Increase of PCO2 during blooms of Emiliania huxleyi: Theoretical considerations on the asymmetry between acquisition of HCO3‐ and respiration of free CO2

The principal source of dissolved inorganic carbon (CT) of most marine phytoplankton is thought to be free CO2, but Emiliania huxleyi can utilize HCO3− ion both for calcification (C) and photosynthesis (P) . We present a simple model that simulates continuous diurnal removal of CT as either free CO2 or HCO3− across the range of CT typically encountered in seawater (2,100–1,800 µmol kgsw−1). Removal as free CO2 forces a drawdown in partial pressure of dissolved carbon dioxide (Pco2), at given CT, of up to 170 µatm greater than that forced by removal as HCO3− by E. huxleyi (assuming Cnet : Pnet = 1.0). Diurnal Cnet : Pnet must exceed 1.5 for Pco2 to increase during removal of HCO3−. When nocturnal respiration of free CO2 (R) is incorporated into the model, the respective drawdowns in Pco2 forced by diurnal removal of free CO2 and HCO3 are balanced by diel ratios R : Pnet = 1.0 and R : (P + C)net ≈ 0.15. Therefore, with diel R : (P + C)net > 0.15 but diurnal Cnet : Pnet = 1.0, E. huxleyi can theoretically grow with net diel increase in Pco2; however, diel Cnet : Pnet > 1.0 is forced by nocturnal loss of particulate organic carbon but not particulate inorganic carbon. Published values for C : P do not necessarily represent diel Cnet : Pnet and caution is required when extrapolating these ratios to predict changes in CT and Pco2 during natural blooms of E. huxleyi.