Ocean acidification affects pigment concentration and photoprotection of marine phytoplankton

Ocean acidification produces significant changes on phytoplankton physiology that can affect their growth and primary production. Among them, a downregulation of the enzymatic activity and the production of different cellular metabolites, including chlorophyll a (Chl a), has been observed in high CO2 cultures under stable conditions. However, the extent of how phytoplankton metabolism regulation under high CO2 conditions affects pigment pools and patterns is unknown. This study shows the effect of the atmospheric CO2 increase on pigment concentration of three important marine primary producers: Thalassiosira pseudonana, Skeletonema costatum, and Emiliania huxleyi. Cultures grown under saturating photosynthetically active radiation were aerated for at least 3 weeks with current concentrations of atmospheric CO2 (0.04% CO2 in air) and with CO2 concentrations expected for future scenarios of climate change (0.1% CO2 in air) to assess the effect of CO2 under acclimated metabolism and stable conditions. Moreover, cultures were also subjected to a perturbation (4 h without aeration) to assess responses under non-stable conditions. The results showed that light harvesting and photoprotective pigment concentrations (i.e., Chl a, Chl c(2), beta beta-carotene, diadinoxanthin, diatoxanthin, fucoxanthin, among others) decreased significantly under high CO2 and stable conditions, but the response reversed after the perturbation. The de-epoxidation state of xanthophylls, also showed similar patterns, indicating an increase in phytoplankton sensitivity under high CO2 and stable conditions. The results demonstrate the relevance of CO2 concentration and acclimation status for phytoplankton light absorption and photoprotective response. They also identify fucoxanthin and Chl c(2) as suitable biomarkers of phytoplankton carbon metabolism under ocean acidification conditions.