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Abstract

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

Rapid adjustments of the photosynthetic machinery and efficient antioxidant mechanisms to scavenge harmful ROS are physiologic adaptions exhibited by intertidal seaweeds to persist in temperate regions. This study examines short-term (3 h) responses of three large kelps from the cold-temperate coast of Chile, normally adapted to water temperatures <16°C, but exposed abruptly to simultaneous high temperatures and UV radiation during low tide in summer. The kelps were exposed in the laboratory to three temperatures (10, 20 and 28°C) with and without UV radiation, and photochemical reactions, concentration of phlorotannins and antioxidant activity were examined. The exposure to elevated temperature (slightly exacerbated by the presence of UV radiation) decreased photochemical processes (measured as fluorescence kinetics) in the three studied species and increased lipid peroxidation in two of them. The concentration of total soluble phlorotannins was variable and correlated with the antioxidant activity in the presence of UV radiation. Insoluble phlorotannins did not change during the exposure. In all, the downregulation of the photochemical machinery, which was expressed as dynamic photoinhibition, and the rapid induction of soluble phlorotannins triggered by UV radiation minimized the effects of oxidative stress and maintained the operation of photochemical processes during short-term thermal stress.


Introduction

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

Intertidal seaweeds from temperate regions are naturally exposed to broad fluctuations in the environment, e.g. desiccation, high solar radiation, broad and rapid changes in salinity, etc., and thus they are ecophysiologically well equipped to deal with such stressful conditions (1). However, the examination of physiologic adaptations in organisms living in these systems is difficult as many responses can be triggered by multiple factors acting simultaneously, resulting in e.g. synergistic or antagonistic effects (2). This is relevant especially in enzyme-based thermal acclimation, e.g. synthesis of stress proteins and related reactions (3,4), DNA repair via photolyase and excision enzymes (5,6), activation of antioxidant enzymes (7–9), downregulation of photosynthetic enzymes (e.g. RuBisCo) and the whole process of carbon fixation (10,11). In the case of abrupt changes in environmental conditions during emersion, algae can respond through photochemical downregulation (e.g. dynamic photoinhibition), which serves to dissipate excess energy (12–14) and complementarily, by activating the whole suite of radical scavenging mechanisms, e.g. antioxidative enzymes (7,8). Apparently, an efficient and rapid ROS scavenging during exposure to high solar irradiation is regarded as an important physiologic adaptation of intertidal species (15).

Brown algal phlorotannins (polymers of phloroglucinol; 1,3,5-trihydroxybenzene) are inducible phenolic compounds associated mainly with antiherbivory defense (16,17), similar to the condensed tannins and flavonols of higher plants (18). However, they are also essential components of the cell wall and can be rapidly synthesized during wound-healing and sealing after thallus amputations (19), thus serving as primary structural compounds during growth (20). Due to the fact that these compounds absorb UV wavelengths and are mostly located in the periphery of the cell, their role as UV-screening substances has been demonstrated (21,22). Furthermore, these compounds have chemical characteristics that make them effective antioxidants that can scavenge harmful reactive oxygen species (ROS) (23,24). As the formation of ROS is one of the primary expressions of photodamage by UV radiation (7,23), the role of phlorotannins beyond a putative UV shielding function requires further examination in an ecological context. In fact, phenolic compounds of higher plants are known to act not only as UV-screening substances (24,25) but also as highly efficient ROS scavengers (26,27). Thus, studies on the examination of the relationship between phlorotannins and short-term adjustments of the physiologic machinery of seaweeds might give valuable insights into large-scale responses, e.g. those driven by global climate changes (2,28).

