To improve biomass productivity and CO2 fixation of microalgae under 15% (v/v) CO2 of flue gas, Chlorella species were mutated by nuclear irradiation and domesticated with high concentrations of CO2. The biomass yield of Chlorella pyrenoidosa mutated using 500 Gy of Co-60 gamma irradiation increased by 53.1% (to 1.12 g L-1) under air bubbling. The mutants were domesticated with gradually increased high concentrations of CO2 [from 0.038% (v/v) to 15% (v/v)], which increased the biomass yield to 2.41 g L-1. When light transmission and culture mixing in photo-bioreactors were enhanced at 15% (v/v) CO2, the peak growth rate of the domesticated mutant (named Chlorella PY-ZU1) was increased to 0.68 g L-1 d(-1). When the ratio of gas flow rate (L min(-1)) to 1 L of microalgae culture was 0.011, the peak CO2 fixation rate and the efficiency of Chlorella PY-ZU1 were 1.54 g L-1 d(-1) and 32.7%, respectively. (C) 2013 Elsevier Ltd. All rights reserved.