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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Radiation damage can inter alia result in lipid peroxidation of macroalgal cell membranes. To prevent photo-oxidation within the cells, photoprotective substances such as phlorotannins are synthesized. In the present study, changes in total fatty acids (FA), FA composition and intra/extracellular phlorotannin contents were determined by gas chromatography and the Folin-Ciocalteu method to investigate the photoprotective potential of phlorotannins to prevent lipid peroxidation. Alaria esculenta juveniles (Phaeophyceae) were exposed over 20 days to high/low photosynthetically active radiation (PAR) in combination with UV radiation (UVR) in the treatments: PAB (low/high PAR + UV-B + UV-A), PA (low/high PAR + UV-A) or low/high PAR only. While extracellular phlorotannins increased after 10 days, intracellular phlorotannins increased with exposure time and PA and decreased under PAB. Interactive effects of time:radiation wavebands, time:PAR dose as well as radiation wavebands:PAR dose were observed. Low FA contents were detected in the PA and PAB treatments; interactive effects were observed between time:high PAR and PAB:high PAR. Total FA contents were correlated to extra/intracellular phlorotannin contents. Our results suggest that phlorotannins might play a role in intra/extracellular protection by absorption and oxidation processes. Changes in FA content/composition upon UVR and high PAR might be considered as an adaptive mechanism of the A. esculenta juveniles subjected to variations in solar irradiance.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

UV-B radiation and high photosynthetically active radiation (PAR) have been shown to have detrimental effects on marine organisms (1–6). The impact of UV-B radiation on organisms can be manifold and ranges from the cellular to the community level. On the cellular level, damages on the DNA, changes in cell wall morphology, cellular stoichiometry, inhibition of photosynthesis (7) and changes in fatty acid (FA) content (8,9) are described while effects within the community are determined by different UV sensitivities of species and their developmental stages (10,11). In particular sensitive to UV radiation (UVR) are unicellular organisms e.g. many phytoplankton species (12,13) and early developmental stages of brown algae (14–22).

Studies investigating FA composition in few-celled marine organisms are scarce (9,12,13). Polyunsaturated FA (PUFA), major constituents of cell membranes, are known to be oxidized and produce lipid peroxidation products upon UVR (23). Some of the lipid oxidation products such as malonaldehyde exhibit toxic activities by reacting with biological nucleophiles and crosslink upon UVR to proteins and bind covalently to nucleic acids (24). Irradiance levels are known to affect ratios of polyunsaturated and saturated FA (25,26). Consequently, changes in FA, driven by UV-B and low/high PAR, can influence the entire metabolism of an organism, its protective mechanisms such as membrane fluidity in polar species and formation of UV-protective substances against biotic and abiotic environmental factors, their nutrient uptake as well as the nutritional quality of the organism itself (13).

To prevent photo-oxidation within the cells, photoprotective substances such as phlorotannins are synthesized. Among brown algae, only one group of polyphenolics, the phlorotannins, is present (27,28) which might serve as an indicator of UV-protection and acclimation potential toward UVR. Within spores, phlorotannins are located in spherical membrane-bound vesicles (physodes) (29–31) which are randomly distributed throughout the cell. Phlorotannins in adult algae are located in the cell walls (32) and the cytoplasma (31) as well as in the outer epidermal cell layer, cortical cells and in the innermost medullary (33). Phlorotannins are based on the monomer phloroglucinol (34) and discussed to be formed via the acetate–malonate pathway (35) involving a polyketide synthase-type enzyme complex (36). It was suggested for the brown alga Saccharina latissima that phlorotannin building blocks are derived from storage lipids within the early developmental stages linking lipid metabolism with potential photoprotective mechanisms (37). Phlorotannins have a multifunctional role in ecology and respond to several environmental parameters, such as salinity, nutrient (38) and light availability as well as UVR and high irradiances (39). However, the most important property of phlorotannins for this study was their ability to absorb in the UV wavelength range (6,11,18,19,40–42) and their radical scavenging (antioxidant) activity (43–46). Phlorotannins can be either integrated into cell walls as supporting substances (32) or released via exudation processes into the surrounding water (34,41,47) creating so-called UV refugia (41) to protect spores from harmful radiation and to deter grazers. Especially polar species are exposed to extraordinary circumstances such as low temperatures, ice cover, shifts in light availability (polar day and night) and a high interannual variability in UVR (48) and can therefore serve as a role model.

This study investigates for the first time FA and phlorotannins in spores of a dominant Arctic brown alga (Alaria esculenta, Phaeophyceae) from Spitsbergen (Norway) upon UVR and low/high PAR exposure over a study period of 20 days. The major aim was to gain insights whether:

  • 1
     The FA composition and total FA change in early life stages.
  • 2
     Phlorotannin production in spores/juvenile gametophytes of A. esculenta is affected by low vs high PAR in combination with and without UVR.
  • 3
     The factor of time is involved in phlorotannin production and exudation processes as well as lipid consumption and change in FA composition.
  • 4
     Storage FA of A. esculenta might correlate and serve as potential fuel for phlorotannin building blocks in early developmental stages of seaweeds.

