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 )

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 (mrec) 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.


INTRODUCTION
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 km 2 , e.g., in the years 1998, 2000, 2003and 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 UVradiation 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 UVradiation 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 (Häder 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.

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 16 th and 29 th 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 -2 s -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 ( w w w .s h o a .c l / c e n d h o c / a t l a s / i n d e x .h t m ) .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 -2 s -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 UVirradiances 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 m -2 .The biologically effective dose of 0.82 kJ m -2 4 h -1 [according to the model for generalized plant damage of Caldwell (1971)] was chosen  Light regime in the experimental treatments measured by an UV/VIS spectroradiometer (Ramses ACC, TriOS GmbH, Oldenburg, Germany).Weighted UV-B irradiances were calculated by multiplying the action spectra for generalized plant damage response according to Caldwell (1971) and unweighted UV-B-radiation.Dose of unweighted and weighted UV-B-radiation during the entire experimental time of PAR + UV-A + UV-B exposure were 25.55 kJ m -2 4 h -1 and 0.82 kJ m -2 4 h -1 , respectively.Different letters behind data represent statistically significant differences between light treatments Condiciones de luz en los tratamientos experimentales medidos usando un espectroradiómetro UV/VIS (Ramses ACC, TriOS GmbH, Oldenburg, Alemania).Las radiaciones UV-B ponderadas fueron calculadas multiplicando el espectro de acción de la respuesta de daño generalizado en plantas de acuerdo a Caldwell (1971) y la radiación UV-B experimental.Dosis de radiación UV-B experimentales (no ponderadas) y ponderadas durante el periodo de experimentación a exposiciones a PAR + UV-A + UV-B fueron 25,55 kJ m -2 4 h -1 y 0,82 kJ m -2 4 h -1 , respectivamente.Diferentes letras representan diferencias estadísticamente significativas entre tratamientos de iluminación Light treatment PAR (μmol photons m -2 s -1 ) UV-A (W m -2 ) UV-B (W m -2 ) unweighted UV-B (W m -2 ) weighted PAR 17.4 ± 0.9 a 1.2 ± 0.0 a 0.00 ± 0.00 a 0.00 ± 0.00 a PAR + UV-A 16.0 ± 0.9 a 9.8 ± 0.4 b 0.16 ± 0.00 b 0.00 ± 0.00 a PAR + UV-A + UV-B 16.9 ± 0.9 a 11.6 ± 0.4 c 1.77 ± 0.01 c 0.06 ± 0.00 b 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°i n December, database of National Science Foundation UV Monitoring Network, USA (http://www.biospherical.com/nsf/)].After UVexposure, algae were cultivated under low PAR intensities of 20 μmol photons m -2 s -1 for recovery.Measurements of chlorophyll a fluorescence were performed before (t 0 ), 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 custommade cuvettes filled with seawater at 8 ± 1 °C.The end of the fiber optic probe of the PAMfluorometer 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, F 0 , 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). (1) The change of Fv/Fm over the experimental time was expressed as percentage of inhibition (%) and calculated by the formula:

Fv
(Fm' -F t ) Fv/Fm UV represents photosynthetic efficiency of samples exposed to PAR + UV-A and PAR + UV-A + UV-B.Fv/Fm control is photosynthetic efficiency of samples exposed under PAR alone.
Linear regression revealed as the best fit (y = mx + n, r 2 ≥ 0.61) to determine the rates of inhibition of PSII (m inh ) after 4 h of UVexposure (Fig. 8).Rates of recovery (m rec ), however, were obtained by fitting data to an exponential function (y = a • e -bx , r 2 ≥ 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 (ϕ PSII ): Calculation of relative electron transport rate (rel.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 (I k ).Initial linear slopes (α) of P-I curves do not represent photosynthetic quantum efficiency because the amount of absorbed photons was not determined (Saroussi & Beer 2007).

Data treatment
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.
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).

Photosynthetic 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 UVexposure 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).Photosynthetic versus irradiance (P-I) parameters determined from relative maximal electron transport rate (ETRmax) at continuously increasing actinic light intensities measured by pulseamplitude modulated (PAM) chlorophyll fluorescence.Tested field-grown macroalgal species were measured under unstressed conditions.Data were submitted to one-way ANOVA (P < 0.05, Tukey-Kramer HSD test).Different letters denote statistical differences between each species for a corresponding P-I parameter Parámetros de curvas fotosíntesis versus irradianza (P-I) determinados a partir de la tasa máxima de transporte de electrones (ETRmax) durante aumento continuo de intensidades de luz actínica usando fluorescencia de clorofilas de pulso/ amplitud modulada (PAM).Las mediciones de algas crecidas en ambiente natural se realizaron mientras estas no se encontraban sometidas a estrés solar.Los datos fueron tratados aplicando ANOVA de una vía (P < 0,05; prueba de Tukey-Kramer HSD).Diferentes letras denotan diferencias estadísticas entre cada especie para cada parámetro P-I Species Initial linear slope, Relative ETRmax(r.u.)I k (μmol photons m -2 s -1 ) n α[(μmol photons m -2 s -1 )     Highest rates of inhibition (m inh ) 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 -2 s -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 (m rec ) were not statistically significant between species and UV-treatment (Fig. 9).

