A novel in situ system to evaluate the effect of high CO2 on photosynthesis and biochemistry of seaweeds

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.