Journal of Experimental Marine Biology and Ecology 411 (2012) 45–51

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Journal of Experimental Marine Biology and Ecology

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UV radiation and its effects on P-uptake in arctic diatoms

Dag O. Hessen a,⁎, Helene Frigstad b,c, Per J. Færøvig a, Marcin W. Wojewodzic a, Eva Leu d

a University of Oslo, Dept. Biology, P.O. Box 1066 Blindern, 0316 Oslo, Norwayb Geophysical Institute, University of Bergen, Allégaten 70, 5007 Bergen, Norwayc Bjerknes Centre for Climate Research, University of Bergen, Allégaten 55, 5007 Bergen, Norwayd Norwegian Polar Institute, N-9296 Tromsø, Norway

⁎ Corresponding author. Tel.: +47 22854553.E-mail address: dag.hessen@bio.uio.no (D.O. Hessen

0022-0981/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.jembe.2011.10.028

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 July 2011Received in revised form 25 October 2011Accepted 28 October 2011Available online 25 November 2011

Keywords:Arctic diatomsNutrient uptakeSolar radiationStoichiometry

Due to severe alterations of the physical conditions in Arctic ice-covered ecosystems (decrease of sea ice,thinning of the ozone layer), the underwater light climate is changing both with respect to its intensityand spectral composition. It is commonly observed that phytoplankton respond differently to photosynthet-ically active radiation (PAR, 400-700 nm) and ultraviolet radiation (UVR, 280–400 nm) in terms of their ele-mental stoichiometry, where high levels of PAR tend to increase the ratios of carbon to phosphorus (C:P),while UVR has the opposite effect. Since this has importance not only for elemental cycling and P-sequestration, but also for the algal food quality for grazers, it is of considerable interest to reveal the roleof different spectral regimes of elemental uptake and stoichiometry. There are ambiguous evidence as towhether UVR stimulates or reduces the uptake of inorganic P, and to test this for three common, arctic marinediatoms; Porosira glacialis, Thalassiosira sp. Synedropsis hyperborean, we performed 33P-uptake assays withexposure to either PAR alone or PAR+UVR. Neither of the species showed strong P-uptake responses toUVR exposure, yet the two former species had a negative trend, while Synedropsis, showed a slight increasein its P-uptake under moderate UVR exposure. The latter species also had a remarkably fast P-uptake kinetics.These mixed results indicate a species-specific complex role of P in the algal response to UVR induced lightstress, where not only species affinities but also ambient conditions may yield different outcomes.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The cycling of key elements like carbon (C), nitrogen (N) and phos-phorus (P) in marine systems depends to a large extent on productivityand fate of marine autotrophs. Light is the key factor regulating the ab-solute and relative uptake of these elements in autotrophs, both withrespect to its intensity or quantity, but also in terms of its spectral prop-erties. Recent studies have demonstrated an inverse effect of ultravioletradiation (UVR, 280–400 nm) and photosynthetically active radiation(PAR, 400–700 nm)with regard to elemental ratios, notably C:P, in phy-toplankton (Hessen et al., 2008). Under high PAR intensities but lowlevels of inorganic phosphorus (P), a disproportionate fixation of C rel-ative to P has been demonstrated for freshwater phytoplankton, yield-ing an increased cellular C:P-ratio (Hessen et al., 2002; Sterner et al.,1997; Urabe and Sterner, 1996; Urabe et al., 2002). A skewed uptakeof C relative to P or N commonly causes an accumulation of C rich stor-age compounds (Berman-Frank and Dubinsky, 1999), which may haveconsequences for autotroph C-sequestration as well as the transport ofbiomass and energy through the food web. (Sterner and Hessen, 1994;Sterner and Elser, 2002). UVR and P combinedmay also affect algal fatty

).

rights reserved.

acid composition, especially the highly unsaturated fatty acids whichare crucial for zooplankton nutrition (cf. Leu et al., 2006a,b; Villar-Argaiz et al., 2009).

