Differential responses to UVR by bacterioplankton and phytoplankton from the surface and the base of the mixed layer

SUMMARY 1. We tested the influence of ultraviolet radiation (UVR) and shallow stratification on phytoplankton and bacterioplankton from the surface and the base of the mixed layer in two boreal lakes in north-western Ontario, Canada.
2. We measured phytoplankton biomass and production, bacterioplankton production and plankton respiration after transplantation under three solar radiation treatments: ambient radiation (Photosynthetically active radiation (PAR) + ultraviolet-A (UVA) + ultraviolet-B (UVB)), minus UVB (PAR + UVA) and PAR only. We repeated this experiment on three occasions in each lake during the summer.
3. Solar stress (measured as reduced growth and photoinhibition) was generally only found in the 'base phytoplankton' (i.e. originating from the base of the mixed layer). No inhibition of photosynthesis by UVB exposure was found in near-surface phytoplankton. On the other hand, production of near-surface bacterioplankton was reduced following a 4-h UVR exposure but had increased after a 48-h exposure to both UVA and UVB compared with the PAR only treatment.
4. Negative effects of UVR on phytoplankton and bacterioplankton were not ubiquitous. We emphasise the importance of conducting experiments repeatedly, particularly those which test the effects of UVR on different community assemblages from different lakes.     

Effects of ultraviolet radiation on the eggs of landlocked Galaxias maculatus (Galaxiidae, Pisces) in northwestern Patagonia

• 1 Ultraviolet radiation (UVR) damages early life stages of several fish species. Galaxias maculatus is a small catadromous fish, with landlocked forms occurring in many lakes within the Nahuel Huapi National Park (Patagonia, Argentina). In this work, the vulnerability of G. maculatus eggs exposed to both natural and artificial UVR was investigated in relation to water transparency.
• 2 Field experiments were performed in two lakes differing in UVR attenuation. Galaxias maculatus eggs were exposed to in situ levels of UVR in quartz tubes incubated at various depths. For laboratory experiments, the eggs were exposed to five levels of artificial UVB radiation.
• 3 Exposure to natural UVR causes various degrees of egg mortality depending on water transparency and incubation depth. In the less transparent lake (K_d320 = 3.08 m^‐1), almost complete mortality was observed near the surface. At a depth of 43 cm the observed mortality was only 22%, but was still significantly different from the dark control. In the most transparent lake (K_d320 = 0.438 m^‐1), almost total mortality was observed in tubes incubated at 2.56 m or shallower. A gradual decline in mortality was recorded from that depth to 3.78 m where the values approached those in the dark control treatments.
• 4 A monotonic relationship between mortality and UV exposure could be observed both in field and laboratory experiments. Using the results from field incubations, a LD₅₀ of 2.5 J cm^‐2 nm^‐1 was estimated. In a few mountain lakes, this value would be exceeded even if the eggs were laid at the maximum depth of the lake. Thus UVR seems a sufficient cause to explain the absence of G. maculatus populations in some mountain lakes. For most lakes, however, UVR is probably one of several important environmental factors, which together determine the habitat suitability.

     

Yeasts from an oligotrophic lake in Patagonia (Argentina): diversity, distribution and synthesis of photoprotective compounds and extracellular enzymes

Nahuel Huapi (NH) Lake is an oligotrophic temperate lake of glacial origin with high transparency, surrounded by well-developed forests and located at San Carlos de Bariloche, Nahuel Huapi National Park, in Patagonia, Argentina. In this lake, we characterized yeast distribution and diversity along a south-to-north transect and established a relationship between the ability to produce photoprotective compounds (PPCs) (carotenoid pigments and mycosporines) and the occurrence of yeast at different collection points. Subsurface water samples were filtered for yeast isolation. Total yeast counts ranged between 22 and 141?CFU?L(-1) , and the highest values corresponded to the most impacted sites. Littoral sites had a low proportion of yeast-producing PPCs and this group prevailed in pelagic sites. This is probably a result of the high transparency of the water and the increased UV exposure. The yeast community from NH Lake showed a high species richness and a uniform distribution of taxa between pelagic and border collection points. Yeasts were identified as belonging to 14 genera and 34 species. Rhodotorula mucilaginosa and Cryptococcus victoriae were the most frequently found species, representing 14.4% and 13.6% of the total yeast isolates, respectively. Most of the yeast isolates demonstrated at least one extracellular enzymatic activity (mainly cellulase and lipase activities), which suggested that these microorganisms are metabolically active in the lake.     

