The effects of pH-induced changes in seawater carbonate chemistry on inorganic carbon (Ci) acquisition and domoic acid (DA) production were studied in two potentially toxic diatom species, Pseudo-nitzschia multiseries and Nitzschia navis-varingica, and the non-toxic Stellarima stellaris. In vivo activities of carbonic anhydrase (CA), photosynthetic O2 evolution and CO2 and HCO3− uptake rates were measured by membrane inlet MS in cells acclimated to low (7.9) and high pH (8.4 or 8.9). Species-specific differences in the mode of carbon acquisition were found. While extracellular carbonic anhydrase (eCA) activities increased with pH in P. multiseries and S. stellaris, N. navis-varingica exhibited low eCA activities independent of pH. Half-saturation concentrations (K1/2) for photosynthetic O2 evolution, which were highest in S. stellaris and lowest in P. multiseries, generally decreased with increasing pH. In terms of carbon source, all species took up both CO2 and HCO3−. K1/2 values for inorganic carbon uptake decreased with increasing pH in two species, while in N. navis-varingica apparent affinities did not change. While the contribution of HCO3− to net fixation was more than 85% in S. stellaris, it was about 55% in P. multiseries and only approximately 30% in N. navis-varingica. The intracellular content of DA increased in P. multiseries and N. navis-varingica with increasing pH. Based on our data, we propose a novel role for eCA acting as Ci-recycling mechanism. With regard to pH-dependence of growth, the ‘HCO3− user’S. stellaris was as sensitive as the ‘CO2 user’N. navis-varingica. The suggested relationship between DA and carbon acquisition/Ci limitation could not be confirmed.