In contrast, UVR seems to promote reduced C:P-ratios both infreshwater and marine phytoplankton (Hessen et al., 2008; Leu etal., 2006a, 2007), yet the mechanisms behind this are not fully under-stood. Whereas the capacity of UVR to reduce photosynthetic activityand thus C-uptake is clearly documented (Neale et al., 2003), its rolein nutrient uptake and cellular stoichiometry is less well understood.Phytoplankton cells exposed to UVR undergo a series of physiologicalchanges that influence cell volume and intracellular morphology, aswell as biochemical pathways and products, eventually affecting nu-trient uptake as well. Reduced uptake of inorganic N under UV-stress has been reported by some authors (Braune and Döhler,1994; Döhler and Biermann, 1987), while other studies found ratherweak or even no effects of UV on C:N ratios in phytoplankton(Fauchot et al., 2000; Mousseau et al., 2000). Wängberg et al.(1998) reported decreased photosynthesis but increased P-uptakein marine phytoplankton communities under UV exposure. Recentfield studies have also demonstrated decreased C:P-ratios in phyto-plankton and epilithic communities in lakes under UV-stress (Tanket al., 2003; Watkins et al., 2001; Xenopoulos et al., 2002), althoughin situ experiments suggest that this effect is weak under low ambientP concentrations (Frost and Xenopoulos, 2002).

46 D.O. Hessen et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 45–51

The response to UVR in general differs between taxa and species,and typically small species of diatoms are more vulnerable thanlarge species (e.g. Karentz et al. 1991). Also their nutritional statusand not the least the intensity or dose-rate of UVR may affect thenet uptake of nutrients. Hessen et al. (1995) observed increased up-take of 33P in flagellates under low to moderate UV exposure, andaccredited this to increased demands for P for nucleotide or mem-brane repair. At very high doses of UV, the uptake of P was impaired,however, in line with the findings of Aubriot et al. (2004). These ob-servations suggest that not only the photon flux per se, but also thePAR:UV ratio may play a crucial role for productivity and biomass ofautotrophs, and also govern their nutritional quality in terms of ele-mental composition. This in turn will affect nutrient cycling and tro-phic transfer efficiency to higher trophic levels. To the extent thatUVR affect P-uptake and growth rate, it should also be assumed to af-fect RNA and RNA:protein ratio (cf. Sterner and Elser, 2002), andhence we included an assays on RNA:protein dynamics related tolight exposure.

Both with regard to the physiological response in the phytoplank-ton cells, and due to its relevance for the uptake and cycling of C and Pin the sea and nutrient competition between autotrophs and prokary-otic heterotrophs, it is of considerable interest to reveal if thereported decrease in C:P under UVR exposure relates primarily to re-duced C-fixation, or whether the exposed cells also have an enhanceduptake of P. This is accentuated by the continued ice loss in the Arcticas well as ozone anomalies and minima (Manney et al., 2011).

To test for a potential stimulating effect of UVR on P-uptake inhigh latitude diatoms, we performed radioactive 33P-tracer uptakeexperiments with and without UVR for three common diatom speciesisolated from the high Arctic.

2. Materials and methods

The unialgal cultures used in this experiment were isolated fromwaters around the Svalbard archipelago in 2007. Two of the threespecies are pelagic, centric diatoms that are usually occurring inchain-forming colonies: Porosira glacialis, and Thalassiosira sp.,where the latter has a rather small cell size for diatoms (~10 μm).The third species, Nitzschia frigida: is a pennate diatom, most often oc-curring as an epiphyte on the common sea ice algae species.

All cultures were kept at 3 °C in f/10 media (a fivefold dilution off/2 medium, Guillard, 1975). In the treatment with surplus phospho-rus (marked+P), 1 mg L−1 NaH2PO4 according to the f/10 concen-trations was added, while the medium for the algae grown under Plimitation (marked −P) contained only background concentrationsof P found in filtered seawater that was used for media preparation.All other nutrients were added as in f/10. A light:dark cycle of16:8 h was applied, with 55 μmol m−2 s−1 of PAR.