Preferential accumulation of carotenoids rather than of mycosporine-like amino acids in copepods from high altitude Himalayan lakes

Zooplankton species have evolved several adaptive strategies to minimize damage caused by exposure to solar ultraviolet radiation, but the environmental conditions favoring one strategy or another are not yet fully understood. Here, I quantified the concentration of photoprotective compounds (carotenoids and mycosporine-like amino acids or MAAs) and assessed the photorepair activity (photolyase assay) in populations of the calanoid copepod, Arctodiaptomus jurisowitchi and the cladocerans, Daphnia himalaya and D. longispina, from five high altitude lakes located in the Himalayan Region (Khumbu Valley, Nepal) between 4890 and 5440 m above sea level. The concentration and diversity of MAAs were low in copepods, as well as in seston samples. Significant differences in the concentration of MAAs among the five copepod populations were largely explained (96%) by the lake depth refuge (i.e., the fraction of the water column to which 1% of the surface UVR at 320 nm penetrates). Concentrations of carotenoids (mostly free astaxanthin) in copepods were among the highest reported in the literature. Similar to MAAs, the carotenoid concentration was inversely related to the lake depth refuge. The lowest concentration of photoprotective compounds in copepods was observed in a turbid glacier lake, whereas the highest was found in a shallow water body dominated by a benthic mat of filamentous green algae. Except for the presence of melanin in D. himalaya, no other photoprotective compounds were found in cladocerans. The assay of photolyase activity in A. jurisowitchi and D. himalaya suggested the absence of a photorepair mechanism. The results of this study indicate that the copepod populations from this relatively pristine alpine region rely mainly on the accumulation of carotenoids to minimize damage by UV radiation, a pattern that strongly contrasts with what is known for copepods from other alpine lakes, for instance, in the Alps. I hypothesize that this difference is attributed to nitrogen limitation of the MAA synthesis in phytoplankton from remote Himalayan lakes.     

UV radiation and potential biological effects beneath the perennial ice cover of an antarctic lake

High-resolution spectral scans of solar ultraviolet radiation (UVR) were obtained directly beneath the 4.0–5.0 m thick, perennial ice cover of Lake Hoare, South Victoria Land, Antarctica. Both UVA (320–400 nm) and UVB (280–320 nm) radiation were detectable beneath the ice using a diver-deployed, underwater scanning spectroradiometer which permitted accurate measurement in the 280–340 nm range, while avoiding effects of surface shading and/or hole effects. UVR at wavelengths <310 nm was not detectable below the ice. This lower wavelength UVB appears to penetrate the Lake Hoare ice to depths of no more than 1.5 m during relatively cloud-free austral summer days. Based upon estimated biologically effective UVR dosages and DNA dosimeter data, exposure of benthic and planktonic microbes to the UVR encountered immediately beneath the ice is unlikely to inhibit microbial metabolism. Although waters of oligotrophic antarctic lakes are highly transparent to UVR, the thick, high scattering and optically dense ice covers on many of these lakes offers organisms a degree of protection largely unavailable in temperate and tropical systems. Thinning or complete loss of these overlying ice covers is likely to have major consequences for the structure of antarctic lake microbial communities.     

Role of dissolved organic carbon in the attenuation of photosynthetically active and ultraviolet radiation in Adirondack lakes

