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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  p CO 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  p CO 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  p CO 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.

plos one

Effect of CO2-induced seawater acidification on growth, photosynthesis and inorganic carbon acquisition of the harmful bloom-forming marine microalga, Karenia mikimotoi