Material and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

In late May 2009, fertile sporophytes of A. esculenta (Linnaeus) Greville were collected from 8 to 9 m water depth by SCUBA diving in Kongsfjorden (Ny Ålesund, Spitsbergen, Norway). To obtain zoospores, the fertile tissue (sorus) of seven individuals was cleaned and kept at 7°C in a moist and dark chamber (16). Upon immersion in 0.2 μm filtered seawater, the fertile tissue released zoospores after 1.5 days at temperatures 5°C above the temperature the fertile tissue was stored. Individual spore suspensions were mixed and spore density was adjusted with 0.2 μm filtered seawater to 12.9 × 105 spores mL−1 for lipid and phlorotannin analysis (counted with Neubauer chamber “improved,” Brand, Germany). Obtaining three replicates per treatment and sampling day for phlorotannin (40 mL) and lipid analysis (10 mL), the spore suspension was distributed over glass Petri-dishes. Petri-dishes containing spores were then exposed to the radiation conditions summarized in Table 1, applying a light cycle of 24 h:12 h (PAR:UVR) according to the polar day. In an orthogonal experimental set-up, both high and low PAR in combination with and without UVR were applied to distinguish between radiation effects caused by UVR only (low PAR + UVR) or high PAR (high PAR-UVR) or in their respective combinations (low PAR-UVR and high PAR + UVR). Low PAR was provided by three white light fluorescent tubes (36 W true light® II Powertwist) while high PAR was provided by three Osram Biolux (36 W/965; München, Germany). PAR was measured with a LiCor 250A and a Li-190 Quantum sensor (LiCor, Lincoln, NE). UVR was generated by three fluorescent tubes (Q-Panel UVA 340, 40 W; Cleveland, OH) and measured with a Solar light PMA 2100 (Solar Light Co., PA).

Table 1.   Summary of radiation treatments applied over the exposure time of 20 days. Irradiances are given as mean values (W m−2). For a better comparison of data, UV-A, UV-B and UVery over the exposure period of 8 h are shown in dose [J m−2], additionally. Ratios of UVR:PAR are given for the low PAR laboratory (2.06) and the high PAR field treatment (0.58).
 Time (days)Irradiances (W m−2)Doses (J m−2)
PARUV-AUV-BPARUV-AUV-BUVery
UVR:PAR 2.0605.5 ± 1.210.9 ± 0.40.45 ± 0.0    
52.3 × 1062.3 × 1069.7 × 1041.2 × 104
104.7 × 1064.7 × 1061.9 × 1052.4 × 104
157.1 × 1067.0 × 1062.9 × 1053.7 × 104
209.5 × 1069.4 × 1063.8 × 1054.9 × 104
UVR:PAR 0.58020.6 ± 3.011.6 ± 1.90.48 ± 0.0    
58.8 × 1062.5 × 1061.0 × 1051.3 × 104
101.7 × 1075.0 × 1062.0 × 1052.6 × 104
152.6 × 1077.5 × 1063.1 × 1053.9 × 104
203.5 × 1071.0 × 1074.1 × 1055.2 × 104

Petri-dishes containing spore solutions were covered with one of three different cut-off filters allowing the following radiation spectra to pass: only PAR (400–700 nm, Ultraphan URUV farblos; Digefra, Munich, Germany), PAR and UV-A radiation (PA, 320–700 nm; Folex PR Montage Folie, Dr. Schleussner, Dreieich, Germany) and PAR and UV-A and UV-B radiation (PAB, 290–700 nm, Ultraphan URT 300 foil; Digefra). For a better comparison of data, doses of PAR, UV-A and UV-B over the exposure period of 24/12 h a day were calculated (Table 1). Additionally, erythemally weighted radiation (49) was calculated (UVery, Table 1). Samples were taken directly after release (day-0) and after 5, 10, 15 and 20 days. To avoid nutrient limitation, spore/gametophyte suspensions were enriched with an artificial medium (50).

After exposure, samples for FA analysis (10 mL each) were filtered on GF/C Filters (Whatman, Maidstone, UK, precombusted at 450°C for 4 h), covered with chloroform/methanol (2:1 vol/vol, Merck, Darmstadt, Germany) and frozen at −80°C before further extraction and gas chromatographic analysis.

Phlorotannin analysis

For quantitative phlorotannin analysis, spores/juvenile gametophytes were detached gently from the Petri-dish bottom after exposure. Then, spore/juvenile gametophyte solutions (40 mL each) were filtered under low pressure on GF/C filters and frozen in liquid nitrogen. After extracting spore filters with 7:3 acetone/water (analysis grade, Merck), phlorotannin extractions were freeze-dried and redissolved in 1 mL MilliQ water for further measurements (37). Subsequently, total phlorotannin content of spores was measured (35,51) applying the Folin-Ciocalteu method (Folin-Ciocalteu solution and sodium carbonate, Merck) using phloroglucinol (Sigma-Aldrich, St. Louis) as a standard agent.

