Short-term ecophysiological and biochemical responses of Cystoseira tamariscifolia and Ellisolandia elongata to changes in solar irradiance and nutrient levels were analyzed in situ in oligotrophic coastal waters by transferring macroalgae collected at 0.5 and 2.0 m depth and exposing them to 2 irradiance levels (100 and 70% of surface irradiance) and nutrient conditions (nutrient-enriched and non-enriched). Both species were affected by changes in irradiance and nutrient levels. Few interactive effects between these 2 physical stressors were found, suggesting major additive effects on both species. C. tamariscifolia collected at 0.5 m and exposed to 70% irradiance had the highest maximal electron transport rate (ETR max ), saturated irradiance (Ek ETR ) and chl a content and the lowest antioxidant activity. Under the same conditions, E. elongata had increased Ek ETR , antheraxanthin and β-carotene content. At 100% irradiance, C. tamariscifolia collected at 2.0 m had higher maximal quantum yield (F v /F m ), photosynthetic efficiency (α ETR ), ETR max , maximal non-photochemical quenching (NPQ max ), saturation irradiance for NPQ (Ek NPQ ), and antheraxanthin and polyphenol content increased, whereas in E. elongata only α ETR increased. In nutrient-enriched conditions, phenolic compounds, several carotenoids and N content increased in C. tamariscifolia at both depths. E. elongata from 2.0 m depth at 100% irradiance and nutrient-enriched conditions showed increased N content and total mycosporine-like amino acids (MAAs). Our results show rapid photophysiological responses of C. tamariscifolia to variations in in situ irradiance and nutrient conditions, suggesting efficient photoacclimation to environmental changes. In E. elongata, F v /F m and ETR max did not change in the transplant experiment; in contrast, N content, pigment and MAAs (biochemical variables) changed. The responses of these macroalgae to nutrient enrichment indicate oligotrophic conditions at the study site and environmental stress.

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