Revista chilena de historia natural
versión impresa ISSN 0716-078X
Rev. chil. hist. nat. v.82 n.1 Santiago mar. 2009
Revista Chilena de Historia Natural 82:43-61, 2009
Photosynthetic responses to UV-radiation of intertidal macroalgae from the Strait of Magellan (Chile)
Respuestas fotosintéticas a la radiación UV en algas intermareales del Estrecho de Magallanes (Chile)
RALF RAUTENBERGER1,2, *, ANDRÉS MANSILLA4, IVÁN GÓMEZ5, CHRISTIAN WIENCKE6 & KAI BISCHOF2.3
1 Institute for Polar Ecology, Christian Albrechts University of Kiel, Wischhofstrasse 1-3, Building 12, D-24148 Kiel, Germany.
2 Department of Marine Botany, University of Bremen, FB 2 (Biology/Chemistry), Leobener Strasse NW 2, D-28359 Bremen, Germany.
3 Center for Tropical Marine Ecology, Fahrenheitstrasse 6, D-28359 Bremen, Germany.
4 Instituto de la Patagonia, Universidad de Magallanes, Avenida Bulnes 01855, Casilla 113-D, Punta Arenas, Chile.
5 Instituto de Biología Marina, Universidad Austral de Chile, Facultad de Ciencias, Casilla 567, Valdivia, Chile.
6 Alfred Wegener Institute for Polar and Marine Research, Section Seaweed Biology, Am Handelshafen 12, D-27570 Bremerhaven, Germany.
*e-mail for correspondence: firstname.lastname@example.org
For the first time, the photosynthetic performance of field-grown macroalgae from the Magellan Strait was evaluated with respect to their photoadaptation in the field and acclimation potential to ultraviolet radiation. Five macroalgal species were collected in the eulittoral and the upper sublittoral: Ulva intestinalis, Porphyra columbina, Adenocystis utricularis, Desmarestia confervoides and D. ligulata. Photosynthesis vs. irradiance (P-I) curves of macroalgae cultivated in the laboratory under low white light intensities more than a week were used to assess the acclimation potential to irradiance conditions in the field. Both, photosynthetic parameters referred to as maximum electron transport rate (ETRmax) and light saturation points of photosynthesis (Ik) were species-specific and did not correlate with the position on the shore. Ik ranging between 80 and 215 μmol photons m-2 s-1 was indicative of moderate sun-adaptation in all studied species. The acclimation potential to UV-radiation was determined as the extent and the rates (minh) of PSII inhibition (resulting from decreased maximum PSII-quantum yield (Fv/Fm)) after four hours of UV-exposure and as the subsequent rates of recovery (m ) in dim PAR. The results suggest that photosynthesis of each species during UV-exposure was transiently down regulated rather than damaged. Furthermore, no reductions in ETRmax were detected after 4 h exposure to UV-A and UV-B-radiation, and consequently the Calvin cycle could not be affected. Thus, Fv/Fm was a more sensitive parameter than ETRmax explaining the photosynthetic UV acclimation. Both, the rates of inhibition and the rates of recovery were not correlated with the morpho-functional groups of macroalgae nor to their vertical distribution. Thus, although the capacity to acclimate to UV-radiation varied among the different species, data suggest that such responses are based on metabolic adjustments or possibly photoprotective strategies. In conclusion and taking into account the light adaptation characteristics, all species collected in the midlittoral and the upper sublittoral at the Strait of Magellan seem to be well acclimated to UV-B doses occurring at their habitat, which might be a pre-requisite to withstand enhanced solar UV-B during ozone depletion or summertime irradiance conditions.
Key words: UV-radiation, macroalgae, photosynthesis, photoacclimation, Strait of Magellan.