FA analysis

For FA analyses, filters of the three replicates per treatment were pooled due to the extreme low lipid concentrations. The filters were homogenized by ultrasonification and extracted in dichloromethane: methanol (2:1, vol/vol) following the method after Folch et al. (52). Prior to extraction, an internal standard was added (19:0 FA methyl esters [FAME]). For gas liquid chromatography of FA, methyl esters were prepared from aliquots of the extracted spores/gametophytes by transesterification with 3% sulfuric acid in methanol for 4 h at 80°C. After extraction with hexane, FAME were analyzed with a gas liquid chromatograph (HP 6890; Hewlett-Packard GmbH, Waldbronn, Germany) on a capillary column (30 m × 0.25 mm I.D.; film thickness: 0.25 μm; liquid phase: DB-FFAP; J&W, Cologne, Germany) using temperature programming (53). FAMEs were identified by comparison with known standard mixtures. If necessary, identification of FAMEs was confirmed by gas chromatography-mass spectrometry (GC-MS) measurements. Total lipid concentration referred to the sum of total FAME. During sample processing, we unfortunately lost the FA treatment low PAR + UV-A + UV-B as indicated in Table 3 and Fig. 2 by dashes.

Statistical analysis and multivariate regression model

Statistical analysis was done with R (version 2.11.0) (54). First we did a two-way ANOVA, applying a post hoc test (Tukey HSD) of pair-wise comparison to test for differences among treatments for the phlorotannin measurements. Obtained P-values are listed for the phlorotannin data in a separate table. Significant differences among phlorotannin treatments (P < 0.05) are highlighted in gray (Table 2a,b). This however did not allow us to model the effects that the treatments have on the phlorotannin and total FA or whether there might be a correlation between them. We therefore studied them under a multivariate regression setting as a trivariate response to the treatments with correlated residual structure. Additionally, all 10 FA (Fig. 3) were tested separately to obtain the individual responses to the applied environmental conditions. For each FA, an ANOVA comparison between two models, a model without any interaction in the predictors and one model with all predictors interacting, was carried out. The treatments are time (considered continuous), PAR dose (discrete two levels low and high PAR) and radiation wavebands (discrete three levels PAR, PA, PAB). In the following, the model is described in more detail to trace the mathematical analysis if required. The model can be written as

  • image

where [y1 y2 y3 ] is the response vector (phlorotannin in spore solution, phlorotannin in spore and total FA [TFA]), x is the design vector of length depending on the considered model (e.g. if we allow for interactions in predictors or not), B is the unknown effect matrix to be estimated with three columns and number of rows equal to the length of x and [ε1ε2ε3] are the residuals jointly distributed as Normal(0, Σ), where Σ is the 3 by 3 covariance matrix, with entries Σii = Var(εi) = σii2 and Σij = Cov(εij) = σij. Significantly nonzero entries in B will tell us how the predictors influence the responses and nonzero off-diagonal entries of Σ will tell about the interactions in the joint response. As we assumed the residuals are normally distributed the joint distribution of all of the data is multivariate normal. Due to the missing values and the pooling of the TFA data we could not estimate B by the least-squares formula and Σ as the covariance of the residuals. However as we assumed normality we can write the likelihood function (ignoring missing measurements and repeated TFA measurements). To get a measure of a model’s relative goodness of fit, we use the Akaike information criterion (AIC) for model selection (interactions in predictors and nondiagonal elements in Σ). To obtain parameter estimates we use an iterated generalized least squares approach (GLS) maximizing the log-likelihood function using R’s built-in optim() function over the parameters describing Σ. At each candidate for Σ we estimate Bvia the GLS formula

  • image
Table 2. P-values derived from Tukey HSD post hoc test of the phlorotannin analysis within spores/early gametophytes and their filtrates for low PAR (a) and high PAR (b). Gray colored boxes describe significant deviation between treatments tested (P ≤ 0.05). Thumbnail image of

where vec is the vectorization (stacking of columns) operator, X is the model appropriate design matrix, V the model appropriate residual covariance structure and Y is the vector of measurements of all three responses. The significance of the estimated element Bij was assessed by approximate P-values coming from the t distribution with n-k degrees of freedom of the statistic inline image where n is the length of Y and k is the number of elements in B. The mathematical theory behind such iterated GLS procedures is complex and can be deepened in Ref. 55.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Phlorotannin content

