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

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.