La actividad fotosintética de algas provenientes del Estrecho de Magallanes fue medida por primera vez con el propósito de determinar su fotoadaptación en terreno y su potencial de aclimatación a la radiación ultravioleta. Cinco especies fueron recolectadas desde la zona eulitoral y sublitoral superior: Ulva intestinalis, Porphyra columbina, Adenocystis utricularis, Desmarestia confervoides y D. ligulata. Curvas de fotosíntesis versus irradiancia (P-I) medidas en algas cultivadas en el laboratorio por más de una semana fueron usadas para determinar la aclimatación potencial de las algas en su habitat. Tanto las capacidades fotosintéticas, determinadas a través de la tasa máxima de transporte de electrones (ETRmax), como los puntos de saturación de fotosíntesis (Ik) fueron especies-específicas, no observándose correlaciones con la distribución vertical de las algas. Valores de Ik con rangos entre 80 y 215 μmol m-2 s-1 indicaron una moderada adaptación a la luz en todas las especies estudiadas. La aclimatación a la radiación UV fue determinada a través del grado de disminución del rendimiento cuántico máximo del PSII (Fv/Fm) y su subsiguiente tasa de recuperación. Los resultados sugieren que bajo exposición a la radiación UV, la fotosíntesis no es dañada sino más bien regulada a la baja de forma transitoria. Además, no se observaron reducciones en ETRmax después de 4 h de exposición a la radiación UV-A y UV-B, y por consiguiente el ciclo de Calvin pudo no ser afectado. De este modo, Fv/Fm fue un parámetro mucho más sensitivo que ETRmax para explicar la aclimatación de fotosíntesis a la radiación UV. Tanto las tasas de inhibición como las tasas de recuperación no estuvieron correlacionadas con grupos morfofuncionales ni con la distribución vertical. Por tanto, aunque la capacidad para aclimatar a la radiación UV varió entre las diferentes especies, los resultados sugieren que estas respuestas están basadas en ajustamientos metabólicos o posiblemente estrategias fotoprotectoras. En conclusión, y tomando en cuenta las características de adaptación a la luz, todas las especies colectadas en el mesolitoral y sublitoral del Estrecho de Magallanes están bien aclimatadas a las dosis de UV-B que ocurren en su habitat, lo cual es un prerrequisito para tolerar aumentos de UV-B durante disminución de ozono o durante condiciones de alta radiación en verano.
Palabras clave: radiación UV, macroalgas, fotosíntesis, fotoaclimatación, Estrecho de Magallanes.
Chemical processes in the stratosphere over Antarctica lead to an annual depletion of ozone concentration in austral springtime since 1979 (Farman et al. 1985). When the ozone concentration in the air-column falls below the threshold of 220 Dobson Units, the so-called ozone hole develops (Stolarski et al. 1990). Ground-based measurements revealed a dramatic springtime loss of total ozone by ≈=30 % over Antarctica in 1979 compared with normal conditions (Farman et al. 1985). As soon as the polar vortex over the Antarctic breaks down in late spring, the ozone-poor air mass may move towards the southern tip of South America (Thompson 1991, Casiccia et al. 2003). Several times, the area of the Antarctic ozone hole has reached record sizes of more than 28 million km2, e.g., in the years 1998, 2000, 2003 and 2006 (Newman et al. 2007). Since the beginning of the 1990s, in September and October the Antarctic ozone hole has transiently extended as far north as to the southernmost regions of South America, e.g., Ushuaia (in 1990) and Punta Arenas (in 1993) (Casiccia et al. 2003). The most important consequence of low stratospheric ozone concentrations is the increase of solar UV-B-radiation on the Earth's surface (Casiccia et al. 2003), which is also affecting to aquatic environments (Tedetti & Sempéré 2006).
Irradiance conditions in aquatic environments depend on multiple factors such as optical properties of the water body, the time of day, season and cover of the sky by clouds (Kirk 1994). Thus, marine organisms are exposed to rapidly changing irradiance conditions. In the case of (adult) macroalgae. which are strictly sessile, diverse metabolic mechanisms (e.g., photoacclimation and inhibition of photosynthesis) have been identified as a way to avoid impairments induced by UV-radiation (reviewed by Bischof et al. 2006).
Although various studies have hitherto investigated the response of macroalgae to UV-radiation in relation to their vertical distribution in temperate and polar regions (Bischof et al. 1999, Gómez et al. 2004, Rautenberger & Bischof 2006), no information is available about photoadaptation and acclimation potential of photosynthesis to UV-radiation of adult macroalgae occurring at Strait of Magellan (Mansilla et al. 2006) or southernmost Patagonia (Rautenberger & Bischof 2006), a region especially affected by transient Antarctic ozone hole (Casiccia et al. 2003).
