Absence Of UV Radiation Research Articles

Ultraviolet Radiation Reduces the Photoprotective Capacity of the Marine Diatom Phaeodactylum tricornutum (Bacillariophyceae, Heterokontophyta)

This study demonstrates that UV radiation (UVR) reduces the photoprotective capacity of the diatom Phaeodactylum tricornutum by affecting xanthophyll cycle (XC) activity. The short-term reduction of photosystem II (PSII) maximum efficiency of charge separation (F(v)/F(m)) in cells exposed to UVR could be explained mainly by a reduced photoprotective capacity under this condition. Phaeodactylum tricornutum cells acclimated to two different photosynthetically active radiation (PAR) intensities, high light (HL, 200 micromol quanta m(-2) s(-1)) and low light (LL, 50 micromol quanta m(-2) s(-1)), were exposed to saturating irradiance (1100 micromol quanta m(-2) s(-1)) in the presence (PAR + UVR) and absence of UVR (PAR). HL cells exhibited a greater reduction in F(v)/F(m) in PAR + UVR when compared with the PAR treatment that was related to a reduction in the de-epoxidation of XC pigments. In contrast, in LL cells, UVR did not considerably affect XC de-epoxidation even though the reduction in F(v)/F(m) was greater than in HL cells. The negative effect of UVR on photoprotection was more pronounced in HL cells because they synthesized more XC pigments than LL cells. This was confirmed when XC activity was blocked with dithiothreitol and when PSII repair was inhibited with chloramphenicol (CAP). The differential reduction of F(v)/F(m) between PAR + UVR and PAR treatments disappeared when XC was blocked in HL cells. A higher reduction and an incomplete recovery of F(v)/F(m) were observed in cells incubated with CAP in the presence of UVR. Such responses confirm that UVR had a negative effect on photoprotective mechanisms causing an enhancement of damage by PAR, especially in HL-acclimated cells in which heat dissipation is important for PSII regulation.

TGF-β Targets PAX3 to Control Melanocyte Differentiation

In the absence of UV radiation, keratinocytes secrete TGF-beta which blocks melanocyte differentiation. Yang et al. (2008), in a recent issue of Molecular Cell, show that TGF-beta induces Smad signaling in melanocytes to repress PAX3, which encodes a transcription factor crucial for melanocyte differentiation. In the presence of UV radiation, a Jnk/AP-1 pathway represses TGF-beta, which together with a UV-induced p53 pathway promotes melanocyte differentiation.

Mercury Emission to the Atmosphere from Experimental Manipulation of DOC and UVR in Mesoscale Field Chambers in a Freshwater Lake

Mesocosm experiments in an optically transparent lake allow the manipulation of both dissolved organic carbon (DOC) and incident ultraviolet radiation (UVR) in order to study mercury reduction and emission processes. In the absence of UVR and the presence of visible light, mercury emission is very low (approximately0.3 ng/m2/h). When UVR is permitted in the mesocosm chambers, mercury emission increases, with emission rates ranging from 0.3 to 2.5 ng/m2/h. At concentrations between 1.5 and 2.5 mg/L DOC, mercury emission does not appear to depend on either the concentration or the optical properties of the DOC. In particular, the addition of 1.0 mg/L DOC from a nearby wetland to a photobleached mesocosm did not increase the emission of mercury. The similarities between mercury emission from highly photobleached 1.5 mg/L DOC and from terrestrially enriched 2.5 mg/L DOC suggest that the moieties responsible for mercury reduction are far in excess of that needed for mercury reduction. Using the measured flux rate of mercury from the water surface, we calculated a dissolved gaseous mercury (DGM) concentration thatwould need to be present to drive the emissive flux. The buildup of DGM was used to approximate a kinetic rate constant for the net mercury reduction in this system of approximately 0.17 h(-1), which is consistent with existing published values.