The experimental protocol followed that of Leu et al. (2007), anddetails of experimental setup can be found here. In brief, the experi-ments were conducted in a thermostatically controlled room at3−4 °C, using 500 mL quartz bottles (i.e. UV-transparent) coveredwith different cutoff foils (see below). Photosynthetically active radi-ation (PAR) was provided by one daylight fluorescent tube (OSRAMLumilux de luxe 36W/950 daylight) under a 16 h light:8 h darkcycle, and measured with a 4 π sensor (QSL-100, Biospherical Instru-ments, Inc.). UVR was provided by two Q-Panel UVA-340 fluorescenttubes (Q-Panel Lab Products, Cleveland, USA), and measured with a IL1400A radiometer (International Lights, Inc., Newburyport, MA, USA)equipped with a SPS 300 sensor (UVB) and a SUL 033 sensor for UVA.Since the SUL 033 sensor has no sharp cutoff at 320 nm, we usedMylar foil for measuring the exact UVA intensities. The whole setupwas coated with aluminum foil, in order to provide a light field as ho-mogenous as possible.

PAR exposure lasted daily from 5:00 a.m. to 9 p.m. In the middle ofthis daylight period, cultures were exposed to UVR for 8 h from 9 a.m.

to 5 p.m. Two different irradiation treatments were applied in threereplicates each:

(1) no UVR (referred to as PAR), shielded from UVA and UVB radi-ation by an Ultraphan 400 foil (Digefra, Munich, Germany)

(2) PAR+UVR (referred to as UV), covered with cellulose acetate(Tamboer & Co Chemie B.V., Heemstede, Netherlands) to cor-rect for the ca. 10% absorption of Ultraphan 400 in the PARspectrum. The absorption spectra for these foils can be foundin Leu et al. 2007.

In the UV experiments, we applied UVR intensities near 50% of max-imum surface UVR-levels at a latitude of 80° N, the latitude fromwhichthe algae was isolated, at noon in mid-summer (referred to as lowlight), and an elevated level twice of this (i.e. corresponding to peak sur-face values), denoted high light (cf. Leu et al., 2006b). Measurements ofapplied UVA and UVB intensities were carried out with a IL 1400A radi-ometer (International Lights, Inc., Newburyport, MA, USA), PAR-levelsranged between 56 and 64 μmol m−2 s−1. For UVA the average mea-sured intensity was 11.3 Wm−2, UVB was 1.0 Wm−2. The high UVtreatments had close to 200 μmol m−2 s−1 PAR, and the UV-was in-creased correspondingly, so that the ratio between PAR, UVA, and UVBwas roughly: 2:1:0.1 under both radiation regimes. The difference in ra-diation intensitywas obtained by changing distance to the lamps. To en-sure equal exposure for all treatments, the flasks were exchangedrandomly between the 18 positions marked on the shelf every day.

Since the initial counts showed no signs of enhanced P-uptakeunder UVR exposure in neither Porosira, nor Thalassiosira grownunder surplus P conditions, we did not run experiments with elevatedlevels of P for these species (i.e. only the −P culture was used). InSynedropsis, we found signs of an elevated P-uptake under UVRunder both conditions (P replete and P depleted). Hence, we also per-formed a test with elevated levels of P to see if this effect was eitherweakened or stimulated by increased access to P. Due to the extreme-ly fast kinetics for P-uptake in this species, we run an additional ex-periment with higher time resolution to study the initial responsein more detail. The cultures were diluted with the respective medium(+P or −P) prior to the start of the experiment to the final concen-tration. The algal concentration at the start of the experiment wasin the range of 1–2 mg CL−1 or 30–40 μg chlorophyll a L−1.

For labeling of the cultures we added 10 μL of carrier-free 33P orto-phosphate (Amersham BF1003) yielding initial specific activity of10 μCi l−1 in the algal cultures, representing tracer amounts relativeto 31P in the media even at ambient concentrations of P (−P), 33Pwould represent an insignificant change in total P. Quenching was lowand counting efficiency exceeded 90% for all cultures, meaning thatthe reported counts per minute (CPM) mL−1 should be equivalent touptake of 33P. The radiotracer was added one hour after UV-lampswere turned on. Generally 15 mL from each flask was filtered on25mmGF/F filters and washed after 1 h, 6 h and 21 h, yet some exper-iments have additional samples (see Table 1 for an overview). After fil-tration each GF/F filter was placed in a 20 mL scintillation vial, and 1 mLof filtrate was added into another 20 mL scintillation vial. 20 mL liquidscintillation cocktail (Ultima Gold: High Flash-point, Universal LSC-cocktail, Perkin Elmer cat. No: 6013329) was added to both vials, andthe activity was counted on a Packard TriCarb liquid scintillation coun-ter. Due to the very fast uptake kinetics of Synedropsis, we performed asecond set of experiments with higher time resolution for this species,with sampling after 10, 30, 50, 70 and 100 min.