The goal of this study was to examine the light adaptation status of macroalgal assemblages of the Magellan region, which may be primary targets of enhanced solar UV-B radiation with important consequences for the whole coastal ecosystem (Hader et al. 2007). An experiment to evaluate UV-susceptibility of photosynthesis of macroalgae growing at Strait of Magellan was conducted. This study represents a first attempt in order to gain insights into potential mechanisms of photobiological acclimation and tolerance in macroalgal assemblages exposed naturally to enhanced UV-B radiation.
MATERIAL AND METHODS
Algal material and experimental conditions
Five different macroalgal species (Table 1) from the tidal zone of the Strait of Magellan were collected at Punta Santa Ana (53.50° S, 70.93° W) during late austral summer on 16th and 29th March 2005 (Fig. 1). Immediately after sampling, specimens were transported in a cooled and dark box to the laboratory of the Department of Science and Natural Resources of the Institute of Patagonia at the Universidad de Magallanes in Punta Arenas (Chile). Macrothalli were raised in air-bubbled Provasoli enriched seawater (Provasoli 1968) in a climate room at 8 ± 1 °C with artificial radiation of 20 μmol photons m-2s-1 PAR (provided by one True Light Natural Daylight fluorescent tube, Osram, Germany). The temperature of the cultivation room was set up to the water temperature of the Strait of Magellan during summer according to the Meteorological Service of the Chilean Navy (www.shoa.cl/cendhoc/atlas/index.htm). Macroalgae were cultivated in the laboratory between eight and 13 days until experimentation. However, Desmarestia ligulata was used for UV-experiment one day after sampling. For UV-exposure experiments, algae were divided and spread out in a thin layer of macrothalli in Petri dishes to minimize self-shading. After 12 h of pre-incubation at 15 μmol photons m-2s-1 PAR (provided by one.
True Light Natural Daylight fluorescent tube, Osram, Germany), radiation was supplemented with UV-A (provided by three UV-A-340 fluorescent tubes, Q-Panel Lab Products, Cleveland, USA) and UV-B radiation (provided by three TL20W/12RS, Philips, The Netherlands) for 4 h. By the use of different cut-off filters algae were either exposed to PAR alone (controls, λ ≥400 nm, achieved by Ultraphan URUV, Digefra, Munich, Germany), to PAR + UV-A λ ≥320 nm, Folanorm SF-AS, Folex, Cologne, Germany) or to PAR + UV-A + UV-B λ ≥295 nm, Ultraphan URT, Digefra, Munich, Germany). The light regime of the experimental set-up during UV-exposure is summarized in Table 2 and the spectral composition in Fig. 2. In order to study effects of UV radiation (particularly UV-B radiation) on macroalgal photosynthesis exclusively, intensities of PAR were kept low at about 17 μmol photons m-2 s-1. Under such unrealistic radiation conditions, UV-effects on photosynthesis remain unmasked by high PAR induced effects as observed under natural radiation conditions. Light intensities and UV-irradiances were measured using an UV/VIS spectroradiometer (Ramses ACC Hyperspectral Sensor, TriOS GmbH, Oldenburg, Germany). The integral dose of exposed UV-B radiation (PAR + UV-A + UV-B, unweighted) over the entire experimental time (4 h) was 25.55 kJ nr2. The biologically effective dose of 0.82 kJ m 24 h-1 [according to the model for generalized plant damage of Caldwell (1971)] was chosen to simulate daily UV-B doses occurring both under ozone depletion in early spring (October) and under non depleted conditions in summertime caused by low a solar zenith angle over southernmost South America [around 31° in December, database of National Science Foundation UV Monitoring Network, USA (http://www.biospherical.com/nsf/)]. After UV-exposure, algae were cultivated under low PAR intensities of 20 μmol photons m-2s-1 for recovery. Measurements of chlorophyll a fluorescence were performed before (t0), after two and 4 h of UV-exposure and after four and 24 h of recovery. The experiment performed with P. columbina was completed after 4 h of recovery; therefore data after 24 h of recovery are not available.
Chlorophyll a fluorescence measurements
Measurements of in vivo chlorophyll a fluorescence were performed with a portable pulse amplitude modulated fluorometer PAM-2100 (Walz GmbH, Effeltrich, Germany) according to the protocol of Hanelt (1998). Pieces of algal material were taken from Petri dishes and samples were inserted into custom-made cuvettes filled with seawater at 8 ± 1 °C. The end of the fiber optic probe of the PAM-fluorometer was mounted to the algal samples at a distance of 5 mm. After the application of a 5 s far-red pulse, the brown and green algae were darkened for 5 min. Measuring the red alga, a second short, red, actinic pulse was given after 5 min. of dark adaptation. Basal fluorescence, F0, was determined by using the measuring beam and maximal fluorescence, Fm, was induced after applying a saturation pulse. Photosynthetic efficiencies were calculated according to the equation of Schreiber et al. (1994).