Light, literacy and the absence of ultraviolet radiation in the development of myopia

As the prevalence of myopia steadily increases, reaching as high as 90% in some populations, investigators continue to look for causative factors other than family history. Most current research suggests an association of axial myopia with reading or either the presence or absence of light. Even though these studies are frequently inconsistent, non-reproducible or contradictory, many clinicians utilize them in recommending treatments for children, such as bifocals or atropine. By reviewing the biologic effects of non-ionizing electromagnetic fields, we may gain insight into these discrepancies as well as unify the combined role of literacy and light in the pathogenesis of myopia. These biologic effects are wavelength specific. The wavelength of artificial (either incandescent or fluorescent) light is primarily 700-400 nm, while the wavelength of natural light is 700-200 nm, inclusive of the ultraviolet spectrum. So the opposite findings of myopia resulting from either accommodation under continuous light or under darkness (form deprivation) can be reconciled by restating it: Close focusing in the absence of UV light may provoke axial myopia. Experimental evidence exhibiting both scleral remodeling under accommodation as well as the inhibition of scleral remodeling by the hardening of collagen under ultraviolet exposure may support this concept. Perhaps new research can look into the role of the presence or absence of UV light in animal models of myopia.

Induction of Primary Cutaneous Melanomas in C3H Mice by Combined Treatment with Ultraviolet Radiation, Ethanol and Aloe Emodin ¶

The role of ultraviolet (UV) radiation in the induction of nonmelanoma skin cancer is widely accepted, although its precise contribution to the development of primary cutaneous melanoma skin cancer requires further definition. We found that painting aloe emodin, a trihydroxyanthraquinone from Aloe barbadensis, in ethyl alcohol vehicle on the skin of mice in conjunction with exposure to UVB (280–320 nm) radiation results in the development of melanin-containing skin tumors. C3H/HeN mice were treated thrice weekly with aloe emodin in a 25% ethanol in water vehicle and exposed to 15 kJ/m2 UV radiation. Neither ethanol vehicle nor aloe emodin alone induced skin tumors in the absence of UV radiation. In two separate experiments, 20–30% of the mice treated with a combination of UV radiation and ethanol vehicle and 50–67% of the UV-irradiated animals given aloe emodin in ethanol vehicle developed primary cutaneous melanin-containing tumors. The diagnosis of melanoma was established using Fontana silver stain for melanin; these tumors were negative for vimentin and keratin. Melanin-containing melanosomes were observed by transmission electron microscopy in tumors diagnosed as melanomas. Although the mechanism of carcinogenesis in these mice is currently unknown, our findings have led to the development of the first facile murine model for the induction of primary melanoma. This model has the potential to clarify the role of UV radiation in the etiology of malignant melanoma.

Toxicity of cobalt-complexed cyanide to Oncorhynchus mykiss, Daphnia magna, and Ceriodaphnia dubia

Cobalt cyanide complexes often result when ore is treated with cyanide solutions to extract gold and other metals. These have recently been discovered in low but significant concentrations in effluents from gold leach operations. This study was conducted to determine the potential toxicity of cobalt-cyanide complexes to freshwater organisms and the extent to which ultraviolet radiation (UV) potentiates this toxicity. Tests were also conducted to determine if humic acids or if adaptation to UV influenced sensitivity to the cyanide complexes. Rainbow trout (Oncorhynchus mykiss), Daphnia magna, and Ceriodaphnia dubia were exposed to potassium hexacyanocobaltate in the presence and absence of UV radiation, in the presence and absence of humic acids. Cyano-cobalt exposures were also conducted with C. dubia from cultures adapted to elevated UV. With an LC50 concentration of 0.38 mg/L, cyanocobalt was over a 1000 times more toxic to rainbow trout in the presence of UV at a low, environmentally relevant irradiance level (4 microW/cm2 as UVB) than exposure to this compound in the absence of UV with an LC50 of 112.9 mg/L. Toxicity was immediately apparent, with mortality occurring within an hour of the onset of exposure at the highest concentration. Fish were unaffected by exposure to UV alone. Weak-acid dissociable cyanide concentrations were observed in irradiated aqueous solutions of cyanocobaltate within hours of UV exposure and persisted in the presence of UV for at least 96 hours, whereas negligible concentrations were observed in the absence of UV. The presence of humic acids significantly diminished cyanocobalt toxicity to D. magna and reduced mortality from UV exposure. Humic acids did not significantly influence survival among C. dubia. C. dubia from UV-adapted populations were less sensitive to metallocyanide compounds than organisms from unadapted populations. The results indicate that metallocyanide complexes may pose a hazard to aquatic life through photochemically induced processes. Factors that decrease UV exposure such as dissolved organic carbon or increased pigmentation would diminish toxicity.

