Photosynthetic Adaptation by Synechococcus leopoliensis in Response to Exogenous Dissolved Inorganic Carbon

Synechococcus leopoliensis was grown over a wide range of dissolved inorganic carbon (DIC) concentrations (4-25,000 micromolar) which were obtained by varying culture pH (6.2-9.6) and the CO₂ concentration of the gas stream (36-50,000 microliters per liter). The DIC] required to half-saturate photosynthesis (K_½^DIC) was found to vary depending upon the ambient DIC concentration at which the cells were grown. Low DIC] grown cells exhibited low values of K_½^DIC (4.7 micromolar) whereas cells grown at high DIC] exhibited high values of K_½^DIC (1-2.5 millimolar). Intermediate concentrations of DIC produced intermediate values. Changes in K_½^DIC appeared to be solely a function of DIC] and were independent of both culture pH and CO₂ concentration. As changes in K_½^DIC occur in response to DIC concentrations commonly found in natural systems we suggest this adaptation may be of ecological significance.