QUENCHING OF CHLOROPHYLL-A FLUORESCENCE IN RESPONSE TO NA-DEPENDENT HCO3- TRANSPORT-MEDIATED ACCUMULATION OF INORGANIC CARBON IN THE CYANOBACTERIUM-SYNECHOCOCCUS UTEX-625()

In the cyanobacterium Synechococcus UTEX 625, the yield of chlorophyll a fluorescence decreased in response to the transport-mediated accumu lation of intracellular inorganic carbon (CO2 + HCO3- + CO32- = dissol ved inorganic carbon [DIC]) and subsequently increased to a near-maxim um level following photosynthetic depletion of the DIC pool. When DIC accumulation was mediated by the active Na+-dependent HCO3- transport system, the initial rate of fluorescence quenching was found to be hig hly correlated with the initial rate of (HCO3-)-C-14 transport (r = 0. 96), and the extent of fluorescence quenching was correlated with the size of the internal DIC pool (r = 0.99). Na+-dependent HCO3- transpor t-mediated accumulation of DIC caused fluorescence quenching in either the presence or absence of the CO2 fixation inhibitor glycolaldehyde, indicating that quenching was not due simply to NADP(+) reduction. Th e concentration of Na+ required to attain one-half the maximum rate of (HCO3-)-C-14 transport, at 20 mu M external HCO3- declined from 9 to 1 mM as the external pH increased from 8 to 9.6. A similar pH dependen cy was observed when fluorescence quenching was used to determine the kinetic constants for HCO3- transport. In cells capable of Na+-depende nt HCO3- transport, both the initial rate and extent of fluorescence q uenching increased with increasing external HCO3-, saturating at about 150 mu M. In contrast Na+-independent HCO3- transport-mediated fluore scence quenching saturated at an HCO3- concentration of about 10 mu M. It was concluded that measurement of chlorophyll a fluorescence emiss ion provided a convenient, but indirect, means of following Na+-depend ent HCO3- transport and accumulation in Synechococcus.