Nonequilibrium Liquid Crystalline Layered Phase Stabilized by Light

The ability of light to alter/stabilize a particular thermodynamic phase is a power tool to investigate condensed matter from a new dimension. This field of photoinduced phase transitions is currently an important area of research. Being elastically soft and having subtle changes between its many phases a liquid crystal material is an attractive medium to investigate such light-driven phase transitions. The attraction is partly due to the large birefringence changes accompanying these transitions that are useful in developing photonic devices. In all of the cases reported to date, the photoinduced transition always leads to a phase that in any case exists in the thermal cycle. Recently we reported the first exception to such an established phenomenon (Adv. Mater. 2005, 17, 2086). The guest-host ternary mixture consisting of the photoactive azobenzene guest molecules does not exhibit smectic A phase in the absence of UV radiation. However, the smectic A phase is induced and stabilized only in the presence of UV light. In this paper, we map out hitherto unexplored temperature versus UV intensity phase diagrams for various mixtures, which illustrate that light mimics, in a limited sense, the role of a thermodynamic parameter such as, for example, pressure. The threshold UV intensity required to photodrive the appearance of the smectic A phase is seen to have a strong concentration dependence. Our studies also suggest the possibility of observing a double critical point by employing the UV intensity as a fine-tuning parameter.

UV effects on stoichiometry and PUFAs of Selenastrum capricornutum and their consequences for the grazer Daphnia magna

Summary1. The effect of ultraviolet (UV) radiation (280–400 nm) on fatty acid composition and elemental ratios of carbon (C), nitrogen (N) and phosphorus (P) of a unialgal culture of the chlorophyte Selenastrum capricornutum was investigated. Algae were cultured in the presence or absence of UV radiation and were subsequently fed to Daphnia magna to assess potential effects of UV on zooplankton fatty acid composition, growth and reproduction.2. Algal growth rate was substantially reduced by UV radiation, probably because of a severe inhibition of photosynthetic efficiency (measured as optimal quantum yield).3. Algae exposed to UV radiation had a significantly reduced content of 18 : 1 n‐9, while C18 polyunsaturated fatty acids (PUFAs) were higher under UV radiation. These observations point at an increased demand for and synthesis of PUFAs under UV stress.4. The C : P and N : P ratios showed a remarkable decrease in UV‐exposed cells primarily owing to an increased uptake of P. The nutritional quality in terms of both fatty acid composition and stoichiometry was therefore higher in the UV treatment relative to the control.5. Despite the UV‐induced changes in nutritional quality of S. capricornutum, no significant effects on D. magna growth or reproduction were detected. The fatty acid composition of Daphnia fed on UV irradiated algae showed a significantly lower content of 18 : 1 n‐9, but no changes in the essential PUFAs.

Ultraviolet vision and foraging in juvenile bluegill (Lepomis macrochirus)

Ultraviolet (UV) photoreceptors have been reported in a wide variety of freshwater and marine organisms, suggesting that UV vision is prominent in aquatic ecosystems. However, its adaptive significance remains speculative. The present study tested whether the foraging of juvenile bluegill (Lepomis macrochirus) is enhanced in the presence of UV radiation (UVR). Laboratory feeding trials were conducted in a laminar flow tank in which L. macrochirus juveniles between 2.3 and 3.5 cm standard length were fed the cladoceran Daphnia magna. Sighting and striking distances, as well as capture success, were measured in the presence and absence of UVR. Mean sighting and striking distances and capture success did not differ significantly between the two light treatments. There were also no significant differences in the frequency distributions of sighting and striking distance. These results suggest that UV vision may not be used to enhance foraging in L. macrochirus on Daphnia within the size class tested.

