Carbon Dioxide vs. Bicarbonate Utilisation

While CO2 is the external inorganic carbon (Ci) form used for photosynthesis in terrestrial plants, the higher concentration of bicarbonate (HCO3−) in most water bodies renders this ionic Ci form the preferred external Ci source for most cyanobacteria, microalgae, and submerged macrophytes. The equilibrium concentrations of these two Ci forms depend largely on pH. For example, at today’s average seawater pH of 8.1, the HCO3− concentration is approximately 120 times that of dissolved CO2; in most lakes and streams pH values are lower so this ratio is reduced in favour of CO2 (but this Ci form is seldom higher than that of HCO3−), and HCO3− utilisation becomes less advantageous. On the other hand, the affinity of aquatic phototrophs for CO2 is much higher than that for HCO3−, and the preferred Ci form will thus be a compromise between the affinities for, and concentrations of, the two Ci forms. This chapter will outline ways to determine whether, and to what degree, external CO2 or HCO3− is utilised by various aquatic phototrophs.