Abstract The ability of Spirulina platensis LB2340 to grow on carbon dioxide and mixtures of bicarbonate/carbonate was compared under light irradiation conditions simulating those naturally present in temperate latitudes. Batch tests performed at 25 °C in open tanks suggested that inorganic carbon is preferentially assimilated in the form of bicarbonate and that its utilization efficiency depends either on pH or final biomass level. The efficiencies of photosynthesis (PE) and carbon utilization (CUE) on carbon dioxide reached maximum values (>6 and 38%) after 4 and ≥7 days, respectively. They then progressively decreased. Fed-batch tests performed on carbon dioxide showed the highest values of biomass concentration (1.50 g l −1) and PE (1.1%) under pseudo steady-state conditions at 0.25 l per day feeding rate.