A study on the optimization of CO2 usage during outdoor microalgae cultivation in order to further maximize the CO2 to biomass conversion efficiency is presented. A constant supply of CO2 was found to be non-essential for culturing microalgae outdoors in 80 L (8 L x 10 sets) bubble columns. Among the different CO2 input conditions that were studied, 2% CO2 with intermittent supply and 2% + 4% CO2 alternation did not affect the algal growth as compared to having a constant supply of 2% CO2. However, during both input conditions, the CO2 to biomass conversion efficiency was doubled while the amount of CO2 used was reduced by 50%. The algal biomass obtained was found to have a higher carbohydrate yield but a lower protein yield as compared to previously published studies. The findings from this study could be applied for large-scale microalgae production so as to minimize cultivation and energy costs. (C) 2015 Elsevier Ltd. All rights reserved.