A Model for HCO(3) Accumulation and Photosynthesis in the Cyanobacterium Synechococcus sp: Theoretical Predictions and Experimental Observations

A simple model based on HCO₃⁻ 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, CO₂ fixation, internal carbonic anhydrase activity, and leakage of CO₂ 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 CO₂ changes (using a mass spectrometer) during light/dark transients. At just saturating CO₂ conditions, total inorganic carbon transport did not exceed net CO₂ fixation by more than 30%. This indicates CO₂ leakage similar to that estimated for C₄ plants.

For this leakage rate, the model predicts the cell would need a conductance to CO₂ of around 10⁻⁵ centimeters per second. This is similar to estimates made for the same cells using inorganic carbon pool sizes and CO₂ efflux measurements. The model predicts that carbonic anhydrase is necessary internally to allow a sufficiently fast rate of CO₂ production to prevent a large accumulation of HCO₃⁻. Intact cells show light stimulated carbonic anhydrase activity when assayed using ¹⁸O-labeled CO₂ 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.