1. We surveyed eighty-five lakes located in the Adirondack Mountain Region of New York State, U.S.A., to characterize the attenuation of photosynthetically active (PAR) and ultraviolet radiation (UVR) in relation to dissolved organic carbon (DOC) concentrations and pH. Attenuation of PAR was quantified in situ . Attenuation was also inferred by measuring the light absorption of filtered lake water samples at wavelengths (300, 340 and 440 nm) representing UV-B, UV-A and PAR.
2. Substantial variation in transparency was observed among lakes in this region. Attenuation depths ( z _1%) for PAR ranged from 0.5 to greater than 20 m, while inferred values for UV-B and UV-A ranged from a few centimetres to > 5 m. Median values of UV-A penetration (0.75 m) and UV-B penetration (0.45 m) corresponded to 11% (UV-A) and 6% (UV-B) of lake maximum depth.
3. Much of the variation in PAR and UVR attenuation was explained by differences in lake DOC. Univariate power models based solely on DOC accounted for 85% (PAR), 90% (UV-A) and 91% (UV-B) of the variation in absorption.
4. Attenuation and absorption coefficients were generally lower for recently acidified lakes compared to acidic and circumneutral lakes which have not undergone recent acidification. However, differences among these three groups of lakes were not statistically significant. Our results suggest that the effects of acidification on the optical properties of a regional population of lakes, even in an area experiencing widespread acidification, are relatively subtle in comparison with other factors contributing to inter-lake variability.
5. The presence of near-shore wetlands is probably a key factor influencing regional variability in DOC and light climate among Adirondack lakes. Temporal variability in climatic factors influencing wetland DOC production and export may mask more subtle influences on lake DOC associated with anthropogenic acidification.     

Photoprotective compounds in weakly and strongly pigmented copepods and co-occurring cladocerans

1. The prevalence of mycosporine‐like amino acids (MAAs) – a group of potential ultraviolet (UV)‐photoprotective compounds – was surveyed across 11 species of freshwater copepods from 20 lakes of varying ultraviolet radiation (UVR) transparency in North America, New Zealand and Argentina. Co‐occurring cladocerans were also analysed (seven species from 12 lakes). Many of the calanoid copepod populations were red with carotenoid pigmentation, allowing comparison of MAA and carotenoid accumulation as photoprotective strategies. 2. In two Pennsylvania (U.S.A.) lakes, MAA and carotenoid contents were followed during the early spring to mid‐summer period of lake warming. A pronounced seasonal pattern of higher carotenoid/low MAA content in spring, shifting to low carotenoid/higher MAA content in summer, was observed in calanoids from the more UV transparent lake. 3. All copepod samples contained MAAs. Visibly red calanoids, especially southern Hemisphere Boeckella, often had moderate to high concentrations (2.5–11 μg MAA mg^?1 dw), but low concentrations (0.04–1 μg MAA mg^?1 dw) in some N. American red calanoids show that high carotenoid pigmentation (e.g. 5–10 μg carotenoid mg^?1 dw) does not necessarily imply high MAA content. 4. No cladoceran sample had more than trace amounts of MAAs (<0.05 μg mg^?1 dw). Therefore, MAA accumulation does not seem to be a photoprotective strategy utilized by Daphnia (five species from nine lakes) or other cladocerans. 5. Seven identifiable MAAs were widely distributed among both calanoids and cyclopoids. Shinorine was ubiquitous and was usually the most abundant MAA in N. American samples. In contrast, porphyra‐334 was the predominant MAA in the southern Hemisphere Boeckella. 6. Copepods from higher UVR lakes tended to have a higher MAA content, but this relationship was statistically weak overall and taxon‐specific when found.     

Mycosporines from freshwater yeasts: a trophic cul-de-sac?

Mycosporine-like amino-acids (MAAs) are found in aquatic bacteria, algae, and animals. A related compound, the mycosporine-glutaminol-glucoside (myc-glu-glu), has recently been reported in freshwater yeasts. Although animals depend on other organisms as their source of MAAs, they can efficiently accumulate them in their tissues. In this work we assessed the potential transfer of the yeast mycosporine myc-glu-glu from the diet into the copepod Boeckella antiqua and the ciliate Paramecium bursaria. For this purpose, we performed experiments to study the feeding of B. antiqua and P. bursaria on the yeast Rhodotorula minuta and their ability to bioaccumulate myc-glu-glu. Bioaccumulation of myc-glu-glu in B. antiqua was assessed through long-term factorial experiments manipulating the diet (Chlamydomonas reinhardii and C. reinhardii + yeasts) and radiation exposure (PAR and PAR + UVR). Shorter term experiments were designed in the case of P. bursaria. The composition and concentration of MAAs in the diet and in the consumers were determined by HPLC analyses. Our results showed that even though both consumers ingested yeast cells, they were unable to accumulate myc-glu-glu. Moreover, when exposed to conditions that stimulated the accumulation of photoprotective compounds (i.e. UVR exposure), an increase in MAAs concentration occurred in copepods fed C. reinhardii plus yeasts as well as in those fed only C. reinhardii. This suggests that the copepods were able to modify their tissue concentrations of MAAs in response to environmental clues but also that the contribution of yeast mycosporines to total MAAs concentration was negligible.     