Microwave-assisted photo-Fenton decomposition of chlorfenvinphos and cypermethrin in residual water

The coupling of microwave radiation with photo-Fenton reaction for wastewater treatment was evaluated. This strategy was performed in a focused microwave digestion oven with open vessels. Ultraviolet radiation was obtained from microwave lamps (MWL) activated by microwave radiation. The operational conditions were established considering the extent of degradation of chlorfenviphos and cypermethrin used for bovine ticks ( Boophilus michoplus) control. The results were based on residual carbon content, always comparing the procedures in the presence or absence of UV radiation. The combination of MWL and Fenton reaction degraded pesticide residues efficiently (>98%) in about 4 min.

Positive effects of UV radiation on a calanoid copepod in a transparent lake: do competition, predation or food availability play a role?

Zooplankton tolerant to ultraviolet radiation (UVR) could be indirectly affected by UVR through interactions with UV-sensitive species in the same ecosystem. In Lake Giles, Pennsylvania, USA, the calanoid copepod Leptodiaptomus minutus is more UVR tolerant than the cohabiting species Daphnia catawba and Cyclops scutifer. We asked whether L. minutus is affected by UV-induced mortality of a food competitor (D. catawba) or a predator of its nauplii (C. scutifier). We conducted two in situ enclosure experiments with six treatments: L. minutus alone, L . minutus + Daphnia and L. minutus + Cyclops in the presence and absence of UVR. There were few differences in survival among treatments in Experiment 1, which had enhanced food and a cumulative UVR (320 nm) dose of 9.3 kJ m -2 . In Experiment 2, which had ambient food and a UVR (320 nm) dose of 20.0 kJ m -2 , L. minutus survival and reproduction were higher in the + UVR compared to -UVR, regardless of competitors or predators. Chlorophyll a (Chl a) in Experiment 2 was higher in the +UVR than -UVR. While interactions between zooplankton species of differing UVR tolerances are potentially important, these results instead demonstrate that the beneficial UVR effect on L. minutus is independent of concurrent detrimental UVR effects on competitors and predators. Further research on the phytoplankton community is necessary to determine whether UVR alleviates bacterial competition, increases nutrient availability or affects phytoplankton by other mechanisms.

Induction of defenses and within-alga variation of palatability in two brown algae from the northern-central coast of Chile: Effects of mesograzers and UV radiation

Macroalgae possess different defense mechanisms in response to herbivory. Some species produce anti-herbivore secondary metabolites, but production of these substances can be costly. Therefore, algae may produce defensive metabolites only in response to herbivory (inducible defense) or defend particular parts of the alga differentially (within-alga variation). In the present study, we examined whether two species of brown algae from the SE-Pacific show evidence of inducible chemical defense (non-polar compounds) or within-alga variation of defense, which we estimated in form of palatability of differently treated algae to amphipod grazers (with live algae and agar-based food containing non-polar algal extracts). In Glossophora kunthii (C. Agardh) J. Agardh, we observed an increase in palatability after algae were acclimated for 12 days without grazers. Subsequent addition of grazers for 12 days then resulted in a reduction of palatability indicating the existence of inducible defense. After removal of grazers for 12 days, these induced effects again disappeared. The reaction of G. kunthii was triggered even by the mere presence of grazers, which suggests that this alga can respond to waterborne cues by reducing palatability. Effects were only found for agar-based food containing non-polar extracts, but not for live algae, suggesting that some parts of the algae are undefended. Our second experiment on within-alga variation confirmed that only apical (growth region) and basal parts (near the holdfast region) of G. kunthii are defended against herbivores. For the second species, Macrocystis integrifolia Bory, the first experiment revealed no induction of defense, while the second experiment on within-alga variation showed that amphipods avoided basal parts and in particular stipes of M. integrifolia but only in live algae. Although both studied algal species differed substantially in their defensive strategies, their reaction was independent of the presence or absence of UV radiation. Thus, it appears that UV effects play only a minor role in anti-herbivore defense, which is in accordance with most previous studies.