In Fig. 1, the sum of phlorotannin content in the surrounding medium (lower part of the bars) and the spores/gametophytes (upper part of the bars) in A. esculenta at the sampling days 0, 5, 10, 15 and 20 and upon low PAR (a), and high PAR (b) exposure is presented. Upon release, phlorotannin content within the spores was averaged to 0.31 ± 0.04 μg phlorotannin (mL spore solution)−1 and within the filtrate (surrounding medium) 5.64 ± 0.30 μg phlorotannin (mL spore/gametophyte solution)−1 (Fig. 1). In the low PAR treatments (Fig. 1a), spores exhibited significantly higher phlorotannin contents in the 10 PAR and PAR + UV-A, 15 PAR and PAR + UV-A and the 20 PAR and PAR + UV-A treatments compared to the initial and the contents obtained after 5 days (Table 2a). The phlorotannin content was particularly high (Fig. 1) in the 15 PAR and PAR + UV-A (1.25 ± 0.11 μg phlorotannin (mL spore/gametophyte solution)−1) and the 20 PAR + UV-A treatment (1.60 ± 0.09 μg phlorotannin (mL spore/gametophyte solution)−1). Phlorotannin contents in all PAR + UV-A + UV-B treatment did not increase over time (P > 0.05) compared to the initial (except 10 PAR + UV-A). In contrast, within the filtrate all treatments of 10, 15 and 20 days showed significant higher (Table 2a) phlorotannin contents compared to the initial and day 5 contents (9.3–12.7 μg phlorotannin (mL spore/gametophyte solution)−1).

image

Figure 1.  Sum of phlorotannin content (± SD) in the surrounding medium and the spores/gametophytes in Alaria esculenta during the experiment (0–20 days) upon low PAR (photosynthetically active radiation, upper panel [a]) and high PAR, (lower panel [b]). The corresponding statistical analysis of treatments is given in Table 2a,b.

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In the high PAR treatments (Fig. 1b) only gametophytes in the 15 PAR + UV-A and the 20 PAR + UV-A treatments showed significant higher phlorotannin contents with 1.24–1.39 μg phlorotannin (mL spore/gametophyte solution)−1 compared to the initial and phlorotannin contents after 5 and 10 days and all PAR + UV-A + UV-B treatments (Table 2b). Within the high PAR filtrates, we could measure significant higher phlorotannin contents in the 10 PAR and PAR + UV-A and all other treatments of day 15 and 20 compared to the initial and the values obtained after 5 days (Table 2a,b).

In contrast, gametophytes within the low PAR + UV-A treatment had slightly higher phlorotannin contents than their congeners under high PAR + UV-A exposure. Within the exposure treatments of low PAR + UV-A + UV-B and high PAR + UV-A + UV-B phlorotannin contents (0.15 ± 0.05 μg phlorotannin [mL spore/gametophyte solution]−1) did not differ.

Fatty acids

Throughout the text, the saturated FA 14:0, 16:0 and 18:0 are indicated as SAFA, 16:1(n-7) and 18:1(n-9) as monounsaturated FA (MUFA) and 18:2(n-6), 18:3(n-3), 18:4(n-3), 20:4(n-6) and 20:5(n-3) as PUFA. Initial A. esculenta spores were dominated by MUFA and PUFA (both 32%) followed by SAFA with slightly lower values (up to 31%, largely owing to 18:0 and 16:0 FA; Fig. 2). Under high PAR, PUFA were remarkably low already after 5 days of PAR + UV-A + UV-B treatments (9.9–4.3%; Fig. 2). Under low PAR treatment, PUFA had considerable higher values with up to 60% of total FA (20 PAR and 20 PAR + UV-A; Fig. 2). The monounsaturated FA 18:1(n-9) exhibited highest amounts within the high PAR treatment after 5 days (37%) and were remarkably lower toward the end of experiment and ranged between 11% and 5% after 15 and 20 days, respectively. During low PAR the initial proportions of 18:1(n-9) in gametophytes were slightly lower compared to the high PAR (about 25%) and reached proportions of 7–15% after 20 days of experiment (Table 3, Fig. 3). The next most abundant were the SAFA due to consistently high levels of the 16:0 FA in all treatments, particularly in high PAR + UV-A + UV-B treatments with percentages of about 37% (Table 3; Fig. 3). The TFA content upon UVR exposure in the present study revealed that under low PAR, total FA content was higher than high PAR (Fig. 4).

image

Figure 2.  Sum of saturated fatty acids (SAFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) upon low vs high PAR and UVR exposure during the experiment.

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Table 3.   Total fatty acid (FA) proportions of Alaria esculenta spores (day 0) and juvenile gametophytes (day 5–20) under low and high PAR and upon UVR exposure (PA and PAB) in mass % of total FA for a spore/gametophyte density of 12.9 × 105 spores/gametophytes mL−1. Thumbnail image of
image

Figure 3.  (a–j) Individual fatty acids during exposure time. Different treatment combinations of low and high PAR and PAR, PA, PAB treatments are indicated for the measurements. (a) 14:0, (b) 16:0, (c) 16:1(n-7), (d) 18:0, (e) 18:1(n-9), (f) 18:2(n-6), (g) 18:3(n-3), (h) 18:4 (n-3), (i) 20:4(n-6) and (j) 20:5(n-3). The statistical evaluation of each fatty acid is presented in Table 4.