Inhibition of phytoplankton photosynthesis by solar ultraviolet radiation: studies in Lake Titicaca, Bolivia

1. Lake Titicaca is a large, high altitude (3810 m a.s.l.) tropical lake (16°S, 68°W) that lies on the border of Bolivia and Perú, receiving high fluxes of ultraviolet radiation (UVR) throughout the year. Our studies were conducted during September of 1997 with the main objective of studying the impact of solar UVR upon phytoplankton photosynthesis.
2. Water samples were taken daily and incubated in situ (down to 14 m depth) under three radiation treatments to study the relative responses to PAR (Photosynthetic Available Radiation, 400–700 nm), UV-A (320–400 nm), and UV-B (280–320 nm) radiation.
3. Photosynthetic inhibition by UVR in surface waters was about 80%, with UV-A accounting for 60% and UV-B for 20%; the inhibition by high levels of PAR was less than 20%. The inhibition due to UVR decreased with depth so that there were no significant differences between treatments at 8.5 m depth.
4. The amount of inhibition per unit energy received by phytoplankton indicates that even though there was a significant inhibition of photosynthesis due to UVR, species in Lake Titicaca seem to be better adapted than species in high latitude environments.
5. The cellular concentration of UV-absorbing compounds, a possible mechanism of photoadaptation, was low in phytoplanktonic species. However, they were abundant in zooplankton, suggesting a high rate of bioaccumulation through the diet.     

Relationships between Fish Species Abundances and Water Transparency in Hypertrophic Turbid Waters of Temperate Shallow Lakes

We explored the relationships between Secchi disc depth and the abundance of fish species in very shallow, hypertrophic, turbid waters of Pampa Plain lakes, Argentine. We tested whether the abundance of any of the species present was associated with water transparency for lakes where water transparency, as measured by Secchi disc depth, ranged from 0.1 to 0.4 m. Overall, the abundance of five species (Cnesterodon decemmaculatus, Jenynsia multidentata, Corydoras paleatus, Pimelodella laticeps and Odontesthes bonariensis) seemed to be affected by this narrow gradient in water transparency. These findings represent an interesting result for turbid hypertrophic environments where narrow ranges in water transparency are traditionally neglected as important factors for fishes. We show, however, how water transparency patterns may be still important for some species in highly turbid waters with extremely narrow gradients in Secchi disc depth. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Vertical distribution and pigmentation of Antarctic zooplankton determined by a blend of UV radiation, predation and food availability

Selection pressure induced by simultaneously occurring environmental threats is a major evolutionary driver for organisms in terrestrial, as well as in aquatic ecosystems. For example, protection against ultraviolet radiation (UVR) and predation include both morphological and behavioral components. Here we address those selective pressures on zooplankton by performing a latitudinal monitoring, combined with mechanistic experiments in the Antarctic Southern Ocean, where the UVR-threat is extremely high. We assessed vertical distributions of zooplankton along the Antarctic coast showing that animals were most abundant at 20–80 m and tended to avoid the surface at sites with clear water. UVR-threat disappeared at between 9 and 15 m at sites with low and high water transparency, respectively. Light levels were, however, sufficient for visual fish predation down to approximately 19 and 37 m, respectively. The few zooplankton that were present in surface waters had high levels of non-pigmented UVR-protective compounds (mycosporine-like amino acids) compared to deeper dwelling zooplankton. Overall they had low levels of red pigmented UVR-protective compounds (carotenoids), suggesting high predation on pigmented individuals. In a complementary laboratory study we showed that levels of UVR-protective compounds increased considerably when zooplankton were exposed to UVR in the absence of predator cues. The recently developed transparency-regulator hypothesis predicts that UVR avoidance is an important driver to diel vertical migration in transparent waters, such as in Antarctica. We, however, conclude that copepods resided well below the level where UVR had diminished to very low levels and that predator avoidance or food availability are more likely drivers of zooplankton vertical depth distribution in transparent marine systems.     