Since a major part of P in unicellular organisms are allocated to ribo-somes which are the site of protein synthesis, short-term UVR-effects onP-uptake or growth rates would presumably be reflected in changedlevels of RNA. We thus run assays on two of the species, Porosira andSynedropsis, to check for changes in RNA quantities during the courseof the experiment under PAR and PAR+UV exposure. Ten milliliterfrom each triplicated treatment was collected at the beginning, after 6and 21 h of the experiment. Samples were centrifuged at 5000 g for

Table 1Overview of treatments applied and sampling intervals (in hours) for each species.Note that −P actually represents background P.

−P +P

PAR vsUVR

Par vs. high UVR PAR vsUVR

Par vs.high UVR

Synedropsis 1, 6, 21 1, 6, 21(Short-term: 10, 30,50, 70, 100 min)

1, 6, 21 0.5, 1, 6, 21

Porosira 0.5, 2, 20 1, 6, 21Thalassiosira 1, 6, 21 1, 6, 21

Table 2Results of one-way RM-ANOVA on UVR response of Porosira glacialis, Thalassiosira sp.and Synedropsis hyperborea. Significant effects denoted by asterisk.

Light Time Light x Time

F P F P F P

P. glacialisPAR vs. UVR 0.2 0.678 996.8 0.000* 0.1 0.9437PAR vs. high UVR 26.4 0.007* PAR>HUV 6614.5 0.000* 31.1 b0.001*

Thalassiosira sp.PAR vs. UVR 1.1 0.349 2492.7 0.000* 1.6 0.256PAR vs. high UVR 11.2 0.028* PAR>HUV 126.6 0.000* 3.6 0.075

S. hyperborea−PPAR vs. UVR 3.0 0.154 6.3 0.022* 2.1 0.178PAR vs. high UVR 0.4 0.543 9.0 0.008* 11.3 0.004*PAR vs. high UVRshort-term

1.0 0.371 429.4 0.000* 4.7 0.009*

S .hyperborea+PPAR vs. UVR 8.7 0.041* PAR>UV 22.3 0.000* 6.1 0.024*PAR vs. high UVR 170.7 0.000* PAR>HUV 782.8 0.000* 79.2 b0.001*

0.5 2.5 20

Time (Hours) Time (Hours)

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Fig. 1. Uptake of 33P (counts pr min) in P. glacialis over the course of the experiment forPAR vs. UVR and PAR vs high UVR. Bars and vertical lines represents mean±SD (n=3).

47D.O. Hessen et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 45–51

5 min, supernatants were gently aspirated and remained samples werestored at−80 °C until analysis. RNA was extracted and quantified simi-larly to the RiboGreen method (Gorokhova and Kyle, 2002) and theobtained values were normalized to the protein content measured inthe corresponding extracts. Briefly, RNA from the sample was extractedby 2 min sonication in an ice-cold-maintained cuphorn (Brandson101147048) with 60 μL of 1% (v/w) N-lauroysarcosine (Sigma, L-5125).Immediately 300 μL of ice-cold TE buffer (10 mM Tris–HCl, 1 mMEDTA, pH=7.5)was added to the sample. Homogenateswere dispensedin duplicates (75 μL) into RNAse free wells (655076, Greiner Bio-One,USA) where one of the subsample was exposed to half-an-hour RNAsedigestion (0.1 μg, A7973, Promega). After this step 75 μL of 100× dilutedRiboGreen dye (R-11490, Molecular Probe, USA) was dispensed to allwells, mixed for following 5 min and the fluorescence signal was mea-sured at 535 nm after previous excitation at 480 nm (BioTek FL x 800,BioTek, USA). Difference in a fluorescence signal between undigestedand digested with RNAse samples was related to RNA content in theassayed sample. To translate fluorescence values to RNA content(μg mL−1) and to control for a proper digestion of RNA during the quan-tification, commercially available RNA standards were included (R-11490, Molecular Probes). Bicinchoninic acid method was used to mea-sure proteins from all extracts (Smith et al. 1985) using BCA kit (TermoScientific Pierce, 23227) and following the manufacture protocol.