In the littoral of southern Chile (coast of Valdivia, 39°S), three large brown algae, the kelps Lessonia nigrescens and Macrocystis pyrifera and the fucoid Durvillaea antarctica, coexist and dominate at the intertidal zone. This association is unique among the cold-temperate kelp communities in the southern hemisphere and its importance for the whole intertidal community structure has been emphasized in various ecological studies (29–31). Recently, it has been reported that levels of UV radiation recorded in summer affect photosynthesis in these species (11,32,33), which is exacerbated by the simultaneous action of contaminants (34). Interestingly, these adverse effects on algal metabolism are partially counteracted by efficient photoprotective mechanisms such as dynamic photoinhibition (35) and synthesis of phlorotannins (22). However, the effects of temperature, which can become as high as 30°C during low tide in summer, have not been examined.

The present study is an effort to understand the way in which intertidal kelps are physiologically adapted to cope with the combined action of temperature and UV radiation, two major factors affecting the physiology of these organisms. On the basis of short-term exposures (hours) in the laboratory we examined the changes in concentration of phlorotannins in L. nigrescens, M. pyrifera and D. antarctica. The functional role of these compounds was assessed through in vitro measurements of antioxidant activity, lipid peroxidation, as well as rapid adjustments in chlorophyll fluorescence kinetics. Regarding some ecological and morphofunctional differences between these kelps, we also addressed the question whether they share similar stress mechanisms or alternatively, species–specific responses. This aspect is essential to understand how temperature modifies the effect of UV radiation on different processes of kelps underlying competitive adaptations to the intertidal life.

Materials and methods

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

Sampling and algal material.  Fronds from juvenile exemplars of L. nigrescens, D. antarctica and M. pyrifera were collected in austral spring (October) from intertidal rocky shores of Calfuco, coast of Valdivia, southern Chile (39°51′S, 73°23′W). After sampling algae were immediately transferred to the laboratory and acclimatized for approximately 24 h to dim light (50 μmol·m−2·s−1, Daylight, TL Phillips, The Netherlands) at a temperature corresponding to natural surface water temperature in the sampling site (10 ± 1°C) and under vigorous aeration.

Experimental design.  To determine the temperature threshold for photosynthesis, seaweeds were exposed for 15 and 30 min to increasing temperatures between 5 and 40°C in a culture chamber (Bioref-Pitec, Santiago, Chile). Maximal quantum yield of fluorescence (Fv/Fm) was measured with a portable pulse modulated fluorometer (PAM-2000, Walz, Effeltrich, Germany). This parameter is an indicator of the quantum efficiency of photochemistry (36).

To evaluate the effect of temperature on the physiology of the kelps, algae were exposed for 3 h to three temperatures in a thermoregulated water bath (35): 10°C (winter ambient temperature, low range for this latitude), 20°C (high temperature, 3°C above the upper summer range for this latitude) and 28°C (critical temperature occurring during low ride in summer). After the exposure, the algae were returned to the original culture conditions (10°C, under PAR). The algae were illuminated using a combination of UV (Q-Panel-313 and 340 nm; Q-Panel Co., Cleveland, OH) and PAR lamps (Daylight, Philips), and cut-off foils (Ultraphan 295 and 395, Digefra, Munich, Germany). A group of algae was exposed to UV radiation, which was set at 2.3 W m−2 for UV-B and 8.4 W m−2 for UV-A with a PAR background (photosynthetically active radiation) of 85 μmol·m−2·s−1. The PAR control was set using an Ultraphan 395, which cuts off the entire UV waveband (Fig. 1). The UV-B condition is in the range of the maximal levels recorded for this zone in summer (1.68–2.22 W·m−2) (32,33), whereas PAR levels were kept low to avoid masking of UV effects. In each treatment, 10 pieces of fronds collected from three individual algae were used. After the exposure, algae were returned to the original culture conditions (10°C and PAR irradiance of 85 μmol·m−2·s−1) for recovery for 4 h. After exposure, samples were frozen in liquid nitrogen and stored at −20°C or in silica gel until further biochemical analyses.

image

Figure 1.  Spectral irradiance of the UV (Q-Panel) and PAR (Philips) lamps under cut-off filters (Ultraphan 295 and 395) used to obtain two experimental treatments (only PAR and UV + PAR).

