NITRATE AVAILABILITY AND CALCITE PRODUCTION IN EMILIANIA-HUXLEYI LOHMANN

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.