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image

Figure 4.  Total fatty acids (TFA) during exposure time. Different treatment combinations of low and high radiation as well as PAR, PA, PAB treatments are indicated for the measurements. The statistical evaluation of TFA content within the various treatments is displayed in Table 5.

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However, the low PAR and high PAR treatment showed important differences in TFA increase and decrease. In the low PAR treatment, the TFA content of the control (PAR only) was varying during the 20 days of the experiment exhibiting an average amount of 1410.6 μg (mL spore/gametophyte)−1, which is comparable to the initial value (Fig. 4).

Within the other low PAR dose treatments (PA and PAB), a distinct decrease of TFA after 10 days of experiment occurred with average values of 382.9 and 219.1 μg (mL spore/gametophyte solution)−1, for PA and PAB respectively (compare Fig. 4). In contrast, after 15 and 20 days of the high PAR dose experiment, TFA were almost depleted in all treatments (PAR, PA and PAB; Fig. 4).

After 15 days a mean TFA content of 1222.4 μg (mL spore/gametophyte solution)−1 and after 20 days an average value of 1523.3 μg (mL spore/gametophyte solution)−1 was detected (Fig. 4) upon low PAR. Total FA were significantly affected by UV-A and UV-B as well as by the interaction of time and high PAR (Table 5). The results of the regression analysis of the FAs are summarized in Table 4. For each FA, the model comparison procedure resulted in no significant (at the 0.05 level) improvement when considering the model with interactions between the different predictors. This could be due to the fact that the interaction effects might be too weak to be noticed within the small sample size available. The majority of the different FAs showed only minor dependencies on PA (18:1(n-9) and 18:2(n-6)) but major dependencies on PAB (14:0, 16:0, 18:1(n-9), 18:2(n-6), 18:3(n-3)). However, the FA 18:1(n-9) and 18:2(n-6) strongly depended on time and suggest a decrease over time, especially within the high PAR treatments (Fig. 3, Table 4).

Table 4.   Analysis of interactive effects between radiation wavebands (PAR, PA and PAB) and PAR dose (low and high PAR) on the individual fatty acids.† Estimates of regression coefficients for independent regressions of the individual FAs on the different days, radiation and treatment predictors. Significance codes are 0 “***” 0.001 “**” 0.01 “*” 0.05 “” 1.
 14:016:016:1(n-7)18:018:1(n-9)18:2(n-6)18:3(n-3)18:4(n-3)20:4(n-6)20:5(n-3)
  1. †The linear model for investigating the effects of radiation wavebands and PAR dose on the individual fatty acids is described as fatty acids = intercept + time + PAR dose + radiation wavebands + ε, with PAR being the baseline for radiation wavebands and low PAR the baseline for PAR dose.

(Intercept)82.7485***279.673***15.3361**196.580*295.812***119.077***80.4240**82.32067.825*115.156**
Time−0.9394−6.152−0.0525−7.252−9.095**−2.867*0.31763.6591.700−0.005532
High PAR−5.5124−25.1640.1188−19.282−27.117−14.657−20.2999−29.785−20.761−26.223108
PA−27.6577−90.171−0.5355−24.005−91.141*−46.450**−37.2620−54.644−46.453−49.414228
PAB−40.3643*−116.463*−4.5629−70.279−103.432**−55.850**−51.5291*−83.785−60.425−70.87

Analysis of interactions between phlorotannins and TFA data

The AIC model selection criterion indicated the model where all of the predictors (time, PAR dose, radiation wavebands) were interacting, with residual covariance structure as σ12 = 0 (phlorotannins in filtrates responding independently of phlorotannins in spores/gametophytes) and σ13 ≠ 0, σ23 ≠ 0 (phlorotannin response dependent on the response of TFA). In Table 5, the GLS estimates of the elements of B under the best found estimate of inline image are presented. The estimate of ∑ gives a correlation of 0.2563 between TFA and phlorotannin in the filtrates and 0.2567 between TFA and phlorotannin in spores/gametophytes. Consequently, the phlorotannin content within the spore/gametophyte media only depends on time (Fig. 5; Table 5). The phlorotannin content within the spores/gametophytes depends on all of the predictors with all interaction effects apart from high PAR. Consequently, in the absence of UV-A or UV-B radiation, the phlorotannin content within the spores/gametophytes is not affected by the PAR dose. TFA is affected by changes in radiation wavebands and PAR dose. The effects of PAR dose and time are only present upon interaction of these two variables and when UV-B levels are high.