Penetration of solar ultraviolet radiation into New Zealand lakes: influence of dissolved organic carbon and catchment vegetation

Eleven lakes in the South Island of New Zealand were sampled in summer 1996. Water column profiles of ultraviolet radiation (UVR) at four wavelengths and photosynthetically available radiation (PAR) were obtained, along with analyses of dissolved organic carbon (DOC) concentration, total suspended solids (TSS), and catchment vegetation, including forest and natural grassland. Downward attenuation coefficients (K _d) and lake water transparency (1/K _d) for UVR were examined in relation to these variables. Consistent with other regions of the world, DOC concentration and variables related to DOC were the best predictors of UVR penetration. With our data set, we calculated ratios of water column integrals (RI) of UVR/PAR irradiance, using equations from the literature. At DOC concentrations below 4 g m⁻³, a progressive increase in RI shows that lakes become increasingly transparent to UVR. We also normalized chromophoric dissolved organic matter (CDOM) absorption of UVR at 380 nm (a ₃₈₀) to DOC concentration and found that the UVR-absorbing capacity per unit DOC increases with increasing percentage of forest in the catchment area. This indicates that not only DOC concentration but also DOC type or composition is important in determining the transparency of lake water to UVR, and that qualitative differences in DOC are dictated by the type and amount of vegetation present in the lake's catchment area. Received: September 18, 2000 / Accepted: December 14, 2000     

Acute effects of UVR on human eyes and skin

Solar UVR ( approximately 295-400 nm) has acute clinical effects on the eyes and the skin. The only effect on the eye is inflammation of the cornea (photokeratitis), which is caused by UVB (and non-solar UVC) and resolves without long-term consequences within 48 h. The effects on the skin are more extensive and include sunburn (inflammation), tanning and immunosuppression for which UVB is mainly responsible. Tanning is modestly photoprotective against further acute UVR damage. Skin colour is also transiently changed by UVA-dependent immediate pigment darkening, the function of which is unknown. Skin type determines sensitivity to the acute and chronic effects of UVR on the skin. Some of the photochemical events that initiate acute effects are also related to skin cancer. Solar UVB is also responsible for the synthesis of vitamin D.     

UVB photoprotective role of mycosporines in yeast: photostability and antioxidant activity of mycosporine-glutaminol-glucoside

Several yeast species are able to synthesize and accumulate UV-radiation-absorbing mycosporine metabolites that are of unclear physiological function. In this work we analyzed the relationship between mycosporine-glutaminol glucoside (MGG) production, cell survival after UVB irradiation, and formation of cyclobutane pyrimidine dimers (CPDs). We also assessed the photostability and singlet oxygen quenching activity of MGG. A set of nine isolates of the basidiomycetous yeast Cryptococcus steppossus cultured in both dark and light conditions was used for the studies. Survival of the UVB-irradiated isolates and MGG concentration had a linear relationship when the concentration was over 2.5 mg g(-1). CPD accumulation and MGG accumulation were inversely related. MGG in aqueous solution was photostable with a photodecomposition quantum yield of 1.16 × 10(-5). MGG quenching of singlet oxygen was also observed, and the rate constant for the process in D(2)O was 5.9 × 10(7) M(-1) s(-1). Our results support the idea that MGG plays an important role as a UVB photoprotective metabolite in yeasts by protecting against direct damage on DNA and probably against indirect damage by singlet oxygen quenching.     

