Inorganic carbon acquisition in red tide dinoflagellates

ABSTRACT Carbon acquisition was investigated in three marine bloom‐forming dinoflagellates – Prorocentrum minimum , Heterocapsa triquetra and Ceratium lineatum . In vivo activities of extracellular and intracellular carbonic anhydrase (CA), photosynthetic O 2 evolution, CO 2 and HCO 3 – uptake rates were measured by membrane inlet mass spectrometry (MIMS) in cells acclimated to low pH (8.0) and high pH (8.5 or 9.1). A second approach used short‐term 14 C‐disequilibrium incubations to estimate the carbon source utilized by the cells. All three species showed negligible extracellular CA (eCA) activity in cells acclimated to low pH and only slightly higher activity when acclimated to high pH. Intracellular CA (iCA) activity was present in all three species, but it increased only in P. minimum with increasing pH. Half‐saturation concentrations ( K 1/2 ) for photosynthetic O 2 evolution were low compared to ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco) kinetics. Moreover, apparent affinities for inorganic carbon (Ci) increased with increasing pH in the acclimation, indicating the operation of an efficient CO 2 concentration mechanism (CCM) in these dinoflagellates. Rates of CO 2 uptake were comparably low and could not support the observed rates of photosynthesis. Consequently, rates of HCO 3 – uptake were high in the investigated species, contributing more than 80% of the photosynthetic carbon fixation. The affinity for HCO 3 – and maximum uptake rates increased under higher pH. The strong preference for HCO 3 – was also confirmed by the 14 C‐disequilibrium technique. Modes of carbon acquisition were consistent with the 13 C‐fractionation pattern observed and indicated a strong species‐specific difference in leakage. These results suggest that photosynthesis in marine dinoflagellates is not limited by Ci even at high pH, which may occur during red tides in coastal waters.