CALCIFICATION RATE IN EMILIANIA-HUXLEYI LOHMANN IN RESPONSE TO LIGHT, NITRATE AND AVAILABILITY OF INORGANIC CARBON

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 mumol m-2 s-1 calcification and photosynthetic (CO2)-C-14 fixation were carbon saturated at 1 mm DIC (dissolved inorganic carbon) but at a photon flux density of 300 mumol m-2 s-1 calcification and photosynthetic (CO2)-C-14 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 (CO2)-C-14 fixation and calcification was 1:1. In the high-calcifying strain of E. huxleyi the stoichiometry between photosynthetic (CO2)-C-14 fixation and O2 evolution was 2:1 but in a low calcifying strain the stoichiometry was 1:1. High nitrate concentrations, i.e. 1000 muM were required to inhibit calcification. The optimum pH for calcification and photosynthetic (CO2)-C-14 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.