Ultraviolet-radiation-induced graft copolymerization of methyl acrylate onto the sodium salt of partially carboxymethylated guar gum

In an attempt to modify the sodium salt of partially carboxymethylated guar gum (Na-PCMGG; degree of substitution = 0.291), we studied the ultraviolet-radiation-induced graft copolymerization of methyl acrylate with ceric ammonium nitrate as a photoinitiator. The influence of the grafting yield was studied as a function of the different reaction parameters, and the optimum reaction conditions for photografting were determined. The various reaction parameters included the photoinitiator, nitric acid, and monomer (methyl acrylate) concentrations, the reaction time, the temperature, and the amount of the substrate. A kinetic scheme for photografting copolymerization was proposed, and the results were in good agreement with the kinetic scheme. The graft copolymerization of methyl acrylate onto Na-PCMGG (degree of substitution = 0291) in the presence and absence of ultraviolet radiation was also carried out for the study of the efficiency of the photoinitiator. The influence of carboxymethyl groups added to the guar gum molecule on its behavior toward ultraviolet-radiation-induced grafting with methyl acrylate was also investigated. The evidence of photografting was ascertained with IR spectroscopy and scanning electron microscopy techniques. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1977–1986, 2005

Redox transformations of mercury in an Arctic snowpack at springtime

We investigated the springtime temporal dynamics of both total mercury (Hg) and gaseous Hg in snowpacks from the High Arctic. In situ incubation experiments of snow samples indicated that the production of volatile mercury in snow (VMS) was photo-mediated and occurred in the first 3 cm of snow. The newly produced VMS (consisting mainly of elemental Hg) was partly oxidized back to Hg(II) when light intensity declined or in the absence of UV radiation, probably through a chain of reactions involving photo-induced radicals and organic compounds in the surface snow. During a 2 week monitoring of total Hg in surface snow, we observed a sharp increase in total Hg concentrations, reaching levels 11 times higher than background concentrations, likely as a result of an atmospheric mercury depletion event. Stratigraphic depth profiles indicated that this increase was restricted to the first 2 cm of the snowpack. Total Hg levels subsequently decreased by 92%, reaching background concentrations within 2 days after this event. The photoproduction rate of VMS calculated on the basis of this episode could account for subsequent daily loss of total Hg from the surface of the snowpack.

Removing UV‐A and UV‐C Radiation from UV‐B Fluorescent Lamp Emissions. Differences in the Inhibition of Photosynthesis in the Marine Alga Dunaliella tertiolecta Using Chromate Versus Cellulose Acetate‐Polyester Filters¶

ABSTRACTUltraviolet‐B (UV‐B; 280‐320 nm)‐emitting lamps unavoidably emit ultraviolet‐A (UV‐A; 320‐400 nm) and ultraviolet‐C (UV‐C; < 280 nm) radiation. Short‐wavelength‐blocking filters are generally used to limit the wave bands of UV under investigation. The widespread use of such filters means that all exposures to UV‐B radiation will have a significant UV‐A component. Therefore, the physiological effects unique to UV‐B exposure are difficult to clearly isolate. This study presents a method to remove the UV‐A and UV‐C “contamination” using a liquid potassium chromate (K2CrO4) filter, thus allowing more direct assessment of the effects of UV‐B exposure. Cultures of the green marine alga Dunaliella tertiolecta were grown in the absence of UV radiation. Sunlamps supplied the UV radiation for a 24 h exposure (solar radiation was not used in this study). The UV radiation was filtered either by the standard method (i.e. cellulose acetate (CA) with polyester = Mylar controls) or by a liquid filter of potassium chromate. Photosynthetic responses were compared. Major decreases in the ratio of variable to maximal fluorescence in dark‐adapted cells and photosynthetic capacity were observed in CA‐filtered cultures, whereas no change was observed in cells exposed to the same UV‐B flux with the UV‐A removed by K2CrO4. The use of a CA filter with a Mylar control does not link results unequivocally to UV‐B radiation. Such results should be interpreted with caution.

