Large scale precipitation of calcium carbonate in the oceans by coccolithophorids plays an important role in carbon sequestration. However, there is a controversy on the effect of an increase in atmospheric CO₂ concentration on both calcification and photosynthesis of coccolithophorids. Indeed recent experiments, performed under nitrogen limitation, revealed that the associated fluxes may be slowed down, while other authors claim the reverse. We designed models to account for various scenarii of calcification and photosynthesis regulation in chemostat cultures of Emiliania huxleyi, based on different hypotheses on the regulation mechanism. These models consider that either carbon dioxide, bicarbonate, carbonate or calcite saturation state (Ω) is the regulating factor. All were calibrated to predict the same carbon fixation rate in nowadays pCO₂, but they turn out to respond differently to an increase in CO₂ concentration. Thus, using the four possible models, we simulated a large bloom of Emiliania huxleyi. It results that models assuming a regulation by CO₃^2- or Ω predicted much higher carbon fluxes. The response when considering a doubled pCO₂ was different and models controlled by CO₂ or HCO₃ ^- led to increased carbon fluxes. In addition, the variability between the various scenarii proved to be in the same order of magnitude than the response to pCO₂ increase. These sharp discrepancies reveal the consequences of model assumptions on the simulation outcome.