HIGH-AFFINITY TRANSPORT OF CO2 IN THE CYANOBACTERIUM SYNECHOCOCCUS UTEX-625

The active transport of CO2 in Synechococcus UTEX 625 was measured by mass spectrometry under conditions that preclude HCO3- transport. The substrate concentration required to give one half the maximum rate for whole cell CO2 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 CO2 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 O2 evolution observed under these conditions. The initial rate of chlorophyll a fluorescence quenching was highly correlated with the initial rate of CO2 transport (correlation coefficient = 0.98) and could be used as an indirect method to detect CO2 transport and calculate the substrate concentration required to give one half the maximum rate of transport. Little, if any, inhibition of CO2 transport was caused by HCO3- or by Na+-dependent HCO3- transport. However, (CO2)-C-12 readily interfered with (CO2)-C-13 transport. CO2 transport and Na+-dependent HCO3- transport are separate, independent processes and the high affinity CO2 transporter is not only responsible for the initial transport of CO2 into the cell but also for scavenging any CO2 that may leak from the cell during ongoing photosynthesis.