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Measurement of photochemical reactions. In vivo chlorophyll a fluorescence of photosystem II (PSII) was measured with the PAM-2000 fluorometer. For the estimation of the electron transport rate (ETR), thallus discs of 1 cm diameter punched from the middle zone of the fronds were put in a dark chamber and irradiated with increasing intensities of PAR (up to 283 μmol photon·m−2·s−1) provided by the PAM device (36). ETR was then determined through P–I curves by relating effective quantum yield (ΦPSII) and the intensity of the actinic irradiance as follows:

  • image(1)

where E is the incident irradiance of PAR and A the thallus absorptance. The factor 0.5 comes from the assumption that four of the eight electrons required to assimilate one CO2 molecule are supplied by PSII. Absorptance was determined by placing the algae on a cosine corrected PAR sensor (Licor 192 SB, Lincoln), and calculating the light transmission as:

  • image(2)

where Et is the irradiance below the alga (transmitted light) and Eo is the incident irradiance. For defining the ETR parameters, a modified nonlinear function of Jassby and Platt (37) was fitted:

  • image(3)

where ETRmax is the maximal ETR, tanh is the hyperbolic tangent function, α is the initial slope of the P–I curve, which is an indicator of the efficiency of the electron transport and E is the incident irradiance. The saturation irradiance for electron transport (Ek) was calculated as the intercept between α and ETRmax.

Lipid peroxidation.  The content of malondialdehyde (MDA) equivalents, an indicator of peroxidation, was analyzed according to Salama and Pearce (38). Algal segments of approximately 40–70 mg fresh weight were ground in liquid nitrogen and extracted with 1.5 mL of 0.5% wt/vol thiobarbituric acid (TBA) in 20% wt/vol trichloroacetic acid (TCA). The mixture was incubated in a water bath at 95°C for 30 min, cooled in ice, and centrifuged at 8764 g for 20 min. Absorbance at 440, 532 and 600 nm was read in 96-well microplate spectrophotometer (Multiskan Spectrum, Thermo Scientific, Waltham, MA). The amount of MDA was calculated using an extinction coefficient of 1.57 × l05 at 532 nm (39).

Antioxidant activity.  DPPH (2,2-diphenyl-1-picrylhydrasyl) radical scavenging activity was determined using a 96-well microplate assay (40). Samples (50 mg) were previously killed in liquid N2, dried in silicagel and ground in a mortar. The extract was homogenized in 5 mL of 70% acetone for 24 h under shaking at 4°C in darkness. A 150 μm stock solution of DPPH* was prepared in ethanol (80%). For the reaction, a volume of 22 μL of extract (supernatant) and 200 μL of DPPH* solution were mixed directly in the 96-well microplate. The plate was covered and kept in the dark at 22°C. After 5, 10, 15, 20 25, 30, 60, 90 and 180 min, the absorbance was measured at 520 nm using Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) as standard and the antiradical activity was defined as μg of Trolox equivalent/dry weight. The increment of antioxidant activity was calculated contrasting the values after treatment with those initially measured, including recovery, according to the following formula:

  • image(4)

where ACt is the antioxidant activity in treatment, ACc is the antioxidant activity of initial control.

Determination of phlorotannins.  Total soluble and insoluble phlorotannins were determined using the Folin-Ciocalteu method described by Gómez and Huovinen (22) using a 96-well microplate. Purified phloroglucinol is used as a standard. Due to that phlorotannins are practically the only phenolic compounds in brown algae, the interference by other phenols is very low (41). For the determination of the soluble fraction, 10 mg of silica gel dried algal material was homogenized with liquid nitrogen in a mortar. After adding 1 mL acetone (70%), the extract was kept shaking overnight at 4°C. After centrifugation (2500 g, 10 min), 50 μL of supernatant was solubilized in 250 μL of dH2O, 200 μL of 20% NaCO3 and 100 μL of 2 N Folin-Ciocalteu reagent. The samples were incubated for 45 min at room temperature in darkness, centrifuged at 5000 rpm for 3 min and the absorbance read at 730 nm.