Ethanol and aloe emodin alter the p53 mutational spectrum in ultraviolet radiation-induced murine skin tumors.

Mutations in the p53 tumor-suppressor gene contribute to the development of skin cancer, and the spectrum of mutations in this gene correlates with specific physical and chemical carcinogens in the environment. Cosmetics may contain alcohols and/or aloe emodin (AE). Although these compounds are not carcinogenic when applied to the skin, they may increase the carcinogenicity of ultraviolet (UV) radiation. We investigated whether ethanol (EtOH) and AE alone or combined with UV radiation cause mutations in the p53 gene. In the absence of UV radiation, C3H/HeN mice chronically treated for up to 33 wk with AE in 25% EtOH-in-water vehicle or vehicle alone failed to develop tumors and had no mutations in exons 4-8 of the p53 gene. UV radiation alone induced skin tumors, which had mutations predominantly in p53 exons 5 and 8. In contrast, mutations arising in UV + EtOH-or UV + AE-treated groups were more broadly distributed throughout the p53 gene. Mutations were found in exons 4, 6, and 7, as well as in exons 5 and 8. This altered distribution of mutations across the p53 DNA sequence more closely resembles the pattern observed in TP53 from human skin tumors at sun-exposed sites than that in the p53 gene of mice treated with UV alone. Thus, treatment with UV radiation in combination with two chemicals not thought to be carcinogenic, alcohol, and AE results in a broader distribution of mutations in a critical tumor-suppressor gene.

Quercetin enhances melanogenesis by increasing the activity and synthesis of tyrosinase in human melanoma cells and in normal human melanocytes.

Quercetin (3,3',4',5,7-pentahydroxyflavone) is a diphenyl propanoid widely distributed in edible plants. In this study, we examined the effect of quercetin on melanogenesis in human HMVII melanoma cells and in normal human epidermal melanocytes (NHEM) in the absence of ultraviolet radiation. Upon the addition of quercetin to the culture medium, the melanin content in melanoma cells (HMVII) increased remarkably in time- and dose-dependent manners. In addition, quercetin induced melanogenesis in cultured NHEM. As compared with controls, melanin content was increased about sevenfold by treatment with 20 microM (HMVII) or 1 microM (NHEM) quercetin for 7 d. Tyrosinase activity was also increased, to 61.8-fold higher than the control. The expression of tyrosinase protein was slightly increased by the addition of quercetin. However, quercetin did not affect the expression of tyrosinase mRNA. Tyrosinase activation by quercetin was blocked by actinomycin-D or by cycloheximide demonstrating that its actions in stimulating melanogenesis may involve both transcriptional and translational events. Tyrosinase activity was increased dramatically whereas the level of melanogenic inhibitor was remarkably decreased following quercetin treatment. Taken together, these results demonstrate that in human melanoma cells and in NHEM, quercetin stimulates melanogenesis by increasing tyrosinase activity and decreasing other factors such as melanogenic inhibitors.

Removing UV-A and UV-C Radiation from UV-B Fluorescent Lamp Emissions. Differences in the Inhibition of Photosynthesis in the Marine Alga Dunaliella tertiolecta Using Chromate Versus Cellulose Acetate–Polyester Filters

