A simple model based on HCO(3) (-) transport has been developed to relate photosynthesis and inorganic carbon fluxes for the marine cyanobacterium, Synechococcus sp. Nägeli (strain RRIMP N1). Predicted relationships between inorganic carbon transport, CO(2) fixation, internal carbonic anhydrase activity, and leakage of CO(2) out of the cell, allow comparisons to be made with experimentally obtained data. Measurements of inorganic carbon fluxes and internal inorganic carbon pool sizes in these cells were made by monitoring time-courses of CO(2) changes (using a mass spectrometer) during light/dark transients. At just saturating CO(2) conditions, total inorganic carbon transport did not exceed net CO(2) fixation by more than 30%. This indicates CO(2) leakage similar to that estimated for C(4) plants.For this leakage rate, the model predicts the cell would need a conductance to CO(2) of around 10(-5) centimeters per second. This is similar to estimates made for the same cells using inorganic carbon pool sizes and CO(2) efflux measurements. The model predicts that carbonic anhydrase is necessary internally to allow a sufficiently fast rate of CO(2) production to prevent a large accumulation of HCO(3) (-). Intact cells show light stimulated carbonic anhydrase activity when assayed using (18)O-labeled CO(2) techniques. This is also supported by low but detectable levels of carbonic anhydrase activity in cell extracts, sufficient to meet the requirements of the model.