To test the hypothesis that UVR increase the cellular uptake of P,we conducted one-way repeated measures analysis of variance(RM-ANOVA) with time as within-factor (three levels, except forSynedropsis in the short-term experiment (5 levels) and PAR vs.High UVR at+P (four levels)) and light (two levels: PAR and UVR)as between-factor for each species and light intensity. For all threespecies the RM-ANOVAs were performed on the PAR vs. UVR andPAR vs. high UVR at low P concentration. In addition for Synedropsisthese same treatment combinations were tested for the +P cultures.To test if UVR affected the RNA:protein ratio, a RM-ANOVA was alsoperformed on the PAR vs UVR (between-factor) and time (within-fac-tor_ 3 levels) for Porosira and Synedropsis grown at -P. Prior to statis-tical analyses all data were tested for homogeneity of variances usingthe robust Brown–Forsythe Levene-type test, based on the absolutedeviations from the median. The confidence level of each test wasset to 95%. All analyses were performed with the statistical softwareR (R Development Core Team, 2011) on untransformed data.

3. Results

The tested species revealed somewhat mixed responses, but gave ingeneral little support to the hypothesis of enhanced uptake of P underUVR exposure (cf. Table 2 for statistics). For Porosira low UVR yieldedno difference between the PAR and UVR treatments, while there was adecrease in P-uptake (RM-ANOVA, light: p=0.007) under the highUVR treatment (Fig. 1). This species also showed a significant light×timeinteraction (RM-ANOVA, light×time: pb0.001), where the difference inP-uptake between the PAR and UVR exposure increased over time. Asimilar responsewas found for Thalassiosira, where therewere no effectsat low UVR, while a reduced uptake (RM-ANOVA, light: p=0.028) wasobserved under enhanced UVR (Fig. 2). The activity in the filtrate

mirrored the uptake in the particulate fraction in all experiments (datanot shown). The 33P-uptake kinetics was rather slow for both species,and saturated uptake of P clearly exceeded 20 h for both species at thislow temperature. For Synedropsis, however, we recorded an increaseduptake of P under UVR (Fig. 3), with the strongest effect at low UVR,however due to the variability between the replicates this differencewas not significant. This species had a remarkably fast P-uptake kinetics,hence we performed a second set of experiments with higher time reso-lution for the high UVR-treatment at−P (Fig. 4) and also a second essaywith higher concentrations of P (denoted +P), shown in Fig. 5.

These experiments revealed that the half-saturation constant forP-uptake in this species was less than 1 h, and thus strikingly fastercompared to the other two diatoms that also are assumed to be“cold-adapted” species. The high time-resolution experiment (per-formed at −P) showed an increased uptake of P in the high UVRtreatment at the end of the experiment, however this differencewas not significant. However there was a significant interaction effect

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Fig. 2. Uptake of 33P (counts prmin) in Thalassiosira sp. over the course of the experimentfor PAR vs. UVR andPARvs highUVR. Bars and vertical lines representsmean±SD (n=3). 10 30 50 70 100

Time (Minutes)

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Fig. 4. High time-resolution uptake of 33P (counts pr min) in S. hyperborea at low P overthe course of the experiment for PAR vs high UVR. Bars and vertical lines representsmean±SD (n=3).

48 D.O. Hessen et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 45–51

between light and time (RM-ANOVA, light×treatment: p=0.009).The +P experiment showed lower P-uptake both for the UVR andhigh UVR exposure (RM-ANOVA, light: p=0.041 for PAR vs. UVRand pb0.001 for PAR vs. high UVR) relative to the PAR alone in S.hyperborea. The difference in P-uptake also increased over time forboth the UVR and high UVR exposure, as shown by significant light×-time interactions (RM-ANOVA, light×treatment: p=0.024 for PAR vsUVR and pb0.001 for PAR vs high UVR, cf. Table 2).

