Transport and Fixation of Inorganic Carbon during Photosynthesis in Cells of Anabaena Grown under Ordinary Air: III. Some Characteristics of the HCO3--Transport System in Cells Grown under Ordinary Air

In cells of cyanobacterium Anabaena variabilis</it> grown under ordinary air (low-CO<inf>2</inf> cells), the transport of both CO<inf>2</inf> and HCO<inf>3</inf>− was significantly enhanced by Na+. This effect was pronounced as the external pH increased. When low-CO<inf>2</inf> cells were treated with an inhibitor of carbonic anhydrase (CA), only CO<inf>2</inf> transport but not HCO<inf>3</inf>− transport, was inhibited. The initial rate of photosynthetic carbon fixation as a function of the concentration of internal inorganic carbon (IC) was practically the same irrespective of whether CO<inf>2</inf> or HCO<inf>3</inf>− was externally supplied. These results suggest that IC is actively transported through the plasma membrane in a form of HCO<inf>3</inf>− probably by some transporter and that the transmembrane Na+ gradient is involved in this IC transport system. Free CO<inf>2</inf> may be hydrated by CA to HCO<inf>3</inf>− and then transported to the cells by this transporter. On the other hand, CO<inf>2</inf> is actively taken up by cells grown with air containing 5% CO<inf>2</inf> (high-CO<inf>2</inf> cells) though the enhancing effect of Na+ was much smaller in high- CO<inf>2</inf> cells than in low-CO<inf>2</inf> cells. The initial rate of fixation as a function of internal IC concentration indicated that the rate of the carboxylation reaction of accumulated IC is higher in I0W-CO<inf>2</inf> cells than in high-CO<inf>2</inf> cells. The studies with ethoxyzolamide indicated that even in low-CO<inf>2</inf> cells, CA does not function inside Anabaena</it> cells. These results suggest that inside the low-CO<inf>2</inf> cells of Anabaena</it>, some mediator(s) facilitates the transport of IC to RuBPCase.