The aquatic algae have developed a carbon dioxide concentrating mechanism(CCM)to acclimate to the CO₂-limiting environmental conditions,of which carbonic anhydrase is the major component.In order to explore the response pattern of carbonic anhydrase to environmental factors,and to improve the utilization of inorganic carbon and algal biomass of Chlorella,the effects of salinity,CO₂ and bicarbonate on the growth,extracellular carbonic anhydrase activity and 3 subtypes of ca gene expression level of C.pyrenoidosa were investigated by biochemical and real-time quantitative PCR methods in this study.Results showed that the alga grew faster cultured in 15 and 30 salinity,and the algal density in 45 salinity was 0.83-fold of that in 15 salinity on the 9^th day.The extracellular carbonic anhydrase activities were inhibited with the increasing salinity,which was especially obvious after long-time treatment with 45 salinity.However,the expression level of three subtypes of ca genes increased from 15 to 45 salinity,which could vary from 1.52-fold to 2.16-fold of that cultured in 15 salinity.The algal density cultured in double air CO₂(high CO₂ level)could reach 1.23-fold of that in air CO₂(low CO₂ level).But the algal CA activity in high CO₂ level was lower,which was only 0.41-fold of that cultured in low CO₂ level on the 8^th day.The α- and γ-ca mRNA accumulations in high CO₂ level were slightly higher,which were 1.22 times and 1.47 times that in low CO₂ level,respectively.The growth of C.pyrenoidosa was promoted when the bicarbonate concentration ranged from 0 to 10 mmol/L.And the algal CA activity showed maximum value in 5 mmol/L HCO₃^- group.The highest levels of α-,β- and γ-ca were observed in 1 mmol/L HCO₃^- group.It could be concluded that the growth of C.pyrenoidosa 820 was suitable under the conditions of low and moderate salinity,double air CO₂ and higher bicarbonate concentration,and the CA activity was induced by low salinity,air CO₂ and 5 mmol/L HCO₃^-.However,the ca mRNA level was higher under high salinity,double air CO₂ level and 1 mmol/LHCO₃^- conditions.