Table 5.   The linear model refers to the model presented in the section “statistical analysis and multivariate regression model.” It investigates correlations between phlorotannins within the spores/gametophytes and their medium (filtrate) and total fatty acids with PAR being the baseline for radiation wavebands (PAR, PA and PAB) and low PAR the baseline for PAR dose. The generalized least squares approach estimates for B under the numerically found maximum likelihood estimate of Σ. Significance codes: 0 “***” 0.001 “**” 0.01 “*” 0.05 “” 1.
 Phlorotannins within the mediaPr(>|t|)Phlorotannins within the gametophytesPr(>|t|)TFAPr(>|t|)
(Intercept)4.831<2*10−16***0.5031.39*10−6***1.3092.68*10−8***
Time0.392<2*10−16***0.0321.47*10−4***0.0100.595
PA−0.2830.724−0.5360.001**−1.1040.004**
PAB−0.2270.777−0.3930.017*−1.4950.001**
High PAR−0.7290.310−0.2770.068 0.3070.383
Time:PA−0.0580.3070.0538.93*10−6***0.0250.344
Time:high PAR0.0070.876−0.0200.044*−0.0816.04*10−4***
PA:high PAR1.0480.1220.3760.009**0.5460.088
PAB:high PAR−0.1860.7840.5153.31*10−4***1.0640.001**
image

Figure 5.  (a–d) Interactive effects (a) Phlorotannin content within filtrates vs phlorotannin content in spores/gametophytes and (b) total fatty acids vs phlorotannin content in spores/gametophytes with different treatment combinations marked. (c) Total fatty acids (TFA) vs phlorotannin content within the filtrates and (d) TFA vs phlorotannin content within spores/gametophytes over time. The corresponding statistical analysis of interactive effects is displayed in Table 5.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

This study involved a comparable analysis of low vs high PAR in relation to UVR effects and was carried out for the first time on early life stages of A. esculenta. Fatty acid content/composition and intra/extracellular phlorotannin content were highly affected by both low and high PAR and UVR in A. esculenta spores and juvenile gametophytes.

Phlorotannins under low vs high PAR and upon UVR exposure

The variation of phlorotannin content with species, morphology of the algae, habitat and developmental stage as well as with seasonal and environmental parameters have to be taken into consideration when interpreting results and distinguishing effects of applied experiments.

Compared to our study, A. nodosum sporophytes were exposed to significantly higher radiation conditions with 0.6 W m−2 UV-B for 2 weeks leading to a 30% increase in mean phlorotannin concentrations (40). Similar results were obtained in Fucus gardneri (56) and in S. muticum (57) suggesting a response-induced production of phlorotannins. Nevertheless, an increase in intracellular phlorotannin levels in the present studies was only detectable after 10 days exposure upon low PAR and UVR and non-UV treated juvenile gametophytes A. esculenta, while low PAR only treatment showed no effects on S. latissima juveniles (37). Upon combined high PAR and UVR exposure, juvenile gametophytes under PA exhibited significantly higher intracellular phlorotannin levels than their representatives in the high PAR controls and among the PAB treatments (Tables 2a,b and 5). Consequently, it can be assumed that UV-A might be able to either compensate high PAR impact to some extent or that phlorotannin synthesis might be stimulated by wavelengths within the UV-A spectrum.

Different developmental stages are known to exhibit species specific levels of phlorotannin contents. While phlorotannin contents in juveniles of E. menziesii, F. gardneri, H. sessile and Lessoniopsis littoralis were higher relative to adult concentrations, phlorotannin concentrations in Nereocystis luetkeana were higher in adult algae (58). However, less phenolic compounds in juveniles compared to adult stages were observed in two tropical brown algae species (59). Due to the small cell size of spores, intracellular phlorotannin contents were rather small and increased in juvenile gametophytes 10 days after release under low PAR and 15 days under high PAR conditions suggesting a coupling either between mortality/hindered growth and high PAR or decreased phlorotannin formation/phlorotannin oxidation under high PAR compared to low PAR exposure (significant interactive effect of time and high PAR, Table 5).

Exposure time, level of irradiance, nutrients and developmental stage on FA composition and total FA

Nutrient limitation is known to additionally increase sensitivity toward UVR. Although it is suggested that lipid synthesis is not governed by nutrient deficiency in natural aquatic ecosystems but rather by abiotic factors as irradiation and day length (60,61) other studies have revealed that a reduced nutrient uptake changes the C:N:P ratios upon UVR after 2 days (13). A phosphorus deficiency could for instance reduce RNA and slow down the transcription process and associated delayed or hindered cell division would induce accumulation of various primary and secondary photoproducts (13). To avoid these side effects and to focus entirely on interactive effects of UVR and high/low PAR in the early developmental macroalgal stages, the spore (gametophyte) suspensions were enriched with Provasoli enriched seawater (50) and nutrient deficiency related effects can therefore be excluded.