Mycosporine-like amino acids from coral dinoflagellates

Coral reefs are one of the most important marine ecosystems, providing habitat for approximately a quarter of all marine organisms. Within the foundation of this ecosystem, reef-building corals form mutualistic symbioses with unicellular photosynthetic dinoflagellates of the genus Symbiodinium. Exposure to UV radiation (UVR) (280 to 400 nm) especially when combined with thermal stress has been recognized as an important abiotic factor leading to the loss of algal symbionts from coral tissue and/or a reduction in their pigment concentration and coral bleaching. UVR may damage biological macromolecules, increase the level of mutagenesis in cells, and destabilize the symbiosis between the coral host and their dinoflagellate symbionts. In nature, corals and other marine organisms are protected from harmful UVR through several important photoprotective mechanisms that include the synthesis of UV-absorbing compounds such as mycosporine-like amino acids (MAAs). MAAs are small (<400-Da), colorless, water-soluble compounds made of a cyclohexenone or cyclohexenimine chromophore that is bound to an amino acid residue or its imino alcohol. These secondary metabolites are natural biological sunscreens characterized by a maximum absorbance in the UVA and UVB ranges of 310 to 362 nm. In addition to their photoprotective role, MAAs act as antioxidants scavenging reactive oxygen species (ROS) and suppressing singlet oxygen-induced damage. It has been proposed that MAAs are synthesized during the first part of the shikimate pathway, and recently, it has been suggested that they are synthesized in the pentose phosphate pathway. The shikimate pathway is not found in animals, but in plants and microbes, it connects the metabolism of carbohydrates to the biosynthesis of aromatic compounds. However, both the complete enzymatic pathway of MAA synthesis and the extent of their regulation by environmental conditions are not known. This minireview discusses the current knowledge of MAA synthesis, illustrates the diversity of MAA functions, and opens new perspectives for future applications of MAAs in biotechnology.     

Interactive effects of UV radiation and water availability on seedlings of six woody Mediterranean species

To assess the effects of UV radiation and its interaction with water availability on Mediterranean plants, we performed an experiment with seedlings of six Mediterranean species (three mesophytes vs three xerophytes) grown in a glasshouse from May to October under three UV conditions (without UV, with UVA and with UVA+UVB) and two irrigation levels (watered to saturation and low watered). Morphological, physiological and biochemical measures were taken. Exposure to UVA+UVB increased the overall leaf mass per area (LMA) and the leaf carotenoids/chlorophyll a + b ratio of plants in relation to plants grown without UV or with UVA, respectively. In contrast, we did not find a general effect of UV on the leaf content of phenols or UVB‐absorbing compounds of the studied species. Regarding plant growth, UV inhibited the above‐ground biomass production of well‐watered plants of Pistacia lentiscus. Conversely, under low irrigation, UVA tended to abolish the reduction in growth experienced by P. lentiscus plants growing in a UV‐free environment, in accordance with UVA‐enhanced apparent electron transport rate (ETR) values under drought in this species. UVA also induced an overall increase in root biomass when plants of the studied species were grown under a low water supply. In conclusion, while plant exposition to UVA favored root growth under water shortage, UVB addition only gave rise to photoprotective responses, such as the increase in LMA or in the leaf carotenoids/chlorophyll a + b ratio of plants. Species‐specific responses to UV were not related with the xerophytic or mesophytic character of the studied species.     

In vivo relevance for photoprotection by the vitamin D rapid response pathway

Vitamin D is produced by exposure of 7-dehydrocholesterol in the skin to UV irradiation (UVR) and further converted in the skin to the biologically active metabolite, 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) and other compounds. UVR also results in DNA damage producing cyclobutane pyrimidine dimers (CPD). We previously reported that 1,25(OH)₂D₃ at picomolar concentrations, protects human skin cells from UVR-induced apoptosis, and decreases CPD in surviving cells. 1,25(OH)₂D₃ has been shown to generate biological responses via two pathways—the classical steroid receptor/genomic pathway or a rapid, non-genomic pathway mediated by a putative membrane receptor. Whether the rapid response pathway is physiologically relevant is unclear. A cis-locked, rapid-acting agonist 1,25(OH)₂lumisterol₃ (JN), entirely mimicked the actions of 1,25(OH)₂D₃ to reduce fibroblast and keratinocyte loss and CPD damage after UVR. The effects of 1,25(OH)₂D₃ were abolished by a rapid-acting antagonist, but not by a genomic antagonist. Skh:hr1 mice exposed to three times the minimal erythemal dose of solar-simulated UVR and treated topically with 1,25(OH)₂D₃ or JN immediately after UVR showed reduction in UVR-induced UVR-induced sunburn cells (p < 0.01 and <0.05, respectively), CPD (p < 0.01 for both) and immunosuppression (p < 0.001 for both) compared with vehicle-treated mice. These results show for the first time an in vivo biological response mediated by a rapid-acting analog of the vitamin D system. The data support the hypothesis that 1,25(OH)₂D₃ exerts its photoprotective effects via the rapid pathway and raise the possibility that other D compounds produced in skin may contribute to the photoprotective effects.     

