The function of carbonic anhydrase in aquatic photosynthesis

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Carbonic anhydrase (CA, EC 4.2.1.1) is universally distributed in photoautotrophic aquatic organisms, terrestrial plants and animals. Only a little information about the molecular properties and reaction characteristics has been obtained for this enzyme in aquatic plants. It is known that there is variation in its intracellular location and its molecular characteristics; some species have CA both inside the cells and on their cell surface, while others have only an internal enzyme. Most CA in the cells of Chlamydomonas reinhardtii Dangeard in soluble, while about 80% is insoluble in those of Chlorella vulgaris Beijerinck 11h. The insoluble CA from Chlorella vulgaris C-3 is heat-stable, while the soluble CA from Chlamydomonas reinhardtii is heat labile. Chlamydomonas CA does not form a precipitate against antibody of spinach CA, or vice versa. Cells grown under ordinary air (low-CO 2 cells) show higher activity of CA than those grown under high CO 2 conditions (high-CO 2 cells). Synthesis of the CA starts when high-CO 2 cells are transferred into low CO 2 conditions, while synthesis stops when low-CO 2 cells are transferred to high CO 2 conditions. In aquatic angiosperms such as Myriophyllum, CA is induced under summer-like conditions (high temperature and long day length). CA is synthesized on cytoplasmic ribosomes and is therefore encoded in the nuclear genome. CA on the cell surface of Chlamydomonas responds to changes in the CO 2 concentration used for growth in a similar manner to the internal CA. However, the external CA in some species of Chlorella shows comparable activities between low- and high-CO 2 cells. Since the apparent K m (CO 2) for photosynthesis in low-CO 2 cells in always lower than that in high-CO 2 cells, CA inside the cells seems to play a more important role in decreasing the apparent K m (CO 2) for photosynthesis than the external enzyme. The CA on the cell surface is important for the conversion of HCO 3 − to CO 2. The converted CO 2 is then absorbed by green algae. In this respect, the CA may play a role in HCO 3 − utilization for those species where HCO 3 − is the dominant form assimilated. It has also been established that various microalgae and cyanobacteria can accumulate inorganic carbon into low-CO 2 cells during photosynthesis, to an extent far in excess of its concentration in the suspending medium. The nature and intracellular location of the accumulating system of inorganic carbon remains to be elucidated. Possible functions of CA and the accumulating system are discussed.