QUANTIFICATION OF THE CONTRIBUTION OF CO2, HCO3-, AND EXTERNAL CARBONIC-ANHYDRASE TO PHOTOSYNTHESIS AT LOW DISSOLVED INORGANIC CARBON IN CHLORELLA-SACCHAROPHILA

An equation has been developed incorporating whole-cell rate constants for CO2 and HCO3- that describes accurately photosynthesis (Phs) in s uspensions of unicellular algae at low dissolved inorganic carbon. At pH 8.0 the concentration of CO2 available to the algal cells depends o n the rate of supply from, and the loss to, HCO3- and the rate of use by the cells. At elevated cell densities (>30 mg chlorophyll [Chl] L(- 1)), at which CO2 use by the cells is high, the slope of a graph of ab solute Phs versus Chl concentration approaches the rate of Phs on a mi lligram of Chl basis because of HCO3- use alone. The slope of a graph of Phs versus HCO3- will be the rate constant for HCO3- and for Chlore lla saccharophila it was 0.16 L mg(-1) Chl h(-1). The difference betwe en the constants for dissolved inorganic carbon (measured in cells wit h external carbonic anhydrase) and HCO3-1 is the constant for CO2, whi ch was 26 L mg(-1) Chl h(-1). This difference causes the half-saturati on constant for Phs to increase 5- to 6-fold at high cell densities. T he increase in CO2 use as a result of external carbonic anhydrase is d escribed mathematically as a function of cell density.