Abstract Among the microbial phototrophs, those belonging to the cyanobacteria utilize CO2 and HCO−3 for photosynthesis. Some Chlorophyceae mainly take up CO2 in photosynthesis, and others, which have carbonic anhydrase (CA) on their cell surface can utilize HCO−3 as well as CO2. Kinetic studies revealed that most of the HCO−3 is utilized after this ion is converted to CO2 via CA located on the cell surface. Therefore, the actual molecular species which crosses the plasmalemma is mostly free CO2. There is apparent variation in the mode of utilization of dissolved inorganic carbon (DIC) for photosynthesis in microalgae in other classes. The apparent Km(CO2) values for photosynthesis in most microalgae grown in ordinary air (low-CO2 cells) are as low as in terrestrial C4 plants, although the algal cells fix CO2 via the C3 pathway. In contrast, the apparent Km(CO2) values in cells grown on CO2-enriched air (high-CO2 cells) are as high as those in the terrestrial C3 plants. Most low-CO2 cells show low photorespiration; a low CO2 compensation point, low rates of glycolate excretion and no or low O2 inhibition of photosynthesis. These results indicate that the efficiency of DIC utilization for photosynthesis in low-CO2 cells is very high. The activity of CA in low-CO2 cells is higher than that in high-CO2 cells, while no difference has been confirmed in the activities of other photosynthetic enzymes between low- and high-CO2 cells. In addition, low-CO2 cells can accumulate large amounts of DIC internally, indicating the existence of CO2-concentrating mechanisms in these cells. When CA activity or CO2 concentrating ability is reduced by inhibitors or by mutation, the apparent Km(CO2) values for photosynthesis and the rate of photorespiration increased notably. These results indicate that the high efficiency of DIC utilization in low-CO2 cells depends on both CA and a CO2-concentrating mechanism. It is concluded that CA facilitates the diffusion of DIC from outside the cells to the site(s) of the carboxylation reaction and the concentration of DIC is achieved via an active transporter.