Ultraviolet-B (UV-B; 280–320 nm)–emitting lamps unavoidably emit ultraviolet-A (UV-A; 320–400 nm) and ultraviolet-C (UV-C; <280 nm) radiation. Short-wavelength–blocking filters are generally used to limit the wave bands of UV under investigation. The widespread use of such filters means that all exposures to UV-B radiation will have a significant UV-A component. Therefore, the physiological effects unique to UV-B exposure are difficult to clearly isolate. This study presents a method to remove the UV-A and UV-C “contamination” using a liquid potassium chromate (K2CrO4) filter, thus allowing more direct assessment of the effects of UV-B exposure. Cultures of the green marine alga Dunaliella tertiolecta were grown in the absence of UV radiation. Sunlamps supplied the UV radiation for a 24 h exposure (solar radiation was not used in this study). The UV radiation was filtered either by the standard method (i.e. cellulose acetate (CA) with polyester = Mylar controls) or by a liquid filter of potassium chromate. Photosynthetic responses were compared. Major decreases in the ratio of variable to maximal fluorescence in dark-adapted cells and photosynthetic capacity were observed in CA-filtered cultures, whereas no change was observed in cells exposed to the same UV-B flux with the UV-A removed by K2CrO4. The use of a CA filter with a Mylar control does not link results unequivocally to UV-B radiation. Such results should be interpreted with caution.

Removing UV-A and UV-C radiation from UV-B fluorescent lamp emissions. Differences in the inhibition of photosynthesis in the marine alga Dunaliella tertiolecta using chromate versus cellulose acetate-polyester filters.

Ultraviolet-B (UV-B; 280-320 nm)-emitting lamps unavoidably emit ultraviolet-A (UV-A; 320-400 nm) and ultraviolet-C (UV-C; <280 nm) radiation. Short-wavelength-blocking filters are generally used to limit the wave bands of UV under investigation. The widespread use of such filters means that all exposures to UV-B radiation will have a significant UV-A component. Therefore, the physiological effects unique to UV-B exposure are difficult to clearly isolate. This study presents a method to remove the UV-A and UV-C "contamination" using a liquid potassium chromate (K(2)CrO(4)) filter, thus allowing more direct assessment of the effects of UV-B exposure. Cultures of the green marine alga Dunaliella tertiolecta were grown in the absence of UV radiation. Sunlamps supplied the UV radiation for a 24 h exposure (solar radiation was not used in this study). The UV radiation was filtered either by the standard method (i.e. cellulose acetate (CA) with polyester = Mylar controls) or by a liquid filter of potassium chromate. Photosynthetic responses were compared. Major decreases in the ratio of variable to maximal fluorescence in dark-adapted cells and photosynthetic capacity were observed in CA-filtered cultures, whereas no change was observed in cells exposed to the same UV-B flux with the UV-A removed by K(2)CrO(4). The use of a CA filter with a Mylar control does not link results unequivocally to UV-B radiation. Such results should be interpreted with caution.

INTERACTIONS BETWEEN UV RADIATION AND TEMPERATURE LIMIT INFERENCES FROM SINGLE‐FACTOR EXPERIMENTS1

The global environment is changing. Substantial shifts in temperature, rainfall, cloud cover, and UV radiation (UVR) are all predicted as a result of anthropogenic activity. Although the actual and potential effects of changes in single environmental variables are being studied intensively, the interactive effects of multiple stressors have received little attention. Here we offer the first experimental evidence of interactive effects between UVR and temperature on germination and growth in multicellular organisms. To address the question of how temperature affects survival and growth of organisms in the presence of UVR, we exposed early life stages of two species of intertidal algae, Alaria marginata Postels et Ruprecht and Fucus gardneri Silva, to four levels of UVR at three temperatures for 56 h. PAR and day length (12:12‐h light:dark) were held constant across all treatments. UVR levels bracketed natural levels, and temperatures were within the range of ambient temperatures. Designated endpoints were germination rate and cell number, and we recorded mortality where survival was nil. Our results support the hypothesis that temperature mediates the net biological effect of UVR and vice versa. For instance, spores of A. marginata were able to survive and grow at 15° C at all UV levels and at 10° C in the absence of UVR but were unable to survive at 10° C in the presence of high levels of UVR. Our results suggest that the ability to predict the effects of global change hinges on understanding interactions among environmental variables, imposing strict limits on inferences made from single‐factor experiments.