FA profiles are more susceptible than overall production parameters as photosynthesis and may be affected by smaller dose rates and shorter exposure times. While short exposure periods to UVR and high visible irradiances might lead to change in FA composition via lipid peroxidation, longer exposure periods might reduce photosynthesis and change in total FA content in phytoplankton (13) but also in early developmental stages of brown algae (this study, Table 6). Decreasing UV-B damages with higher irradiation levels in several microalgae were investigated (12) with lowest damage at 200 μmol photons s−1 m−2 [43 W m−2] and highest at 15 μmol photons s−1 m−2 [3 W m−2]. Although PAR levels were much higher, daily doses of UV-B were calculated to 12 kJ m−2 (12) while in our study, daily doses of UV-B ranged from 19.4 to 20 kJ m−2. In contrast, our low PAR controls (low PAR only) showed varying total FA contents over time while high PAR controls (high PAR only) showed a strong decrease in total FA compared to the initial value. Since a methodological problem can be excluded, the high variations in FA content both in the high and low PAR only treatments (day 10) remain a matter of debate. However, we can support a positive UV-A effect only upon low PAR + UV-A exposure on PUFA while upon high PAR + UV-A exposure, the SAFA and MUFA was much higher and the total FA content decreased dramatically over the exposure period.

Table 6.   Comparison of fatty acids composition in various developmental stages of Alaria esculenta. Values are given in % of total fatty acids. Fatty acid determination in spores was obtained directly after spore release and in juvenile gametophytes, 20 days after spore release in the present study.
Developmental stageFatty acids in % by weight
14:016:016:1(n-7)18:018:1(n-9)18:2(n-6)18:3(n-3)18:4(n-3)20:4(n-6)20:5(n-3)
Spores (present study)6.118.00.56.831.810.05.41.44.411.2
Juvenile Gametophytes (present study)6.614.11.20.810.35.610.018.812.813.2
Adult Sporophytes (73)4.810.11.50.56.54.210.627.911.918.4
Adult Sporophytes (74)4.612.71.51.611.34.39.218.613.210.6

Consequently, high PAR over time seems to have a similar impact on total FA and FA composition as high energetic short wavebands of UV-B (Table 5). Changes in lipid content can be considered as an adaptive and survival mechanism of the juveniles subjected to variations in solar irradiance (62) and related to the developmental stage as summarized in Table 6. Results obtained suggest that interactive effects of high PAR and UV-B on A. esculenta juveniles might enhance FA peroxidation leading to feedback stress responses as formation of ROS and antioxidants (e.g. phlorotannin).

TFA content and composition under low vs high PAR and upon UVR exposure

Fatty acid profiles are known to change under UVR exposure by an increase in lipid peroxidation (63) leading to an increase in short-chained FA and a decrease in PUFA (12,13,24). Nevertheless, UV-related responses are dependent on taxonomy (13), cell-stage (64), nutrient limitation and the UVR:PAR-ratio. In contrast, visible light (PAR) influences primarily the content of saturated and PUFA (25,26,62) but is considered to affect FA composition and content of phytoplankton species specifically (65).

In particular, reactive products found were β-dicarbonyls, α,β-unsaturated aldehydes, 2,4-alkadienals, 4-hydroxy-2 alkenals (66). Additionally, photoproducts derived from arachidonic acid, squalene and linolenic acid: formaldehyde, acetaldehyde, acrolein, malonaldehyde, n-hexanal, 4-hydroxy-2-nonenal were detected (24). Some products such as malonaldehyde exhibit toxic activities by reacting with biological nucleophiles and crosslinks upon UVR to proteins. The additional covalently binding of FA to nucleic acids (24) and the decrease in number and synthesis of FA under UV-B exposure (8,67) may explain the strong decrease of FA content and shift in FA composition upon low PAR and UV-B exposure in our study. An overall increase in MUFA and SAFA was described in earlier studies (8,68), especially of the radiation dependent FA 16:0 (68), while the high susceptible PUFA (16:4(n-1), 18:3(n-3), 20:5(n-3)) decreased about 50%. Although accumulations of short chain SAFA and MUFA as storage lipid constituents were largely unaffected by UV-B in phytoplankton (8), our results suggest a significant impact of UV-B on SAFA, MUFA and PUFA in A. esculenta juveniles.

Upon UV-B exposure, decreases in the omega-3 FA 20:5(n-3) and 22:6(n-3) were observed in microalgae (12). In each case, the decrease was species dependent and was less in UV tolerant species. In the present study, upon low PAR and UV-B exposure, juveniles of A. esculenta showed a decrease of all FA but in particular of 14:0, 16:0, 18:1(n-9), 18:2(n-6) and 18:3(n-3). An increase in short-chained FA and a decrease in PUFA upon UVR exposure were observed in phytoplankton cells (13). Phospholipids (69) and membrane FA such as 20:5(n-3) and 22:6(n-3) are known to be particularly sensitive to UVR due to a reduced biosynthesis and lipid peroxidation processes (13). A decrease in absolute and relative terms of the membrane FA 20:5(n-3) was especially observed upon high PAR + UV-B in our study. However, TFA content upon UVB-exposure in the present study revealed that under high PAR, total FA content was significantly (P = 0.001) higher than low PAR (except sampling day 20). Despite decreasing total FA contents upon both low and high PAR and UVR exposure, distinct lipid peroxidation processes with increasing SAFA contents and decreasing MUFA/PUFA contents could be observed in all PA and PAB treatments (Fig. 2).

