NA+-INDEPENDENT HCO3- TRANSPORT AND ACCUMULATION IN THE CYANOBACTERIUM SYNECHOCOCCUS UTEX-625

The active transport and intracellular accumulation of HCO3- by air-grown cells of the cyanobacterium Synechococcus UTEX 625 (PCC 6301) was strongly promoted by 25 millimolar Na+.Na+-dependent HCO3- accumulation also resulted in a characteristic enhancement in the rate of photosynthetic O2 evolution and CO2 fixation. However, when Synechococcus was grown in standing culture, high rates of HCO3- transport and photosynthesis were observed in the absence of added Na+. The internal HCO3- pool reached levels up to 50 millimolar, and an accumulation ratio as high as 970 was observed. Sodium enhanced HCO3- transport and accumulation in standing culture cells by about 25 to 30% compared with the five- to eightfold enhancement observed with air-grown cells. The ability of standing culture cells to utilize HCO3- from the medium in the absence of Na+ was lost within 16 hours after transfer to air-grown culture and was reacquired during subsequent growth in standing culture. Studies using a mass spectrometer indicated that standing culture cells were also capable of active CO2 transport involving a high-affinity transport system which was reversibly inhibited by H2S, as in the case for air-grown cells. The data are interpreted to indicate that Synechococcus possesses a constitutive CO2 transport system, whereas Na+-dependent and Na+-independent HCO3- transport are inducible, depending upon the conditions of growth. Intracellular accumulation of HCO3- was always accompanied by a quenching of chlorophyll a fluorescence which was independent of CO2 fixation. The extent of fluorescence quenching was highly dependent upon the size of the internal pool of HCO3- + CO2. The pattern of fluorescence quenching observed in response to added HCO3- and Na+ in air-grown and standing culture cells was highly characteristic for Na+-dependent and Na+-independent HCO3- accumulation. It was concluded that measurements of fluorescence quenching provide an indirect means for following HCO3- transport and the dynamics of intracellular HCO3- accumulation and dissipation.