagle Channel seasonally exposed to the ozone hole, the results obtained from the fixed-depth experiments show that species can cope with UVR by means of MAA synthesis, while mixing would primarily promote a change in species composition and defense strategies.     

Resistance of a Lizard (the Green Anole, Anolis carolinensis; Polychridae) to Ultraviolet Radiation–induced Immunosuppression¶

The green anole (Anolis carolinensis) is the most northerly distributed of its Neotropical genus. This lizard avoids a winter hibernation phase by the use of sun basking behaviors. Inevitably, this species is exposed to high doses of ambient solar ultraviolet radiation (UVR). Increases in terrestrial ultraviolet-B (UV-B) radiation secondary to stratospheric ozone depletion and habitat perturbation potentially place this species at risk of UVR-induced immunosuppression. Daily exposure to subinflammatory UVR (8 kJ/m2/day UV-B, 85 kJ/m2/day ultraviolet A [UV-A]), 6 days per week for 4 weeks (total cumulative doses of 192 kJ/m2 UV-B, 2.04 x 10(3) kJ/m2 UV-A) did not suppress the anole's acute or delayed type hypersensitivity (DTH) response to horseshoe crab hemocyanin. In comparison with the available literature UV-B doses as low as 0.1 and 15.9 kJ/m2 induced suppression of DTH responses in mice and humans, respectively. Exposure of anoles to UVR did not result in the inhibition of ex vivo splenocyte phagocytosis of fluorescein labeled Escherichia coli or ex vivo splenocyte nitric oxide production. Doses of UV-B ranging from 0.35 to 45 kJ/m2 have been reported to suppress murine splenic/peritoneal macrophage phagocytosis and nitric oxide production. These preliminary studies demonstrate the resistance of green anoles to UVR-induced immunosuppression. Methanol extracts of anole skin contained two peaks in the ultraviolet wavelength range that could be indicative of photoprotective substances. However, the resistance of green anoles to UVR is probably not completely attributable to absorption by UVR photoprotective substances in the skin but more likely results from a combination of other factors including absorption by the cutis and absorption and reflectance by various components of the dermis.     

Penetration of ultraviolet radiation in the marine environment. A review

UV radiation (UVR) is a significant ecological factor in the marine environment that can have important effects on planktonic organisms and dissolved organic matter (DOM). The penetration of UVR into the water column is likely to change in the near future due to interactions between global warming and ozone depletion. In this study we report underwater instruments employed for the measurement of UVR and we review data dealing with the depth of UVR penetration in different oceanic areas including the open ocean, Antarctic waters and coastal waters. We provide the 10% irradiance depth (Z10%) for UV-A and UV-B as well as for DNA damage effective dose (DNA), which we calculated from the values of diffuse attenuation coefficients or vertical profiles reported in the literature. We observe a clear distinction between open ocean (high Z10%, no variation in the ratio UV-B/UV-A), Antarctic waters (increase in the ratio UV-B/UV-A during ozone hole conditions) and coastal waters (low Z10%, no variation in the ratio UV-B/UV-A). These variations in the penetration of UVR could lead to differences in the relative importance of photobiological/photochemical processes. We also compare in this study the penetration of UV-B (unweighted and weighted by the Setlow action spectrum) and DNA damage effective dose.     

INFLUENCE OF PHOTON FLUX DENSITY IN THE 400–700 nm WAVEBAND ON INHIBITION OF PHOTOSYNTHESIS BY UV-B (280–320 nm) IRRADIATION IN SOYBEAN LEAVES: SEPARATION OF INDIRECT AND IMMEDIATE EFFECTS

Abstract— Visible radiation can substantially influence the degree to which plant photosynthesis is inhibited by UV-B radiation. This study was designed to separate the immediate effects of visible radiation on UV-B photosynthetic inhibition from the indirect influence of visible irradiation on morphological and physiological properties of leaves during leaf development. Soybean plants were pretreated in growth chambers with either high or low visible irradiance (750 and 70 μmol m^-2s^-1 quantum flux in the 400–700 nm waveband, respectively) during the development of leaves used subsequently for UV irradiation. Test leaves still attached to the plant were exposed to 5 h of polychromatic UV-B irradiation and the photosynthetic capacity (net CO₂ exchange) was determined before and after the UV irradiation. During the UV irradiation, plants from both pretreatment groups received either high or low visible flux. Development of leaves in the high visible flux pretreatment conditions resulted in thicker leaves, higher chlorophyll a/b ratios, more UV-absorbing pigments, and reduced sensitivity to the UV-B irradiation. However, higher visible flux during the UV-B irradiation resulted in greater depression of photosynthesis by the UV-B irradiation. The relative magnitude of photosynthetic depression under these treatment combinations was the same when photosynthesis was measured under either light-limited or light-saturated conditions.     