The change of Fv/Fm over the experimental time was expressed as percentage of inhibition (%) and calculated by the formula:
Fv/Fmuv represents photosynthetic efficiency of samples exposed to PAR + UV-A and PAR + UV-A + UV-B. Fv/Fmcontrol is photosynthetic efficiency of samples exposed under PAR alone.
Linear regression revealed as the best fit (y = mx + n, r2 ≥ 0.61) to determine the rates of inhibition of PSII (minh) after 4 h of UV-exposure (Fig. 8). Rates of recovery (mrec). however, were obtained by fitting data to an exponential function (y = a.e -bx, r2 ≥ 0.95).
Specimens were illuminated with continuous increasing actinic white light intensities (PPFD: 42-1,562 μmol photons m-2 s-1). Every 30 s, the actinic light intensity was further increased after application of a saturation pulse to determine effective quantum yield of PSII (φPSII):
Calculation of relative electron transport rate (reí. ETR) was performed by multiplying the effective quantum yield of PSII by corresponding intensities of actinic white light (PPFD = photosynthetically photon flux density) according to Schreiber et al. (1994):
Then, relative ETR was plotted against PPFD of incident light and data points werefitted and calculated according to the model of Eilers & Peeters (1988). Photosynthesis vs. irradiance curves (P-I curves) were used to estimate the photosynthetic activity by determination of maximal electron transport rate (ETRmax) according to Bischof et al. (1999) and light saturation point of photosynthesis (Ik). Initial linear slopes (a) of P-I curves do not represent photosynthetic quantum efficiency because the amount of absorbed photons was not determined (Saroussi & Beer 2007).
For controls (non-photoinhibited cultures exposed to PAR alone), Fv/Fm ratios for U. intestinalis (0.742 ± 0.008 r.u.), A. utricularis (0.744 ± 0.003 r.u.), P. columbina (0.659 ± 0.020 r.u.), D. ligulata (0.728 ± 0.005 r.u.) and D. confervoides (0.676 ± 0.004 r.u.) were determined and remained constant during the whole experimental time. Mean values of respective Fv/Fm values of controls were normalized to 100 % and all the following values were calculated as a percentage of these. Statistical analyses were performed using JMP IN 5.1 (SAS Institute Inc., USA) following an arcsin-transformation of the data set. One-way analysis of variance (ANOVA) with repeated measurements and the Tukey-Kramer honestly significant difference (HSD) post-hoc test was performed in order to identify differences between treatments. A level of probability of P <0.05 was used.
P-I parameters of field-grown macroalgae under non-stressful conditions
Initial linear slope of P-I curves (a) of all five species ranged from 0.47 to 0.80 (μmol photons m-2 s-1)-1 (n = 6 to 9). The green alga U. intestinalis, the red macroalga P. columbina and the brown macroalgal species A. utricularis exhibited a steeper slope ranging from 0.78 to 0.80 (μmol photons m-2 s-1)-1 (Table 3). All these species were collected in the eulittoral (Table 1). In contrast, brown algal species, D. confervoides and D. ligulata, collected from the upper sublittoral had statistically lower initial a-slopes of 0.47 and 0.55 (μmol photons m-2 s-1)-1 (n = 8 and n = 7), respectively.
The highest maximum relative electron transport rates (ETRmax) of 136.1 ± 15.2 r.u. (n = 9) were measured in the green alga U. intestinalis and these were followed by significantly lower ETRmax of 98.6 ± 18.6 r.u. (n = 8) obtained from D. confervoides. In other three species occurring in different tidal locations, ETRmax ranged from 59.1 to 74.8 r.u. (n = 6 or 7).
U. intestinalis and D. confervoides exhibited the highest light saturation points of photosynthesis (Ik): 174.3 ± 23.4 μmol photons m-2 s-1 and 215.0 ± 54.0 μmol photons m-2 s-1 (n = 9 and n = 8), respectively. Ik-values of P. columbina (99.3 ± 21.3 μmol photons m-2 s-1; n = 6), A. utricularis (80.4 ± 33.0 μmol photons m-2 s-1, n = 7) and D. ligulata (115.9 ±11.2 μmol photons m-2 s-1, n = 7) were statistically lower than those of U. intestinalis and D. confervoides but not different among each other (Table 3).
