The effects of changes in CO2 and pH on biomass productivity and carbon uptake of Pleurochrysis carterae and Emiliania huxleyi in open raceway ponds and a plate photobioreactor were studied. The pH of P. carterae cultures increased during day and decreased at night, whereas the pH of E. huxleyi cultures showed no significant diurnal changes. P. carterae coccolith production occurs during the dark period, whereas in E. huxleyi, coccolith production is mainly during the day. Addition of CO2 at constant pH (pH-stat) resulted in an increase in P. carterae biomass and coccolith productivity, while CO2 addition lowered E. huxleyi biomass and coccolith production. Neither of these algae could grow at less than pH 7.5. Species-specific diurnal pH and pCO(2) variations could be indicative of significant differences in carbon uptake between these two species. While E. huxleyi has been suggested to be predominantly a bicarbonate user, our results indicate that P. carterae may be using CO2 as the main C source for photosynthesis and calcification.