Karenia mikimotoi is a widespread, toxic and non-calcifying dinoflagellate, which can release and produce ichthyotoxins and hemolytic toxins affecting the food web within the area of its bloom. Shifts in the physiological characteristics of K. mikimotoi due to CO 2-induced seawater acidification could alter the occurrence, severity and impacts of harmful algal blooms (HABs). Here, we investigated the effects of elevated pCO 2 on the physiology of K. mikimotoi. Using semi-continuous cultures under controlled laboratory conditions, growth, photosynthesis and inorganic carbon acquisition were determined over 4–6 week incubations at ambient (390ppmv) and elevated pCO 2 levels (1000 ppmv and 2000 ppmv). pH-drift and inhibitor-experiments suggested that K. mikimotoi was capable of acquiring HCO 3 -, and that the utilization of HCO 3 - was predominantly mediated by anion-exchange proteins, but that HCO 3 - dehydration catalyzed by external carbonic anhydrase (CA ext) only played a minor role in K. mikimotoi. Even though down-regulated CO 2 concentrating mechanisms (CCMs) and enhanced gross photosynthetic O 2 evolution were observed under 1000 ppmv CO 2 conditions, the saved energy did not stimulate growth of K. mikimotoi under 1000 ppmv CO 2, probably due to the increased dark respiration. However, significantly higher growth and photosynthesis [in terms of photosynthetic oxygen evolution, effective quantum Yield (Yield), photosynthetic efficiency (α), light saturation point (E k) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity] were observed under 2000 ppmv CO 2 conditions. Furthermore, elevated pCO 2 increased the photo-inhibition rate of photosystem II (β) and non-photochemical quenching (NPQ) at high light. We suggest that the energy saved through the down-regulation of CCMs might lead to the additional light stress and photo-damage. Therefore, the response of this species to elevated CO 2 conditions will be determined by more than regulation and efficiency of CCMs.