Transport and Fixation of Inorganic Carbon during Photosynthesis in Cells of Anabaena Grown under Ordinary Air

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Abstract
In cells of cyanobacterium Anabaena variabilis grown under ordinary air (low-CO2 cells), the transport of both CO2 and HCO3− was significantly enhanced by Na+. This effect was pronounced as the external pH increased. When low-CO2 cells were treated with an inhibitor of carbonic anhydrase (CA), only CO2 transport but not HCO3− 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 CO2 or HCO3− was externally supplied. These results suggest that IC is actively transported through the plasma membrane in a form of HCO3− probably by some transporter and that the transmembrane Na+ gradient is involved in this IC transport system. Free CO2 may be hydrated by CA to HCO3− and then transported to the cells by this transporter. On the other hand, CO2 is actively taken up by cells grown with air containing 5% CO2 (high-CO2 cells) though the enhancing effect of Na+ was much smaller in high- CO2 cells than in low-CO2 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-CO2 cells than in high-CO2 cells. The studies with ethoxyzolamide indicated that even in low-CO2 cells, CA does not function inside Anabaena cells. These results suggest that inside the low-CO2 cells of Anabaena, some mediator(s) facilitates the transport of IC to RuBPCase.