A red tide alga grown under ocean acidification upregulates its tolerance to lower pH by increasing its photophysiological functions

Abstract. Phaeocystis globosa, a red tide alga, often forms blooms in or adjacent to coastal waters and experiences changes in pH and seawater carbonate chemistry caused by either diel/periodic fluctuation in biological activity, human activity or, in the longer term, ocean acidification due to atmospheric CO₂ rise. We examined the photosynthetic physiology of this species while growing it under different pH levels induced by CO₂ enrichment and investigated its acclimation to carbonate chemistry changes under different light levels. Short-term exposure to reduced pH_nbs (7.70) decreased the alga's photosynthesis and light use efficiency. However, acclimation to the reduced pH level for 1–19 generations led to recovered photosynthetic activity, being equivalent to that of cells grown under pH 8.07 (control), though such acclimation required a different time span (number of generations) under different light regimes. The low-pH-grown cells increased their contents of chlorophyll and carotenoids with prolonged acclimation to the acidification, with increased photosynthetic quantum yield and decreased non-photochemical quenching. The specific growth rate of the low-pH-grown cells also increased to emulate that grown under the ambient pH level. This study clearly shows that \textit{Phaeocystis globosa} is able to acclimate to seawater acidification by increasing its energy capture and decreasing its non-photochemical energy loss.