PLANT PHYSIOLOGY , Vol 107, Issue 1 245-251, Copyright © 1995 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Quantification of the Contribution of CO2, HCO3-, and External Carbonic Anhydrase to Photosynthesis at Low Dissolved Inorganic Carbon in Chlorella saccharophila
T. G. Williams and B. Colman
Department of Biology, York University, 4700 Keele Street, North York, Ontario, Canada M3J 1P3
An equation has been developed incorporating whole-cell rate constants for
CO2 and HCO3- that describes accurately photosynthesis (Phs) in suspensions
of unicellular algae at low dissolved inorganic carbon. At pH 8.0 the
concentration of CO2 available to the algal cells depends on 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 absolute Phs versus Chl
concentration approaches the rate of Phs on a milligram 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 Chlorella saccharophila it was 0.16
L mg-1 Chl h-1. The difference between the constants for dissolved
inorganic carbon (measured in cells with external carbonic anhydrase) and
HCO3-1 is the constant for CO2, which was 26 L mg-1 Chl h-1. This
difference causes the half-saturation constant for Phs to increase 5- to
6-fold at high cell densities. The increase in CO2 use as a result of
external carbonic anhydrase is described mathematically as a function of
cell density.
