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.