Photo synthetic efficiency under UV-exposure
In all examined macroalgae, both incident UV-A and UV-B radiation caused a decrease of the photosynthetic efficiencies (Fv/Fm) and, thus, an increase of inhibition of PSII. However, exposure to low PAR intensities alone (= control) did not lead to a PSII inhibition. The UV-induced increase of PSII inhibition was already observed after two hours of exposure to UV-A and UV-B radiation. At that time, no statistical differences of inhibition were detected between both UV-treatments in all species with the exception of D. confervoides. Photosystem II of that species was inhibited up to 7 % due to incident UV-A radiation and to 12 % due to UV-B radiation (Fig. 3). After four hours, inhibition did not increase statistically despite its prolonged exposure to UV radiation. Therefore, Desmarestia confervoides exhibited pronounced differences of PSII inhibition between exposure to UV-A (7 %) and UV-B radiation (16 %). Photosystem II of U. intestinalis, however, showed marked differences only after four hours because of increased PSII inhibition due to prolonged exposure to UV-B radiation. PSII was inhibited by UV-A radiation up to 16 % and due to UV-B-exposure to 25 % (Fig. 4). In D. ligulata, inhibition increased by the factor of two from 7 % after two hours to 14 % after four hours of UV-A-exposure. However, increase of inhibition from 12 % after two hours to 26 % after four of UV-B-exposure was not significant (P ≥ 0.05). Nevertheless, impinging UV-B radiation caused a statistically higher inhibition of PSII than UV-A radiation (Fig. 5). In P. columbina and A. utricularis, maximum degrees of inhibition were already reached after two hours under both UV-treatments. They did not further increase despite prolonged UV-exposure of two hours additionally. In both species, neither UV-A radiation nor UV-B radiation caused significant differences of PSII inhibition at both times. UVA and UV-B radiation inhibited PSII to 5 to 13 % in P. columbina, respectively (Fig. 6). In A. utricularis, exposure to UV-A and UV-B radiation lead to an inhibition of 15 to 26 %, respectively (Fig 7).
Highest rates of inhibition (minh) were detected in A. utricularis (UVA: 5.80 ± 1.95. UV-B: 6.91 ± 1.91; n = 3), U. intestinalis (UVA: 3.30 ± 0.63, UV-B: 6.08 ± 0.88; n = 3) and D. ligulata (UVA: 3.83 ±0.38, UV-B: 6.67 ± 1.53; n = 3) exposed to both UV treatments (Fig. 8). The other two species (P. columbina and D. confervoides) exhibited low inhibition rates of 2.17 ± 0.58 and 2.24 ± 0.05 (both n = 3) due to UV-A exposure and 2.75 ± 1.64 and 3.97 ± 0.25 (both n = 3) under UV-B radiation (Fig. 8).
After transfer to low PAR intensities close to 20 μmol photons m-2s-1, first inhibition of PSII decreased again quickly within four hours but, then, it slowed down within subsequent 20 hours in each species. This suggests a two stage recovery of PSII. After four hours in dim PAR, inhibition of PSII in P. columbina and D. confervoides did recover completely (= 0 % of inhibition) (Fig. 3 and 6). Although degrees of inhibition were still approximately 5% in the other three species after four hours, they required 24 hours to recover completely. Specimens of A. utricularis exposed to PAR + UV-A + UV-B, however, recovered to 97 % after 24 hours in comparison to control cultures. However, rates of recovery (mrec) were not statistically significant between species and UV-treatment (Fig. 9).
Photosynthesis versus irradiance (P-I) curves of UV-exposed macroalgae
Comparisons of photosynthesis versus irradiance (P-I) curves after four hours of exposure to UV-A and UV-B radiation of all examined species are displayed in Fig. 10, 11 and 12. Maximal relative electron transport rates (ETRmax) of controls of the chlorophyte U. intestinalis were 151.6 ± 8.1 r.u. (n = 3) but incident UV-A radiation significantly decreased relative ETRmax down to 121.6 ± 9.5 r.u. (n = 3) (Fig. 10A). Exposure of U. intestinalis to UV-B radiation also reduced ETRmax to 130.6 ±11.4 r.u. (n = 3) but this reduction, however, was not statistically different to that of controls. In contrast, ETRmax of other four species (P. columbina (Fig. 10B), A. utricularis (Fig. 11), D. confervoides (Fig. 12A) and D. ligulata) did not change statistically due to exposure neither to PAR + UV-A nor to PAR + UV-A + UV-B.
