Na. Nimer et Mj. Merrett, CALCIFICATION RATE IN EMILIANIA-HUXLEYI LOHMANN IN RESPONSE TO LIGHT,NITRATE AND AVAILABILITY OF INORGANIC CARBON, New phytologist, 123(4), 1993, pp. 673-677
The relationship between photosynthesis and calcification was investig
ated 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 photosynthe
tic (CO2)-C-14 fixation were carbon saturated at 1 mm DIC (dissolved i
norganic carbon) but at a photon flux density of 300 mumol m-2 s-1 cal
cification and photosynthetic (CO2)-C-14 fixation were not saturated a
t the DIC concentration of sea-water, 2 mm. When HCO3- provides the bu
lk 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 fix
ation and O2 evolution was 2:1 but in a low calcifying strain the stoi
chiometry was 1:1. High nitrate concentrations, i.e. 1000 muM were req
uired to inhibit calcification. The optimum pH for calcification and p
hotosynthetic (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 O
H-. The interaction of these processes maintains cytoplasmic pH near n
eutrality.