The influence of predator community composition on photoprotective traits of copepods

Trait expression of natural populations often jointly depends on prevailing abiotic environmental conditions and predation risk. Copepods, for example, can vary their expression of compounds that confer protection against ultraviolet radiation (UVR), such as astaxanthin and mycosporine‐like amino acids (MAAs), in relation to predation risk. Despite ample evidence that copepods accumulate less astaxanthin in the presence of predators, little is known about how the community composition of planktivorous fish can affect the overall expression of photoprotective compounds. Here, we investigate how the (co‐)occurrence of Arctic charr (Salvelinus alpinus) and threespine stickleback (Gasterosteus aculeatus) affects the photoprotective phenotype of the copepod Leptodiaptomus minutus in lake ecosystems in southern Greenland. We found that average astaxanthin and MAA contents were lowest in lakes with stickleback, but we found no evidence that these photoprotective compounds were affected by the presence of charr. Furthermore, variance in astaxanthin among individual copepods was greatest in the presence of stickleback and the astaxanthin content of copepods was negatively correlated with increasing stickleback density. Overall, we show that the presence and density of stickleback jointly affect the content of photoprotective compounds by copepods, illustrating how the community composition of predators in an ecosystem can determine the expression of prey traits that are also influenced by abiotic stressors.     

Zooplankton behavioral responses to solar UV radiation vary within and among lakes

Zooplankton taxa exhibit varying tolerances to ultraviolet radiation(UVR), with rotifers and copepods tending to be more UV tolerantthan cladocerans, regardless of lake UV transparency. On thebasis of these differences, we hypothesized that UV avoidancebehavior would be greatest in zooplankton with low UV tolerance,particularly in high-UV systems. Both downward and upward movementsof zooplankton were examined in cylindrical acrylic columnsthat either transmitted full sunlight or blocked UV-B and shortwavelength UV-A radiation. Columns were suspended verticallyin the surface waters of a high- and low-UV lake at varyingtimes of day and cloud cover. In the high-UV lake, the cladoceranDaphnia catawba displayed a distinct avoidance of the surfacewaters in the UV+ treatment but often showed a preference forthe surface in the UV– treatment. Copepods were less responsiveto UVR with calanoids displaying a somewhat greater UV avoidancethan cyclopoids. In the low-UV lake, no behavioral differenceswere observed among UV treatments. These results suggest thatUVR may influence the vertical distribution and habitat partitioningof certain zooplankton taxa in high-UV lakes while other factors,such as predation and food limitation, may be more importantin low-UV lakes.     

A (too) bright future? Arctic diatoms under radiation stress

Decreasing Arctic sea ice cover and increasing stratification of ocean surface waters make the exposure of pelagic microalgae to high irradiances more likely. Apart from light being a necessary prerequisite for photosynthesis, rapidly changing and/or high irradiances are potentially detrimental. An in situ study was performed in the high Arctic (79°N) to determine the effect of high irradiances in general, and ultraviolet radiation (UVR, 280–400 nm) in particular, on cell concentrations, fatty acid composition, and photoprotective pigments of three diatom species isolated from seawater around Svalbard. Unialgal cultures were exposed in situ at 0.5- and 8 m-depth. After 40 h, cell concentrations of Synedropsis hyperborea and Thalassiosira sp., were lower at 0.5 than at 8 m, and the content of the photoprotective xanthophyll-cycle pigment diatoxanthin in all species (S. hyperborea, Thalassiosira sp., Porosira glacialis) was higher in the 0.5 m exposure compared to 8 m. In S. hyperborea, growth was additionally inhibited by UVR at 0.5-m depth. In situ radiation conditions led, furthermore, to a significant decrease in polyunsaturated fatty acids (PUFAs) in all three species, but UVR had no additional effect. Hence, we conclude that natural radiation conditions close to the surface could reduce growth and PUFA concentrations, but the effects are species specific. The diatoms’ potential to acclimate to these conditions over time has to be evaluated.