DISCUSSION
Here, for the first time, we conducted studies on the photosynthetic performance of five fieldgrown 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 rel.ETR suggest that photosynthetic performance is rather speciesspecific 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 Curvas de luz de las especies de macroalga parda Desmarestia confervoides (A) y Desmarestia ligulata (B) después de cuatro horas de exposición a bajo radiación PAR(línea sólida), PAR + UV-A (línea segmentada) y PAR +UV-A + UV-B (línea punteada).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 surfaceto-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 (I k ) 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 coldtemperate waters, I k -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 I k comparable.Depending on the biogeographic region, I k values 50 and 400 μmol m -2 s -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 shadeadapted (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 UVradiation 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 et al. 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 (m rec ) 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 UVradiation 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
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 mycosporinelike amino acids (Karsten et al. 1998, Hoyer et al. 2003, Huovinen et al. 2004).

Ulva intestinalis
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.

Adenocystis utricularis
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 fieldexperiments 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.

Fig. 1 :
Fig. 1: Sampling site of used macroalgal species at Punta Santa Ana, 60 km south of Punta Arenas at Strait of Magellan, Chile.Sitio de recolección de macroalgas en Punta Santa Ana, 60 km al sur de Punta Arenas en el Estrecho de Magallanes, Chile.

Fig. 6 :
Fig. 6: Porphyra columbina.Degree of photoinhibition (A) due to PAR + UV-A (= PA: solid line) and PAR + UV-A + UV-B-exposure (PAB: dashed line) and its subsequent recovery (B, right).The experiment was cancelled after four hours of recovery; therefore, data of 24 hours of recovery are not available.Photoinhibition data were fitted to a linear regression (y = mx + n).Equation and coefficient of determination is only shown for inhibitory phase.Porphyra columbina.Grado de fotoinhibición (A) debido a la exposición a PAR + UV-A (PA: línea sólida) y PAR + UV-A + UV-B (PAB: línea segmentada) y su subsiguiente recuperación (B) a las 4 h de exposición en luz tenue.Las curvas de fotoinhibición fueron ajustadas a una regresión lineal.Las ecuaciones y coeficientes de determinación son indicados para la fase de inhibición.

Fig. 7 :
Fig. 7: Adenocystis utricularis.Degree of photoinhibition (A) due to PAR + UV-A (= PA: solid line) and PAR + UV-A + UV-B-exposure (PAB: dashed line) and its subsequent recovery (B) up to 24 h in dim PAR.Photoinhibition data were fitted to a linear regression (y = mx + n), while data of recovery phases were adjusted to an exponential function (y = a • e(-bx) ).Equations and coefficients of determination are shown for each phase.

Fig. 8 :
Fig.8: Rates of inhibition of Fv/Fm (m inh ) in five macroalgal species after a four hours exposure to PAR + UV-A (white columns) and PAR + UV-A + UV-B radiation (grey columns).Big and small letters represent statistical differences of inhibition rate between all species exposed either UV-A or UV-B radiation.Asterisks represent statistical differences between UV-A and UV-B effects within a species.Tasas de la fase de inhibición (m inh ) de cinco especies de macroalgas luego de cuatro horas de exposición a radiación PAR + UV-A (columnas blancas) y PAR + UV-A + UV-B (columnas grises).Letras grandes y pequeñas representan diferencias estadísticas en las tasas de inhibición entre todas las especies expuestas ya sea a UV-A o UV-B.Los asteriscos denotan diferencias estadísticas entre los efectos de UV-A y UVB dentro de cada especie.

Fig. 9 :
Fig. 9: Rates of recovery (m rec ) of five macroalgal species which were previously exposure to PAR + UV-A (white columns) and PAR + UV-A + UV-B radiation (grey columns).N.d.indicates no data after 24 h of recovery.

Fig. 11 :
Fig. 11: Light curves of the brown alga Adenocystis utricularis after four hours of exposure to low PAR (solid line), PAR + UV-A (dashed line) and PAR + UV-A + UV-B radiation (dotted line).

Fig. 12 :
Fig. 12: Light curves of the brown macroalgal species Desmarestia confervoides (A) and Desmarestia ligulata (B) after four hours of exposure to low PAR (solid line), PAR + UV-A (dashed line) and PAR + UV-A + UV-B radiation (dotted line).