CHARACTERIZATION OF CARBON-DIOXIDE AND BICARBONATE TRANSPORT DURING STEADY-STATE PHOTOSYNTHESIS IN THE MARINE CYANOBACTERIUM SYNECHOCOCCUS STRAIN PCC7002

Net O-2 evolution, gross CO2 uptake and net HCO3- uptake during steady -state photosynthesis were investigated by a recently developed mass-s pectrometric technique for disequilibrium flux analysis with cells of the marine cyanobacterium Synechococcus PCC7002 grown at different CO, concentrations. Regardless of the CO2 concentration during growth, al l cells had the capacity to transport both CO2 and HCO3- however, the activity of HCO3- transport was more than twofold higher than CO2 tran sport even in cyanobacteria grown at high concentration) of inorganic carbon (C-i = CO2 + HCO3-. In low-C-i cells, the affinities of CO2 and HCO3- transport for their substrates were about 5 (CO2 uptake) and 10 (HCO3- uptake) times higher than in high-C; cells, while air-grown ce lls formed an intermediate state. For the same cells, the intracellula r accumulated C-i pool reached 18, 32 and 55 mM in high-C-i, air-grown and low-C-i cells, respectively, when measured at 1 mM external C-i. Photosynthetic O-2 evolution, maximal CO2 and HCO3- transport activiti es, and consequently their relative contribution to photosynthesis, we re largely unaffected by the CO2 provided during growth. When the cell s were adapted to freshwater medium, results similar to those for arti ficial seawater were obtained for all CO2 concentrations. Transport st udies with high-C-i cells revealed that CO2 and HCO; uptake were equal ly inhibited when CO2 fixation was reduced by the addition of glycolal dehyde. In contrast, in low-C-i cells steady-state CO2 transport was p referably reduced by the same inhibitor. The inhibitor of carbonic anh ydrase ethoxyzolamide inhibited both CO2 and HCO3- uptake as well as O -2 evolution in both cell types. In high-C-i cells, the degree of inhi bition was similar for HCO3- transport and O-2 evolution with 50% inhi bition occurring at around 1 mM ethoxyzolamide. However, the uptake of CO2 was much more sensitive to the inhibitor than HCO3- transport, wi th an apparent I-50 value of around 250 mu M ethoxyzolamide for CO2 up take. The implications of our results are discussed with respect to C- i utilisation in the marine Synechococcus strain.