Here, for the first time, we conducted studies on the photosynthetic performance of five field-grown macroalgal species collected at the Strait of Magellan in southernmost Chile with respect to photoadaptation to their vertical pattern of distribution and their acclimation to ultraviolet radiation. All examined species exhibited a fast acclimation to both UV-A and UV-B radiation as indicated by the fast return of decreased Fv/ Fm to their initial photosynthetic efficiencies. Such high recovery rates of inhibited PSII were shown in various cold-temperate and polar macroalgae inhabiting the eulittoral and upper sublittoral in opposition to macroalgae from deeper growth sites (lower sublittoral) or the subcanopy exhibiting slower proceeding recoveries (Dring et al. 1996, Bischof et al. 1998). Down-regulation of photosynthetic efficiencies as a regulatory mechanism of photoprotection, rather than damage, is assumed due to fast kinetics of recovery after UV-exposure.
Photoadaptive characteristics of algae
P-I parameters of collected field-grown macroalgae (cultivated for 1-2 weeks in the laboratory under non-stressful conditions) give information about the state of photoadaptation, which is considered to be genetically fixed (reviewed by Bischof et al. 2006).
The results of reí. ETR suggest that photosynthetic performance is rather species-specific than a function of tidal position on the shore. A general pattern of ETRmax between species from the eulittoral and the upper sublittoral was absent. Studies of Gómez et al. (2004) confirm this finding as no correlation between ETRmax and position on the shore investigating 18 intertidal macroalgal species from the Chilean coast at Niebla (41° S, near Valdivia) was found. These authors also showed that U. intestinalis had higher photosynthetic capacities than P. columbina and D. ligulata. This is consistent with this study. The results suggest that thallus morphology plays an important role influencing photosynthetic capacity. Ulva intestinalis consists of a sheet-like but tubular thallus with a high content of chlorophyll a (Gómez et al. 2004). This thallus morph has a high surface-to-volume ratio with almost exclusively photosynthetic cells (Weykam et al. 1996). Thus, U. intestinalis may be classified into a group with high photosynthetic rates according to the morpho-functional model described by Littler & Arnold (1982). On the other hand, Johansson & Snoeijs (2002) found that photosynthetic capacity (based on oxygen evolution and normalized to biomass) depends on thallus morphology: thinner and filamentous species had higher rates than coarser and thicker species. Similarly, Poole & Raven (1997) outlined that most of species of the former genus Enteromorpha including E. intestinalis (now Ulva. intestinalis) have high photosynthetic capacities. In our study, the lower photosynthetic capacity (Table 3) measured in A. utricularis and D. ligulata, which exhibit saccate or leathery morphology, respectively, are associated to less chlorophyll a, a lower surface-to-volume ratio and a great proportion of structural tissues (Weykam et al. 1996). The brown alga D. confervoides takes an intermediate position probably because of its terete and branched thallus morphology.
The light saturation point of photosynthesis (Ik) is regarded as an indicator of the photoadaptive status of macroalgae with respect to their position in the littoral zone. For macroalgae from polar and Chilean cold-temperate waters, Ik-values are higher in species occurring in the eulittoral and the upper sublittoral than those determined in seaweeds growing below 10 m water depth (reviewed by Wiencke et al. 2007, Gómez et al. 2004). Thus, macroalgae from the midlittoral are better adapted to strong irradiances than species from the sublittoral. Such a clear general pattern could not be confirmed in this study. One reason might be the small number of species used in this study or alternatively, that all examined macroalgae inhabited shallow sites. Apparently, the studied algae are exposed to high irradiance conditions in summer and may be regarded as moderate sun-adapted organisms, irrespective of their position on the shore. For example, the eulittoral U. intestinalis and the upper sublittoral D. confervoides exhibited ranges of Ik comparable. Depending on the biogeographic region, Ik values 50 and 400 μmol m-2s-1 indicate firstly sun-adapted characteristics and secondly that species are capable to cope with strong solar irradiance conditions by dynamic photoinhibition (Poole & Raven 1997, Gómez et al. 2004, Hanelt et al. 1994). In contrast, only species occurring in deeper waters are characterized as shade-adapted (reviewed by Wiencke et al. 2007).