AMELIORATION OF UV-B DAMAGE UNDER HIGH IRRADIANCE. I: ROLE OF PHOTOSYNTHESIS

Sensitivity to ultraviolet-B radiation (UV-B,280–315 nm) is generally reduced when background irradiance is high. We tested the involvement of photosynthesis in the amelioration of UV-B damage by treating plants at high PAR (photosynthetically-active radiation, 400–700 nm; 1000 μmol m-² s-¹) with supplemental UV-B at double ambient levels of biologically-effective radiation (18 kJ m-²d-¹) and either “ambient” (450 μmol mol-¹) or short term elevated (750 μmol mol-¹) CO₂ levels. Responses to UV-B were assessed by photosynthetic gas exchange, leaf expansion and production of UV-absorbing compounds (presumptive flavonoids) in cultivars of cucumber (Cucumis sativus L.) previously demonstrated to be relatively sensitive (cv. Poinsett) and insensitive (cv. Ashley) to UV-B. Except for marginal leaf interveinal chlorosis observed in Poinsett, both cultivars responded similarly. UV-B had little direct effect on leaf photosynthesis, but it did cause reductions in leaf area and corresponding increases in leaf dry matter per area. Increased CO, stimulated plant growth, counteracting the effect of UV-B on leaf growth and indicating an important role for photosynthesis. In contrast, the accumulation of UV-absorbing flavonoid compounds was enhanced by UV-B exposure but was not affected by COz enrichment.     

DISTORTIONS IN AMPHIBIAN DEVELOPMENT INDUCED BY ULTRAVIOLET-B ENHANCEMENT (290–315 NM) OF A SIMULATED SOLAR SPECTRUM

Abstract— Effects of increased intensity of UV-B radiation (290–315nm) on the systemic development and viability of boreal toad tadpoles were studied. When compared with animals exposed to UV-B deficient radiation of similar irradiance, tadpoles exposed to UV-B enhanced radiation displayed (1) abnormal development of the presumptive cornea, (2) areas of hyperplasia in the integument, (3) an anomalous, concave curvature of the spine, and (4) increased mortality. Daily exposure to photoreactivating radiation (> 315nm) following UV-B insult mitigated the potentially lethal damage to the tadpole population.     

Modeling the effects of ultraviolet radiation on estuarine phytoplankton production: impact of variations in exposure and sensitivity to inhibition.

Spectral ultraviolet (UV) irradiance, water column attenuation and biological weighting functions for inhibition of phytoplankton photosynthesis have been measured for the Rhode River, a subestuary of the Chesapeake Bay. Together, these measurements can be used to estimate UV effects on water column production, but each factor shows a significant range of variability even just considering summer time conditions. A sensitivity analysis of UV inhibition is described which assesses the effect of this variation for different combinations of 28 irradiance spectra, 8 biological weighting functions (BWFs) and 16 water column irradiance profiles. Over all combinations, production averaged about 84% relative to potential production in the absence of UV effects. For a few combinations, relative production was as low as 67%, or as high as 97%, but for most combinations the range was 75-95%. Variations in the sensitivity of the phytoplankton assemblage, i.e. the BWF, and optical properties, represented by a transparency ratio of biologically effective UV to photosynthetically available radiation (PAR), had large effects on water column production. A simple relationship for UV inhibition of water column production is developed based on inhibition at the surface and the ratio of UV and PAR transparency.     

Spectral Modeling of UV Inhibition of In Situ Antarctic Primary Production Using A Field-Derived Biological Weighting Function

Abstract— Our major aim is to illustrate an approach for hindcasting or forecasting UV radiation (UVR, 280–400 nm) effects on in situ rates of aquatic primary production when field measurements do not include estimates of UVR effects. A composite of spectral field measurements is employed to model UVR-dependent rates of photosynthesis in diatomdominated waters in a coastal region of the Southern Ocean. Assumptions, caveats and limitations of the modeling are discussed. Calculations begin with the 1991 Palmer Long Term Ecological Research (LTER) primary production and optical databases, from which daily integrated rates of carbon fixation in the absence of UVR are calculated as a function of depth for a 140 km transect line sampled between dawn and dusk of a single day (14 November 1991). The UVR measurements from the nearby NSF/OPP Polar Network at Palmer Station are used to determine ozone (O₃) concentration on the day of the transect, which is then employed in Madronich's (In UV-B Radiation and Ozone Depletion (Edited by M. Tevini), pp. 17–68. Lewis, Boca Raton, FL, 1993) spectral code to model daytime variations in surface spectral irradiances under clear sky conditions. These data are corrected for cloudiness and then combined with estimates of in-water UVR spectral attenuation coefficients, derived from Icecolors 90 data, to estimate in situ light exposure for phytoplankton collected at different depths and locations. An absolute chlorophyllspecific biological weighting function (BWF), determined under natural solar light fields for Antarctic diatom communities and shown to be reproducible while differing from a laboratory diatom BWF and other in situ BWF determined for other phytoplankton assemblages, is combined with estimates of in situ UVR exposure to derive in situ estimates of chlorophyllspecific losses of carbon fixation due to UVR inhibittion. By repeating calculations for every sampling site along the transect, we derive a spatial map of estimated UVR effects on primary production across the region. We repeat calculations for different O, concentrations expected during the austral spring over Antarctica and illustrate the O, dependency of UVB (280–320 nm) inhibition effects in near surface waters. We estimate ambient UVR reduced carbon fixation rates up to 65% in surface waters, depending upon location, down to undetectable levels at 36 m. Reducing stratospheric O₃ concentrations by 50% further inhibits near surface primary production by 8% and integrated primary production by 5%. Primary production was forced to subsurface maxima across the entire transect line in the presence of UVR.