Abstract: “Microalgae are known to have better growth rate and have higher performance at fixing CO2 compared to other photosynthetic organisms. In addition, microalgae require less space for cultivation, hence, the strategic use of photobioreactor. Cultivation of microalgae in photobioreactor is a practical approach to mitigate CO2 emission. In this study, Characium sp. was cultured in an airlift photobioreactor to determine the effect of CO2 concentrations on its growth. Assessment was also conducted on its biomass, chlorophyll a and lipid contents. Characium sp. presented the highest CO2 fixation rate in 5% CO2 with 1.222 g L-1 d-1. During 21-day interval cultures, the specific growth rate of Characium sp. at CO2 concentrations of 5%, 10%, and 15% were 0.263 d-1,, 0.191 d-1, and 0.181 d-1 respectively. Meanwhile, its biomass productivity at 5%, 10%, and 15% CO2 concentrations was 0.048 g L-1 d-1, 0.038 g L-1 d-1, and 0.028 g L-1 d-1respectively. The highest chlorophyll a recorded was 2.82 µg/m”

Keywords: microalgae, Characium sp., carbon dioxide;, airlift, CO2 fixation