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To investigate the biochemical response of freshwater green algae to elevated CO2 concentrations,Chlorella pyrenoidosaChick andChlamydomonas reinhardtiiDang cells were cultured at different CO2 concentrations within the range 3-186ümol/L and the biochemical composition, carbonic anhydrase (CA), and nitrate reductase activities of the cells were investigated. Chlorophylls (Chl), carotenoids, carbonhydrate, and protein contents were enhanced to varying extents with increasing CO2 concentration from 3-186ümol/L. The CO2 enrichment significantly increased the Chl a/Chlbratio inChlorella pyrenoidosa, but not inChlamydomonas reinhardtii.The CO2 concentration had significant effects on CA and nitrate reductase activity. Elevating CO2 concentration to 186ümol/L caused a decline in intracellular and extracellullar CA activity. Nitrate reductase activity, under either light or dark conditions, inC. reinhardtiiandC. pyrenoidosawas also significantly decreased with CO2 enrichment. From this study, it can be concluded that CO2 enrichment can affect biochemical composition, CA, and nitrate reductase activity, and that the biochemical response was species dependent. [ABSTRACT FROM AUTHOR]