Acclimation to UV-radiation
Photosynthetic efficiencies were directly affected by incident UV radiation in all examined macroalgae collected in the tidal zone of the Strait of Magellan. The ratio of Fv/ Fm is an important and sensitive parameter to assess the physiological state of algae and plants exposed to environmental stress (Krause & Weis 1991). UV-B-radiation leads to a stronger inhibition of PSII because of the higher energy content of UV-B waveband in comparison to UV-A waveband. In contrast to changes of Fv/Fm during UV-exposure, there was only a small impact of incident UV-radiation on photosynthetic capacities, hence, Fv/Fm is revealed as a more sensitive parameter for evaluation of UV-susceptibility of macroalgae than fluorescence-based P-I curves determined. This is in line with other studies performed on UV-mediated stress on brown and red macroalgae (Dring et al. 1996, Bischof et al. 1999). This suggests that the capture of light energy by the light harvesting complexes and energy transfer efficiency from the light harvesting complexes (LHC) to PSII of macroalgae might be more affected (Clendennen et al. 1996) than the photosynthetic reaction centers of PSII which may still remain active (Hanelt et al. 1997, Bischof etal. 1999).
Many studies have been undertaken to reveal correlations between susceptibility or tolerance of macroalgae to UV-radiation and their vertical distribution (reviewed by Bischof et al. 2006). Gómez et al. (2004) demonstrated that UV-tolerance of 18 intertidal macroalgae from Chile is rather related to metabolic adjustments than to morpho-functional groups or to vertical distribution. In our study, neither the morphology of macroalgal thalli nor the tidal distribution of examined species could be correlated to the extent of photoinhibition during UV-exposure or rate of recovery (mrec) in dim PAR. The ability of recovery in dim PAR is regarded as an important parameter for evaluation of UV-susceptibility (Hanelt et al. 1997, Bischof et al. 1999). Despite PAR is important for repair, the results show that the macroalgae recovered after UVR exposures, meaning that low PAR intensities used in this study, despite unrealistic, did not impair repair significantly. The rate of recovery was found to be independent on the extent of UV-induced photoinhibition and suggests that adverse effects of UV radiation on PSII were absent. Hence, this is a hint that acclimation of surveyed Patagonian macroalgae to UV-radiation is based on species-specific adjustments of metabolism and photoprotective strategies. Furthermore, they exhibited a high acclimation potential to UV radiation regardless to their position on the shore.
Porphyra columbina seems to be a very well acclimated species to enhanced UV radiation indicated by the low increase of inhibition of PSII, its complete recovery already after 4 h and unaffected P-I curves.
Porphyra. umbilicalis from the upper eulittoral (0 m) of the Island of Helgoland (Germany) revealed no reduction of Fv/Fm even after 96 h of UV-exposure (Dring et al. 1996). The high UV tolerance determined in several species of Porphyra is certainly due to the high content of UV-absorbing mycosporine-like amino acids (Karsten et al. 1998, Hoyer et al. 2003, Huovinen et al. 2004).
The green macroalga U. intestinalis exhibited a reduction of Fv/Fm followed by a fast recovery completed within 24 h. This indicates a transient and rapidly reversible down-regulation of photosynthetic activity. Down-regulation of the photosynthetic apparatus was interpreted by Russell et al. (1995) as a photoprotective strategy primarily developed to dissipate excess PAR energy, which might be also active as a response to ultraviolet radiation. A small distinct increase of PSII inhibition and a fast subsequent recovery were also reported of the same species from the Northern and Southern hemisphere exposed to artificial and solar radiation (Cordi et al. 1997, Gómez et al. 2004, Huovinen et al. 2006). Furthermore, a decrease of ETRmax after four hours of UV-A-exposure and its subsequent complete recovery after four hours (not shown) suggests that UV-A-radiation did not cause serious detrimental effects on the photosynthetic and enzymatic apparatus of the Calvin-Benson cycle.
