Carbonic anhydrase and CO₂concentrating mechanisms in microalgae and cyanobacteria

Abstract

Among the microbial phototrophs, those belonging to the cyanobacteria utilize CO 2 and HCO 3 for photosynthesis. Some Chlorophyceae mainly take up CO 2 in photosynthesis, and others, which have carbonic anhydrase (CA) on their cell surface can utilize HCO 3 as well as CO 2. Kinetic studies revealed that most of the HCO/ 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 CO 2. There is apparent variation in the mode of utilization of dissolved inorganic carbon (DIC) for photosynthesis in microalgae in other classes. The apparent KIn(CO2) values for photosynthesis in most microalgae grown in ordinary air (low-CO 2 cells) are as low as in terrestrial C 4 plants, although the algal cells fix CO 2 via the C a pathway. In contrast, the apparent Km(CO2) values in cells grown on CO2-enriched air (high-CO 2 cells) are as high as those in the terrestrial C 3 plants. Most low-CO 2 cells show low photorespiration; a low CO 2 compensation point, low rates of glycolate excretion and no or low O 2 inhibition of photosynthesis. These results indicate that the efficiency