The insoluble cell wall-bound fraction was quantified using a modified alkaline method (22,42). The alkaline treatment was repeated four times, and the aliquots of each treatment were analyzed separately. The precipitate was extracted using methanol, H2O, methanol, acetone and diethyl-ether. After drying for 1 h at 60°C, the insoluble residue was resuspended in 800 μL of 1 m aqueous NaOH and stirred for 2.5 h. Samples were centrifuged (3000 rpm, 5 min) and 100 μL aliquots neutralized with 10 μL of H3PO4.

Statistical treatment.  Data were compared using one-way analysis of variance (ANOVA), for temperature treatments with and without UV radiation followed by Tukey’s HSD post hoc analysis when differences were detected. Proportions and percentage data were arcsin transformed to meet the ANOVA requirements. ANOVA assumptions (homogeneity of variances and normal distribution) were examined using the Levene and Shapiro–Wilk W-tests respectively. To determine the correlation between physiologic variables Pearson’s test was performed. Statistical significance was set to P < 0.05.

Results

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

D. antarctica was the species showing the highest Fv/Fm values along a temperature gradient between 5 and 40°C, both after 15 (Fig. 2A) and 30 min (Fig. 2B). In general, the three species showed the highest and relatively constant values of fluorescence (0.58–0.72) between 5 and 25°C (P < 0.05). At 30°C, Fv/Fm started to decrease rapidly, reaching values <0.1 at 40°C in M. pyrifera and L. nigrescens (P > 0.05).

image

Figure 2.  Effect of 15 (A) and 30 (B) min exposure to different temperatures on the maximal quantum yield of fluorescence (Fv/Fm) of three kelps. Values are mean ± SE; n = 10–15.

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Increase of temperature was closely related with the decrease of Fv/Fm in the three studied species (Fig. 3). The highest photoinhibition of photosynthesis was observed in D. antarctica at 28°C (P < 0.05) in the absence of UV radiation. In M. pyrifera, UV exacerbated the effect of temperature at 20 and 28°C (P < 0.05), and in L. nigrescens at 10 and 28°C. Overall, the three species recovered well from the treatments, especially at 10 and 20°C (P < 0.05) (Fig. 3).

image

Figure 3.  Effect of temperature and temperature plus UV radiation on the maximal quantum yield of fluorescence (Fv/Fm) of three kelps. Algae were exposed for 3 h to the different experimental conditions and returned to control culture conditions for 4 h for recovery. Values are means ± SE; n = 10–15. Different letters denote statistically significant differences between different treatments according to ANOVA and Tukey HSD post hoc test.

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ETR was slightly affected by UV and temperature treatments (Fig. 4). Initial ETRmax values of L. nigrescens and D. antarctica were between 35 and 39 μmol·e·m−2·s−1 and higher than those measured from temperature and UV treatments (ranges between 28 and 35 μmol·e·m−2·s−1; P < 0.05). In M. pyrifera, the initial ETRmax values were lower (24 μmol·e·m−2·s−1; P < 0.05) compared with the other two species, and the UV exposure at 28°C caused up to 66% decrease in photosynthetic activity. In general, saturation point for photosynthesis (Ek) ranged from 107 μmol·m−2·s−1 in M. pyrifera to 218 μmol·m−2·s−1 in L. nigrescens, whereas UV radiation at different temperatures decreased Ek values by 33% in all the species (P < 0.05; Fig. 4).

image

Figure 4.  Effect of UV radiation (3 h exposure) at different temperatures on the photosynthesis versus light curves (P–I curves) of three kelps based on the estimation of electron transport rates (ETR). Data are mean ± SE; n = 3–4.

