In order to understand the HCO₃^- utilization in algal photosynthesis, net photosynthetic rate (NPS) under different pH values and photosynthetic bicarbonate uptake rate (PCU_BC) were investigated. The results indicated that the NPS of Scenedesmus quadricauda were all higher than that of Microcystis aeruginosa from weak acid to weak alkaline cultures. The PCU_BC, characterized as a parameter of bicarbonate utilization of alga and calculated as an average of photosynthetic carbonate uptake rate in pH range over pH turning point (pH_TP), were 142.14 and 314.08μmolDIC/(g·h,FW), respectively, for M. aeruginosa and S. quadricauda, which displayed higher ability to utilize bicarbonate in photosynthesis of S. quadricauda. Addition of 0.8mmol/L of AZ (acetazolamide) caused 100% inhibition of PCU_BC for M. aeruginosa; PCU_BC decreased to 0 when 1.6mmol/L of AZ presented for S. quadricauda. Presence of 1.2 mmol/L DIDS (4,4'-diisothio cyanatostilbene-2,2'-disulfonic acid) inhibited PCU_BC by up to 56.10% and 39.74% for M. aeruginosa and S. quadricauda, respectively. These inhibiting results showed that M. aeruginosa was more sensitive to inhibitors than S. quadricauda, and suggested that bicarbonate can be used by these two algae in photosynthesis and that both algae tested are stronger bicarbonate users.