The lack of any strong response in 33P-uptake under UVR was con-firmed by the RNA-assays (Table 3 and Fig. 6). Although there were

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Fig. 3. Uptake of 33P (counts prmin) in S. hyperborea at−Pover the course of the experimentfor PAR vs. UVR and PAR vs high UVR. Bars and vertical lines represents mean±SD (n=3).

apparently slightly higher concentrations of RNA after 6 h of UVR-exposure in Porosira, the differences were insignificant between lightregime (RM-ANOVA, light: p=0.557) and only slightly significant fortime. (p=0.036). For Synedropsis the responses were similar for thePAR and UVR (RM-ANOVA, light: p=0.969), with a significant drop isthe RNA:protein ratio after 6 hour period of light exposure (RM-

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Fig. 5. Uptakeof 33P (counts prmin) in S. hyperborea at+Pover the course of the experimentfor PAR vs. UVR and PAR vs high UVR. Bars and vertical lines represent mean±SD (n=3).

Table 3Results of one-way RM-ANOVA on UVR response of Porosita glacialis and Synedropsishyperborean on RNA:protein ratio.

Light Time Light x Time

F P F P F P

P.glacialisPAR vs. UVR 0.41 0.557 5.17 0.036 * 2.24 0.169

S.hyperboreaPAR vs. UVR 0.0017 0.969 8.62 0.010 * 0.068 0.935

49D.O. Hessen et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 45–51

ANOVA, time: p=0.010) for both PAR and UVR exposure, probablyreflecting an active protein synthesis under light for this fast-growingspecies.

4. Discussion

Light levels and spectral properties both regulate the uptake of in-organic C and nutrients in autotrophs, and thus determine to a largeextent the cellular stoichiometry (in this context the C:N:P-ratio).The algal response to UVR includes typically reduced photosyntheticactivity and thus reduced C-fixation (cf. Neale et al., 2003), whilethere are ambiguous reports on the effect of UVR on nutrient uptake(Hessen et al., 2008). The reduced C:P-ratio that frequently havebeen observed under UVR-stress both in freshwater (Leu et al.2006a; Tank et al., 2003; Watkins et al., 2001, Xenopoulos et al.,2002) and marine algae (Hessen et al. 2008; Leu et al., 2007) couldbe caused by reduced C-uptake, increased P-uptake or both. Previousexperiments (Leu et al., 2007) with other marine diatoms (Thalassio-sira antarctica, Chaetoceros socialis and Bacterosira bathuomphala)yielded reduced C:P-ratios under UVR (while no effect on C:N), andthe authors suggested that this response could reflect an enhancedP-uptake. The three arctic diatoms tested in the current study didnot support this hypothesis however, neither did the RNA-assays forPorosira and Synedropsis.

A B

Fig. 6. RNA:protein ratio (μg μg−1) in (A) P. glacialis and (B) S. hyperborea at high P dur-ing the course of the experiment for PAR vs. UV. Bars and vertical lines representsmean±SD (n=3).

Reduced uptake of inorganic N under UV-stress has been reportedby some authors (Braune and Döhler, 1994; Döhler and Biermann,1987), while other studies found rather weak or even no effects ofUV on C:N ratios in phytoplankton (Fauchot et al., 2000; Mousseauet al., 2000). Similarly, while several studies have demonstrated a de-crease in cellular C:P-content under UVR-stress, there are mixed evi-dence for the role on UVR on P-uptake, especially for natural systems(Sereda et al., 2011). There are different uptake mechanisms and en-zymes involved for N and P in autotrophs, and these may respond dif-ferently to UVR.

There is clearly also a reciprocal effect of ambient concentrations ofP and UVR (Medina-Sánchez et al., 2006, Villar-Argaiz et al., 2009,Xenopoulos et al., 2002), but enhanced levels of P may both mask orenforce the negative effects of UVR (Carrillo et al., 2008). Typically, exper-iments are run in media with nutrient concentrations that aremuch higher than ambient conditions, and typically C:P-ratios, also in re-sponse of UVR, decrease with decreased ambient levels of P (Frost andXenopoulos, 2002, Xenopoulos and Frost, 2003). Likewise, uptake of Phas been found to depend on levels of UVR (Aubriot et al., 2004;Hessen et al., 1995). The interpretation of this would be that low tomod-erate doses of UVR could stimulate an increased uptake of P for photore-pair, or could be used to compensate for a decreased rate of proteinsynthesis under UVR-stress by allocating more P into ribosomes. Underhigher levels of UVR exposure, however, the uptake of nutrients couldbe impaired bymembrane damage or by disruption of alkaline phospha-tase activity (cf. Sereda et al., 2011).

