Carbonic anhydrase and CO 2 concentrating 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 − 3 is utilized after this ion is converted to CO 2 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 K m(CO 2) 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 3 pathway. In contrast, the apparent K m(CO 2) values in cells grown on CO 2-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 of DIC utilization for photosynthesis in low-CO 2 cells is very high. The activity of CA in low-CO 2 cells is higher than that in high-CO 2 cells, while no difference has been confirmed in the activities of other photosynthetic enzymes between low- and high-CO 2 cells. In addition, low-CO 2 cells can accumulate large amounts of DIC internally, indicating the existence of CO 2-concentrating mechanisms in these cells. When CA activity or CO 2 concentrating ability is reduced by inhibitors or by mutation, the apparent K m(CO 2) values for photosynthesis and the rate of photorespiration increased notably. These results indicate that the high efficiency of DIC utilization in low-CO 2 cells depends on both CA and a CO 2-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.