UV-stress experiments conducted by Gómez et al. (2004) and Huovinen et al. (2006) using specimens of U. intestinalis occurring in the upper midlittoral near Valdivia (Chile) showed a less pronounced photoinhibition of approximately 10 % and lower. The authors concluded that this species seems to be less susceptible to UV-radiation because of possibly highly efficient physiological acclimation to prevailing UV-conditions. This is in line with earlier studies by Cordi et al. (1997) who deduced a development of protection mechanism against UV-B-radiation.
Furthermore, in tide pools at the Strait of Magellan (R. Rautenberger, personal observation), U. intestinalis is protected against desiccation and its impairments on photosynthesis. Additionally, rising temperatures of the water of tide pools caused by heating of incident solar radiation might protect photosynthesis of U. intestinalis against UV-radiation, e.g., by more efficiently operating photoprotective mechanisms. This was shown for two Ulva species from southernmost Chile and the Antarctic region (Rautenberger & Bischof 2006). Furthermore, low fluctuations of seasonal photoinhibitory response to ultraviolet radiation reflect a good adaptation to ambient irradiance conditions (Huovinen et al. 2006), which may be operating in Patagonian assemblages of U. intestinalis.
Results of the brown alga A. utricularis suggest that this species was also well acclimated to UV-radiation possibly by regulatory mechanisms. Inhibition of PSII after four hours of UV-exposure was one of the highest compared to the other species. Subsequent (fast) recovery of photosynthetic activity and unaffected photosynthetic capacities after four hours of UV-exposure seems to indicate that photosynthesis was rather down regulated than damaged. Rautenberger & Bischof (2008) also reported such a fast reversible photoinhibition in Antarctic members of this species. Hanelt et al. (1994) concluded from their field-experiments of diurnal changing solar irradiance in the Antarctic that A. utricularis is able to cope with strong radiation stress (including UV-radiation) in virtue of its ability for dynamic photoinhibition, which acts as a photoprotection mechanism (reviewed by Wiencke et al. 2007). Although these findings refer to Antarctic individuals, photosynthesis of Patagonian individuals of A. utricularis might also be protected by a rapidly reversible photoinhibition without further damage of the photosynthetic apparatus. During low tide, A. utricularis is exposed to arising ultraviolet radiation, which is accompanied by desiccation of algae. Apparently, photosynthesis could be additionally protected by the special thallus morphology of A. utricularis. Internal water reservoir inside the saccate thallus keeps it moistened and, thus, it avoids its desiccation (Hanelt et al. 1994).
Desmarestia confervoides and D. ligulata
Apparently, photosynthesis of D. confervoides and D. ligulata collected from the upper sublittoral was also transiently down regulated as shown for the midlittoral species. Nevertheless, the acclimation potential of both species from the same genus and the same tidal position was different and supports the idea of a species-specific phenomenon. Besides metabolic adjustments, UV-sunscreen substances may contribute to protect against UV-radiation. For example, Fairhead et al. (2005) have found phlorotannins, a type of phenolic compounds, in two Antarctic endemic species of Desmarestia (D. anceps and D. menziesii) but they could not provide evidence for their role in UV-protection (Fairhead et al. 2006). Overall, the laboratory experiment was designed to simulate weighted daily UV-B doses according to Caldwell (1971) occurring under ozone depletion in austral springtime and summer irradiance conditions over the southernmost South America in 2004 and 2005, respectively (database from National Science Foundation UV Monitory Network).
In conclusion, our results suggest that adult sporophytes of macroalgal species growing in the intertidal zone of the Strait of Magellan, in virtue of their ability for photoacclimation, are capable of coping with these UV-B doses. Here, a rapid and transient down regulation of photosynthesis seems to be the main mechanism under suddenly increasing UV levels. However, other adverse effects of enhanced UV-radiation, e.g., on small developmental stages of macroalgal species like spores and gametophytes may not be ruled out (Mansilla et al. 2006, Wiencke et al. 2007) and, therefore, further studies are needed.
The authors thank the Alfred Wegener Institute for Polar and Marine Research (Bremerhaven, Germany) for financial support to work in Punta Arenas. Further funding was provided by the Deutsche Forschungsgemeinschaft (BI 772/ 2-1,2) and the Helmholtz-Gemeinschaft deutscher Forschungszentren (VH-NG-059). The collaboration of Nelson Navarro and Mauricio Palacios (both Universidad de Magallanes in Punta Arenas) during collecting algal material and laboratory work is gratefully acknowledged.
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Associate Editor: Vivian Montecino
Received January 1, 2008; accepted September 25, 2008.