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The induction of soluble phlorotannins was variable between treatments and species, although there was a tendency of higher induction associated with UV radiation more than with temperature. In contrast, the concentration of insoluble phlorotannins did not change during the experiment. In L. nigrescens and M. pyrifera, soluble phlorotannins increased by 150 and 200% at 10°C in the presence of UV radiation respectively (P < 0.05) (Fig. 5). In combined treatments of elevated temperature (20 and 28°C) and UV radiation, no induction of phlorotannins was observed, a situation comparable to temperature treatments without UV radiation (Fig. 5). In D. antarctica, the pattern was different with increases of soluble phlorotannins close to 30% being observed in treatments without UV radiation at 20°C (P < 0.05; Fig. 5). On the other hand, in this species, exposure to UV radiation at 10°C did not increase soluble phlorotannins, but it did at 20°C (P < 0.05). After recovery from high temperature (20° and 28°C) and UV radiation, no obvious variations in phlorotannins were observed (Fig. 5).

image

Figure 5.  Effect of temperature and temperature plus UV radiation on the concentration of soluble and insoluble phlorotannins of three kelps. Algae were exposed for 3 h to the different experimental conditions and returned to control culture conditions for 4 h for recovery. Data are mean ± SE; n = 4–6. Different letters (lower case) indicate statistical differences among means from light and temperature treatments after ANOVA and Tukey HSD post hoc test.

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High temperature treatments, alone and in combination with UV radiation, resulted in variable levels of lipid peroxidation, which were also different between the studied species (Fig. 6). M. pyrifera exhibited the highest MDA levels (55–80 nmol·MDA·g−1 FW) followed by L. nigrescens and D. antarctica. Only in M. pyrifera and L. nigrescens a tendency of increasing values after exposure to combined action of high temperature and UV radiation (P < 0.05) was observed. Lipid peroxidation decreased after the 4 h recovery (P < 0.05; Fig. 6).

image

Figure 6.  Effect of temperature and temperature plus UV radiation on lipid peroxidation measured as formation of malondialdehyde (MDA) of three kelps. Algae were exposed for 3 h to the different experimental conditions and returned to control culture conditions for 4 h for recovery. Data are mean ± SE; n = 4. Different lower case letters indicate statistically significant differences among the different treatments after Tukey HSD post hoc test.

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The antioxidant activity of algal extracts was affected by temperature and UV treatments and by the species. The scavenging activity was at its highest after UV exposure at 20°C, especially in L. nigrescens and M. pyrifera (280 and 200% relative to control, respectively, P < 0.05) (Fig. 7). In L. nigrescens, evident antioxidant activity was also measured at 20°C without UV radiation (P < 0.05). Temperature of 28°C strongly inhibited the antioxidant activity of the algal extracts. In general, the capacity for radical scavenging of the algal extracts was significantly correlated with the concentration of soluble phlorotannins with r values <0.7 for the three studied species (Fig. 8).

image

Figure 7.  Effect of temperature and temperature plus UV radiation on the radical scavenging activity (% of the initial value) of three kelps. Algae were exposed for 3 h to the different experimental conditions and returned to control culture conditions for 4 h for recovery. Data are mean ± SE; n = 6. Different letters indicate statistically significant differences among experimental treatments after ANOVA and Tukey HSD post hoc test.

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image

Figure 8.  Relationship between the concentration of soluble phlorotannins (mg g−1 DW) and the antioxidant capacity (%) of extracts of three kelps. Pearson’s coefficients are indicated.

