Calcification rate in Emiliania huxleyi Lohmann in response to light, nitrate and availability of inorganic carbon

SUMMARY The relationship between photosynthesis and calcification was investigated in a high‐calcifying strain of Emiliania huxleyi. At pH 8.3 and a photon flux density of 50 μmol m−2 s−1 calcification and photosynthetic 14CO2 fixation were carbon saturated at 1 mM DIC (dissolved inorganic carbon) but at a photon flux density of 300 μmol m−2 s−1 calcification and photosynthetic 14CO2 fixation were not saturated at the DIC concentration of sea‐water, 2 mM. When HCO3− provides the bulk of inorganic carbon the stoichiometry between photosynthetic 14CO2 fixation and calcification was 1:1. In the high‐calcifying strain of E. huxleyi the stoichiometry between photosynthetic 14CO2 fixation and O2 evolution was 2:1 but in a low calcifying strain the stoichiometry was 1:1. High nitrate concentrations, i.e. 1000 μM were required to inhibit calcification. The optimum pH for calcification and photosynthetic 14CO2 fixation was 7.8. From the results a model is proposed in which a molecule of HCO3− is the precursor of calcite in the coccolith vesicle with the extrusion of H+ into the cytosol, while another HCO3− provides CO2 in the chloroplast with the extrusion of OH−. The interaction of these processes maintains cytoplasmic pH near neutrality.