High-calcifying cells of Emiliania huxleyi were grown on a synthetic s
eawater medium and the effect of nitrate (NO3-) concentration on growt
h, calcite accumulation, calcification rate and DIC (dissolved inorgan
ic carbon) utilisation determined. The stoichiometry between NO3- util
isation and calcite production was 1: 6.5 (mol/mol). Calcification and
growth were tightly coupled: calcite production ceased when cultures
entered the stationary phase due to NO3- depletion, but by adding a pu
lse of NO3- growth and calcification were restored. The initial C/N ra
tio in the medium was important in relation to calcification rate. At
20 muM NO3- the total DIC (2 mM) was rapidly depleted, the calcificati
on rate subsequently declining, whereas at 5 and 10 muM NO3- rates of
calcification were constant at 20 g carbon cell-1 x 10(14) . h-1 throu
ghout culture growth, excess DIC being present relative to the availab
le NO3-. Calcite production per unit NO3- was similar for isolates of
E. huxleyi from neritic, oligotrophic and nitrate-rich waters. In labo
ratory cultures, where the photon flux density is optimised for growth
, the initial NO3- concentration is a reliable indicator of final calc
ite yield.