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Lessonia nigrescens and Durvillaea antarctica, two large sub-Antarctic brown algae from the southern Chilean coast, were exposed to solar UV radiation in an outdoor system during a summer day (for 11 h) as well as to artificial UV radiation under controlled laboratory conditions at two temperatures (15 and 20 °C) for 72 h. Chlorophyll a fluorescence-based photoinhibition of photosynthesis was measured during the outdoor exposure, while electron transport rates, lipid peroxidation, antioxidant activity and content of phlorotannins were determined at different time intervals during the laboratory exposure. Under natural solar irradiances in summer, both species displayed well-developed dynamic photoinhibition: [F.sub.v]/[F.sub.m] values decreased by 70 % at noon coinciding with the levels of PAR [1,500 (µmol [m.sup.-2] [s.sup.-1] and UV-B radiation >1 W [m.sup.-2] and recovered substantially in the afternoon. In treatments including UV radiation, recovery in D. antarctica started already during the highest irradiances at noon. The results from laboratory exposures revealed that (a) elevated temperature of 20 °C exacerbated the detrimental effects of UV radiation on photochemical parameters ([F.sub.v]/[F.sub.m] and ETR); (b) peroxidative damage measured as MDA formation occurred rapidly and was strongly correlated with the decrease in [F.sub.v]/[F.sub.m], especially at elevated temperature of 20 °C; (c) the antioxidant activity and increases in soluble phlorotannins were positively correlated mainly in response to UV radiation; (d) phlorotannins were rapidly induced but strongly impaired at 20 °C. In general, short-term (2-6 h) exposures to enhanced UV radiation and temperature were effective to activate the photochemical and biochemical defenses against oxidative stress, and they continued operative during 72 h, a time span clearly exceeding the tidal or diurnal period. Furthermore, when algae were exposed to dim light and control temperature of 15 °C for 6 h, [F.sub.v]/[F.sub.m] increased and lipid peroxidation decreased, indicating consistently that algae retained their ability for recovery. D. antarctica was the most sensitive species to elevated temperature for prolonged periods in the laboratory. Although no conclusive evidence for the effect of the buoyancy of fronds was found, the interspecific discrepancies in thermo-sensitivity in the UV responses found in this study are consistent with various ecological and biogeographical differences described for these species., Introduction Marine macroalgae inhabiting the intertidal zone are exposed in a short time span to sharp fluctuations of their physical environment. Solar UV radiation and temperature are among the major [...]

Species of the genus Ulva (Chlorophyta) are regarded as opportunistic organisms, which efficiently adjust their metabolism to the prevailing environmental conditions. In this study, changes in chlorophyll‐a fluorescence‐based photoinhibition of photosynthesis, electron transport rates, photosynthetic pigments, lipid peroxidation, total phenolic compounds, and antioxidant metabolism were investigated during a diurnal cycle of natural solar radiation in summer (for 12 h) under two treatments: photosynthetically active radiation (PAR: 400–700 nm) and PAR+ ultraviolet (UV) radiation (280–700 nm). In the presence of PAR alone, Ulva rigida showed dynamic photoinhibition, and photosynthetic parameters and pigment concentrations decreased with the intensification of the radiation. On the other hand, under PAR+UV conditions a substantial decline up to 43% was detected and an incomplete fluorescence recovery, also, P‐I curve values remained low in relation to the initial condition. The phenolic compounds increased their concentration only in UV radiation treatments without showing a correlation with the antioxidant activity. The enzimatic activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX) increased over 2‐fold respect at initial values during the onset of light intensity. In contrast, catalase (CAT) increased its activity rapidly in response to the radiation stress to reach maxima at 10 a.m. and decreasing during solar. The present study suggests that U. rigida is capable of acclimating to natural radiation stress relies on a concerted action of various physiological mechanisms that act at different times of the day and under different levels of environmental stress. [ABSTRACT FROM AUTHOR]

The rhodophyte Gracilaria chilensis is a structuring species of the estuarine and fjord systems in southern Chile and is commercially exploited from natural and farmed populations. Due to its capacity to tolerate extreme variations in environmental conditions, this species is an excellent model organism to examine stress tolerance mechanisms. The present study examined the combined effect of temperature and UV radiation on the reproductive phases of G. chilensis (carposporophyte, tetrasporophyte, and vegetative) at the lower and upper intertidals of the Valdivia River estuary. The concentration of phenolic compounds and their antioxidant capacity as well as the inhibition of primary photochemical reactions, measured as the fluorescence of photosystem II, was determined. The kinetics of the maximal quantum yield (Fv/Fm) between 5 and 50 °C indicated a rapid decrease of photosynthesis from 35 °C upwards in the three reproductive phases. Exposures for 1, 6, 24, and 48 h to photosythetically active radiation (PAR) and PAR + UV at 5, 25, and 35 °C affected photosynthesis in the three reproductive phases, especially at the elevated temperature of 35 °C where the maximum cumulative decreases of Fv/Fm were close to 93 and 98% in tetrasporophytes and carposporophytes. An important finding was the high content of phenols, which increased with increasing UV doses only at 10 °C, especially in carposporophytic and vegetative thalli. The antioxidant activity increased at 10 and 25 °C in algae exposed to UV radiation, while at low (5 °C) and elevated (35 °C) temperatures, it resulted in the inhibition of antioxidant activity. Overall, phenols were positively correlated with the antioxidant activity and Fv/Fm, especially in tetrasporophytes. The study revealed that the different life history phases of G. chilensis showed similar stress tolerance mechanisms, which appeared to be triggered strongly by temperature and less by UV radiation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]