High-CO 2 Levels Rather than Acidification Restrict Emiliania huxleyi Growth and Performance.

The coccolithophore Emiliania huxleyi shows a variety of responses to ocean acidification (OA) and to high-CO 2 concentrations, but there is still controversy on differentiating between these two factors when using different strains and culture methods. A heavily calcified type A strain isolated from the Norwegian Sea was selected and batch cultured in order to understand whether acclimation to OA was mediated mainly by CO 2 or H + , and how it impacted cell growth performance, calcification, and physiological stress management. Emiliania huxleyi responded differently to each acidification method. CO 2 -enriched aeration (1200 µatm, pH 7.62) induced a negative effect on the cells when compared to acidification caused by decreasing pH alone (pH 7.60). The growth rates of the coccolithophore were more negatively affected by high pCO 2 than by low pH without CO 2 enrichment with respect to the control (400 µatm, pH 8.1). High CO 2 also affected cell viability and promoted the accumulation of reactive oxygen species (ROS), which was not observed under low pH. This suggests a possible metabolic imbalance induced by high CO 2 alone. In contrast, the affinity for carbon uptake was negatively affected by both low pH and high CO 2 . Photochemistry was only marginally affected by either acidification method when analysed by PAM fluorometry. The POC and PIC cellular quotas and the PIC:POC ratio shifted along the different phases of the cultures; consequently, calcification did not follow the same pattern observed in cell stress and growth performance. Specifically, acidification by HCl addition caused a higher proportion of severely deformed coccoliths, than CO 2 enrichment. These results highlight the capacity of CO 2 rather than acidification itself to generate metabolic stress, not reducing calcification.