High Affinity Transport of CO(2) in the Cyanobacterium Synechococcus UTEX 625

The active transport of CO(2) in Synechococcus UTEX 625 was measured by mass spectrometry under conditions that preclude HCO(3) (-) transport. The substrate concentration required to give one half the maximum rate for whole cell CO(2) transport was determined to be 0.4 +/- 0.2 micromolar (mean +/- standard deviation; n = 7) with a range between 0.2 and 0.66 micromolar. The maximum rates of CO(2) transport ranged between 400 and 735 micromoles per milligram of chlorophyll per hour with an average rate of 522 for seven experiments. This rate of transport was about three times greater than the dissolved inorganic carbon saturated rate of photosynthetic O(2) evolution observed under these conditions. The initial rate of chlorophyll a fluorescence quenching was highly correlated with the initial rate of CO(2) transport (correlation coefficient = 0.98) and could be used as an indirect method to detect CO(2) transport and calculate the substrate concentration required to give one half the maximum rate of transport. Little, if any, inhibition of CO(2) transport was caused by HCO(3) (-) or by Na(+)-dependent HCO(3) (-) transport. However, (12)CO(2) readily interfered with (13)CO(2) transport. CO(2) transport and Na(+)-dependent HCO(3) (-) transport are separate, independent processes and the high affinity CO(2) transporter is not only responsible for the initial transport of CO(2) into the cell but also for scavenging any CO(2) that may leak from the cell during ongoing photosynthesis.