A Model for HCO3− Accumulation and Photosynthesis in the Cyanobacterium Synechococcus sp

Abstract

A simple model based on HCO3- transport was developed to relate photosynthesis and inorganic C fluxes for the marine cyanobacterium, Synechococcus sp. Nageli (strain RRIMP N1). Predicted relationships between inorganic C 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 C fluxes and internal inorganic C 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 C transport did not exceed net CO2 fixation by > 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 .apprx. 10-5 cms/s. This is similar to estimates made for the same cells using inorganic C 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.