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The estuarine anemone Anthopleura hermaphroditica and its symbiont Philozoon anthopleurum are continuously exposed to intense fluctuations in solar radiation and salinity owing to tidal changes. The aim of this study was to evaluate the effects of the tidal cycle, solar radiation, and salinity fluctuations on the photosynthetic and cellular responses (lipid peroxidation, total phenolic compounds, and antioxidant activity) of the symbiont complex over a 24 h period in the Quempillén River Estuary. Additionally, laboratory experiments were conducted to determine the specific photobiological responses to photosynthetically active radiation (PAR), ultraviolet radiation (UVR), and salinity. Our field results showed that the photosynthetic parameters of the symbiont complex decreased with increasing ambient radiation; however, no relationship was observed with changes in salinity. Increased peroxidative damage, total phenolic compound levels, and antioxidant activity were mainly related to increased UVR and, to a lesser extent, PAR. During the dark period, only PAR-exposed organisms returned to the basal levels of photosynthesis and cell damage. Laboratory exposure confirmed the deleterious effects of UVR on the photosynthetic response. The present study suggests that the ability of A. hermaphroditica to acclimate to natural radiation stress is mediated by the concerted action of various physiological mechanisms that occur at different times of the day, under varying levels of environmental stress. [ABSTRACT FROM AUTHOR]

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]