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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 (ETRmax), saturated irradiance (EkETR) and chl a content and the lowest antioxidant activity. Under the same conditions, E. elongata had increased EkETR, antheraxanthin and β-carotene content. At 100% irradiance, C. tamariscifolia collected at 2.0 m had higher maximal quantum yield (Fv/Fm), photosynthetic efficiency (αETR), ETRmax, maximal non-photochemical quenching (NPQmax), saturation irradiance for NPQ (EkNPQ), and antheraxanthin and polyphenol content increased, whereas in E. elongata only αETR 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, Fv/Fm and ETRmax 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.

Previous studies of the impact of increased CO2 on macroalgae have mainly been done in laboratories or mesocosm systems, placing organisms under both artificial light and seawater conditions. In this study, macroalgae were incubated in situ in UV-transparent cylinders under conditions similar to the external environment. This system was tested in a short-term study (5.5 h incubation) on the effect of 2 partial pressures of CO2 (pCO2): air (ambient CO2) and the pCO2 predicted by the end of the 21st century (700 µatm, high CO2), on photosynthesis, photosynthetic pigments and photoprotection in calcifying (Ellisolandia elongata and Padina pavonica) and non-calcifying (Cystoseira tamariscifolia) macroalgae. The calcifying P. pavonica showed higher net photosynthesis under high CO2 than under ambient CO2 conditions, whereas the opposite occurred in C. tamariscifolia. Both brown algae (P. pavonica and C. tamariscifolia) showed activation of non-photochemical quenching mechanisms under high CO2 conditions. However, in P. pavonica the phenol content was reduced after CO2 enrichment. In contrast to phenols, in E. elongata other photoprotectors such as zeaxanthin and palythine (mycosporine-like amino acid) tended to increase in the high CO2 treatment. The different responses of these species to elevated pCO2 may be due to anatomical and physiological differences and could represent a shift in their relative dominance as key species in the face of ocean acidification (OA). More in situ studies could be carried out to evaluate how macroalgae will respond to increases in pCO2 in a future OA scenario. The in situ incubator system proposed in this work may contribute towards increasing this knowledge.