High Affinity Transport of CO2 in the Cyanobacterium Synechococcus UTEX 625

Abstract

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, ¹²CO₂ readily interfered with ¹³CO₂ 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.