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Discussion

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

Photochemical reactions and photoinhibition

Short-term exposures to temperature revealed that photochemical quantum efficiency measured as Fv/Fm was high and relatively constant between 5 and 25°C. Thereafter, between 30 and 40°C, fluorescence decreased to negligible values. These results confirm the capacity of these organisms to endure high temperatures by short periods despite the fact that they are adapted to water temperatures that normally do not exceed 16°C. When high temperatures were coupled with high UV radiation for 3 h, decreases in Fv/Fm and ETRmax were exacerbated and recovery was delayed, especially in M. pyrifera and L. nigrescens. Short-term effects of temperature on photosynthesis have mostly been studied using O2 evolution and it is well known that photosynthesis increases progressively up to an optimum temperature (Q10 ratios close to 2.0), above which Q10 decreases mainly as a result of the thermal inactivation of some photosynthetic components and increased dark respiration and photorespiration (43). The effect of temperature can be primarily associated with the capacity of algae to photochemically quench excess solar energy (e.g. dynamic photoinhibition), which often occurs during the onset of solar radiation at midday concomitantly with the highest temperatures. Efficient adjustments in the early photosynthetic reactions in a time span of hours reported for intertidal seaweeds in the context of high solar radiation can also be functional in minimizing the effects of thermal stress and other stressful environmental factors. For example, plants acclimated to high temperatures normally exhibit higher capacity for dissipating excess excitation than plants acclimated to low temperatures (44), which has been demonstrated also in various intertidal macroalgae (45,46).

In contrast to species adapted to constant high temperatures, cold-temperate seaweeds not only have to withstand high temperatures in summer but they can also be exposed to very low temperatures, eventually freezing, during low tide in winter. Exposure to low temperatures in conjunction with high irradiance can cause increased formation of ROS, decreased water potential, similar to desiccation and effects on membrane fluidity, which limit repair processes in the thylakoids (47,48). In the case of intertidal kelps from southern Chile, during winter and in conjunction with constant strong winds, algae can be exposed to air temperatures <3°C and thus, episodes of dehydration are perfectly expectable.

Antioxidant activities and phlorotannin content

Lipid peroxidation (measured as MDA concentration) increased only slightly under high temperature (20 and 28°C) and UV radiation, especially in M. pyrifera and L. nigrescens. This response could be explained by an enhanced antioxidant activity of the extracts from these treatments. In fact, results (Fig. 7) indicated that the antioxidant activity increased up to 300% in algae exposed to UV radiation at a temperature of 20°C. UV radiation at a temperature of 10°C was only effective in L. nigrescens. Overall, the marked antioxidant capacity exhibited by the studied species, which resulted in low lipid peroxidation, could be responsible for the low decrease in photochemical reactions occurring at the thylakoids (e.g. Fv/Fm and electron transport). As the integrity of the thylakoid membranes is a prerequisite for the necessary balance between light absorption and damage repair during photoinhibition (49), the ability to photosynthesize at considerable rates during the onset of light and temperature suggest that ROS scavenging is a relevant antistress mechanism in these algae. For example, in the intertidal red alga Stictosiphonia arbuscula the antioxidative metabolism, especially of populations occurring at upper littoral levels and subject to extreme temperature and desiccation, minimizes lipid peroxidation and membrane damage after rehydration via an increase in the antioxidant enzyme activity (8). Moreover, in stress-tolerant species of Fucus lipid peroxidation and increased ROS associated with the exposure to desiccation, freezing and high light were found to be lower than in a more sensitive species (50).

In the present study, a correlation between the phlorotannins and the antioxidant activity during simultaneous exposure to high temperature and UV radiation could be demonstrated. These findings confirm recent studies reporting a similar relationship for the interactive effect of nutrients, light and metals on the photosynthetic metabolism of these three kelps (34). In various temperate species of Fucus and Laminaria, longitudinal gradients in antioxidant capacity were related with the content of phlorotannins (23). Purified forms of phlorotannins from the kelp Ecklonia cava were highly efficient as antioxidants to reduce the H2O2-mediated DNA damage of mouse cells (24). In the present study, the patterns of induction of soluble phlorotannins were highly variable between the treatments and species; however, some major tendencies could be recognized: first, these compounds were rapidly induced after a 3-h UV exposure, whereas the insoluble fraction remained unchanged, and second, treatments at 28°C caused no induction of phlorotannins. The capacity of these algae to mobilize rapidly soluble phlorotannins in response to UV stress confirms the results of a previous study on L. nigrescens, where soluble phlorotannins increased substantially between 2 and 6 h, whereas the insoluble fraction polymerized in the cell wall increased after 24 and 48 h (22). These results suggest that during short-term exposure the soluble fraction acts as a scavenger of ROS caused by the exposure to UV radiation, whereas the insoluble one remains as a cell wall constitutive component and their increase becomes evident only after a longer time span. It must emphasized that many aspects of the phlorotannin dynamics within the cell, and hence, the antioxidative mechanism, are poorly known. On the basis of the concentration of phenolsulphatases during high stress condition, Abdala-Díaz (51) suggested that phlorotannins enclosed in physodes could undergo changes in their chemical configuration, e.g. desulphatation via phenolsulphatases, and thus they could be released to the cytosol allowing their interaction with ROS. However, many questions regarding the link between these reactions and the transport of phlorotannins between different membrane interfaces or the polymerization processes of phlorotannins (52) remain open.