Storage lipids in motile spores seem to fuel swimming after release and germination processes (70,71). Differences in lipid composition and consumption of various spore species might be related to the swimming behavior, photosynthetic efficiency and in the light environment inhabited by spores of the various species and can alter the energy budgets of spores and influence the amount of endogenous reserves needed to fuel spore swimming (69). This is supported by the present study where total FA content in A. esculenta spores and gametophytes was several magnitudes higher than in S. latissima (37). While settlement of Pterygophora californica spores was generally reduced under high PAR conditions, settlement decreased in particular with increasing exposure time but not with irradiance (5). No discrepancies in lipid content between algae grown under low PAR and high PAR were observed in Ulva pertusa (72). In contrast, our low PAR controls showed variations in total FA over time, while high PAR controls showed a strong decrease in total FA compared to the initial value after release with Table 5 showing an interactive effect of exposure time and irradiance levels (PAR) in A. esculenta as discussed previously. Determination of total FA content and composition in spores and juvenile gametophytes of S. latissima obtained under low PAR conditions (37), revealed only minor changes in total FA content but in particular a decrease of 18:1(n-9) over the exposure time of 20 days. Since a similar decrease of 18:1(n-9) was observed in the present study, the previously suggested hypothesis (37) that building blocks for secondary metabolites as phlorotannins might be mainly fueled by this FA can be supported for A. esculenta juveniles as well. In Ulva fenestrata, a PAR impact on the ratio of storage lipids (triacylglycerols) to major chloroplast lipids such as glycolipids and phosphatidylglycerol was investigated (62). Relative proportions of FA present in triacylglycerol, monogalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol did not depend on irradiance conditions. Only variations in FA composition of digalactosyldiacylglycerol and phosphatidylglycerol and changes in amount of lipids were responsible for differences in total FA composition among radiation doses in U. fenestrata (62). While the PUFA 16:4(n-3) showed highest levels under low PAR (24% of incident radiation), highest levels of 16:0 were measured under high PAR (80% of incident radiation; 62). The shift in dominance of PUFA under low PAR to SAFA under high PAR was observed also in the present study within the first 5 days upon high PAR exposure (without UV treatment).

However, these observations were obtained among different divisions (Chlorophyta and Phaeophyta) and different developmental stages (sporophytes, spores/early gametophytes), the impact on FA composition seem to be rather dependent on the interaction of environmental parameters than on single factors.

TFA content interacting with phlorotannins

One of the issues we considered in the regression analysis was whether TFA and phlorotannin respond independently to the predictors or do they interact with each other. The AIC criterion indicated a significant correlation between TFA and phlorotannins. Since high TFA were correlated with high phlorotannin contents, the photoprotective role of phlorotannins with A. esculenta juveniles can be supported indicating a feedback response to environmental radiation conditions. Toward intuitive conclusions, the phlorotannin contents within the juvenile A. esculenta seem to be affected independently from the phlorotannin content in their filtrates (surrounding medium) and should be addressed in further studies.

Our observations suggest that (1) FA composition and TFA content are strongly affected by high PAR and UVR. Changes in lipid content and composition might be considered as an adaptive mechanism of the A. esculenta juveniles subjected to variations in solar irradiance. (2) Intracellular phlorotannins were more affected by the environmental parameters given than extracellular phlorotannin contents indicating an independent impact and different protective roles. (3) Phlorotannins were induced after 10 days both upon PAR only and PAR + UVR exposure in contrast to previous hypotheses of immediate induction mechanisms and induction by UVR only. (4) TFA could be correlated to phlorotannin contents indicating that building blocks for secondary metabolites as phlorotannins might be derived from FA as 18:1(n-9) but should be investigated in more detail in further studies.

Hence, global climate change and stratospheric ozone depletion might influence viability of early life stages more than assumed. Additionally, the decrease in FA content and composition of kelp juveniles upon UVR and high PAR exposure is likely to affect the community level by a decrease in nutritional quality.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Acknowledgements— The authors would like to express their gratitude to two unknown reviewers, the AWI diving group, especially Max Schwanitz for collecting fertile algal material, Dr. M.Y. Roleda and M. Schröder for lab assistance and the Ny-Ålesund International Research and monitoring facility for support. Special thanks go to ARCFAC, supporting this study with a research grant (ID 2008-42) and to the Norwegian Polar Institute, hosting the project in Ny-Ålesund. This study is part of a cumulative PhD dissertation supported by the Bremen International Graduate School for Marine Sciences (GLOMAR) that is funded by the German Research Foundation (DFG) within the frame of the Excellence Initiative by the German federal and state governments to promote science and research at German universities, the Alfred-Wegener-Institute for Polar-and Marine Research, the AWIPEV Base (Dr. M. Schumacher) in Ny-Ålesund and the University of Bremen. We also thank the Centre for Theoretical Biology at the University of Gothenburg for support.

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  6. Discussion
  7. Acknowledgments
  8. References
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