The reason for these and previous inconsistencies with regard tonutrient uptake responses may partly be attributed to strongspecies-specific differences in UVR response (Delgado-Molina et al.,2009; Karentz et al., 1991), as well as the fact that the responses arequite subtle in that they depend on a suite of experimental or ambi-ent conditions. E.g. the time course and light source as well as lightquality and intensity, time for photorepair, algal cell size, physiologi-cal status, growth phase, ambient nutrient conditions and tempera-ture. Finally, there is always a risk that culturing conditions in theabsence of UVR could yield different responses compared to UVR-adapted cultures, especially if UVR-protection is a costly and thus in-ducible trait.

A full factorial test on all parameters that potentiallymight influencethe cellular uptake of P and C:P-ratios is unachievable, and certainly be-yond the scope of this study. Nevertheless, our experiments suggestthat UVR does not trigger an enhanced P-uptake, in support of the find-ings of Carrillo et al. 2008. Thus the frequently observed up-regulationof cellular C:P under UVR-stress seemmost likely to be due to impairedfixation of CO2, or enhanced cellular leakage of organic C. Since P is ascarce element being crucial for nucleic acids and certain membranelipids, it may very well be that most species have evolved mechanismsto protect and regulate P-uptake under UVR-stress.

Given the importance of nutrient uptake and elemental ratios inthe elemental flux and the marine food web, the variability in under-water spectral properties both due to natural variability and expectedperiods of ozone-minima and likely extended loss of ice-cover, thereis a definite need for testing a wider range of species and a widerrange of spectral properties and UV doses to reveal the nature ofcarbon and nutrient uptake in autotrophs. We also need to use dif-ferent approaches that will allow us to understand better the exactphysiological role of P in the algal response to light stress and UVRexposure.

Acknowledgements

This study was financed by grants from the Norklima Programunder The Norwegian Council of Sciences and Letters to projectsMERCLIM and CATCHMENT. We are most thankful to two anonymousreviewers for their most helpful and constructive comments. [SS]

50 D.O. Hessen et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 45–51

Appendix A

Table A1Uptake of 33P (in counts per minute) of Porosira glacialis, Thalassiosira sp and Synedropsis hyperborea for all treatment combinations. Means±SD.

t=1 t=2 t=3 t=4 t=5

P. glacialisPAR vs UVR

PAR 13,164±3706 58,724±13,489 213,221±24,861UVR 17,432±1498 64,382±4662 218,904±9593

PAR vs high UVRPAR 3277±130 9678±425 18,815±631High UVR 3081±57 8516±166 16,615±213

Thalassiosira sp.PAR vs UVR

PAR 9097±355 25,742±328 32,225±737UVR 9285±363 24,734±650 31,877±1099

PAR vs high UVRPAR 6341±269 14,456±769 18,477±1435High UVR 5704±55 13,753±1503 14,578±1791

S. hyperboreaPAR vs UVR at −P

PAR 14,381±2710 18,351±880 15,766±6221UVR 13,333±357 20,824±2710 22,030±1499

PAR vs high UVR at −PPAR 28,752±1076 29,377±3081 24,005±3731High UVR 24,158±520 27,927±1814 26,804±1764

PAR vs high UVR at −P short-termPAR 5411±103 9205±360 11,632±273 14,597±803 14,126±630High UVR 5343±165 8947±152 11,585±133 14,106±816 15,729±266

PAR vs UVR at +PPAR 712±78 1255±21 2253±279UVR 1052±456 1019±138 1509±153

PAR vs high UVR at +PPAR 1320±49 1468±38 2417±140 3819±102High UVR 1413±53 1507±41 2230±49 2709±12

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