With the exception of D. antarctica at 20°C, treatments of high temperature deprived of UV radiation did not stimulate the production of phlorotannins. Although an effect of temperature alone on the synthesis of phlorotannins cannot be ruled out, probably this factor could not be regarded as a trigger for the induction of soluble phlorotannins or alternatively, exposure to high temperature requires a longer time to trigger induction. Temperatures >20°C inhibiting the rapid induction of phlorotannins might be interpreted as a stress condition exceeding the capacity of algae to synthesize these compounds.

Overall, the results summarized in the present study support the role of phlorotannins in metabolic adaptations to environmental stress of brown algae. The high antioxidant capacity of phlorotannins and other phenols described in seaweeds (e.g. tri hydroxy-coumarins) seems to be based on the property of these compounds to inhibit e.g. lipid peroxidation by the transfer of one hydrogen atom to lipid peroxyl radicals (53). In the case of Phaeophyceae, activities of enzymes such as superoxide dismutase, glutathione reductase, ascorbate peroxidase and catalase in response to UV stress have been shown to be lower compared with green and red algae (7). Although these results also revealed differences in the position on the shore and hence, the history of the solar UV stress, clearly suggest that phlorotannins can be essential to maintain antioxidative potential to be used during stress episodes in this group of algae, at least complementary to the pool of antioxidant enzymes.

Ecological implications

The intensity and duration of the thermal and light stress in intertidal macroalgae depend on their position on the shore. Although in large brown algae living at the highly battered littoral zone, where desiccation probably is less severe compared with the situation of brown algae living at the upper littoral zones (e.g. Fucus), enhanced solar UV radiation strongly impairs the photosynthetic performance of these organisms. Regarding the incident summer doses of solar radiation at latitude 40°S, kelps such as L. nigrescens are clearly more sensitive to UV radiation and hence, their photochemical responses more strongly affected, than species inhabiting upper littoral zones (e.g. Ulva intestinalis) (33). It must be emphasized that in the field the effect of high PAR can become the most important photoinhibitory component. However, outdoor experiments carried out in these species revealed relatively low rates of photoinhibition still at PAR levels exceeding 2000 μmol·m−2·s−1 (32).

Comparatively, the three studied kelps show different morphofunctional processes associated with their growth patterns, morphological organization and overall, their ecological strategies to persist in conditions characterized by strong wave action. In the case of D. antarctica and M. pyrifera, their anatomic features allow them to float at the water surface and thus exposure of photosynthetic tissues to high solar radiation and temperature can occur during high tide. This has contrasting consequences for the short-term responses as fronds are exposed to high solar radiation, but not to high temperatures. Finally, morphofunctional adaptations of these large and complex algae lead to the exposure of only some of their tissues (normally the apical parts of the fronds) to high environmental stress, whereas key parts necessary for attachment and growth (basal parts and meristems) are protected by a massive allocation of constitutive defenses (22,35).

Acknowledgments

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

Acknowledgements— This study was supported by a FONDECYT grant no. 1090494 to I.G. and P.H. and a CONICYT PhD fellowship no. 21070148 to E.C. The helpful technical assistance of Marcela Orostegui and Constanza Rosas is gratefully acknowledged.

References

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