Nutrient uptake, and alkaline phosphate (EC 3 : 1 : 3 : 1) activity of Emiliania huxleyi (Prymnesiophyceae) during growth under N and P limitation incontinuous cultures

Emiliania huxleyi (strain L) expressed an exceptional P assimilation capabi lity. Under P limitation, the minimum cell P content was 2.6 fmol P.cell(-1 ), and cell N remained constant at all growth rates at 100 fmol N.cell(-1). Both, calcification of cells and the induction of the phosphate uptake sys tem were inversely correlated with growth rate, The highest (cellular P bas ed) maximum phosphate uptake rate (V-max(P)) was 1400 times (i.e. 8.9 h(-1) ) higher than the actual uptake rate, The affinity of the P-uptake system ( dV/dS) was 19.8 L.mu mol(-1).h(-1) at mu = 0.14 d(-1). This is the highest value ever reported for a phytoplankton species. V-max and dV/dS for phosph ate uptake were 48% and 15% lower in the dark than in the light at the lowe st growth rates. The half-saturation constant for growth was 1.1 nM, The coefficient for lux ury phosphate uptake (Q(maxt)/Q(min)) was 31. Under P limitation, E. huxley i expressed two different types of alkaline phosphatase (APase) enzyme kine tics, One type was synthesized constitutively and possessed a V-max and hal f-saturation constant of 43 fmol MFP.cell(-1).h(-1) and 1.9 mu M, respectiv ely, The other, inducible type of APase ex pressed its highest activity at the lowest growth rates, with a V-max and half-saturation constant of 190 f mol MFP.cell(-1).h(-1) and 12.2 mu M, respectively. Both APase systems were located in a lipid membrane close to the cell wall, Under N-limiting growt h conditions, the minimum N quotum was 43 fmol N.cell(-1). The highest valu e for the cell N-specific maximum nitrate uptake rate (V-max(N)) was 0.075 h(-1); for the affinity of nitrate uptake, 0.37 L mu mol(-1).h(-1). The upt ake rate of nitrate in the dark was 70% lower than in the light, N-limited cells were smaller than P-limited cells and contained 50% less organic and inorganic carbon. In comparison with other algae, E. huxleyi is a poor comp etitor for nitrate under N limitation, As a consequence of its high affinit y for inorganic phosphate, and the presence of two different types of APase in terms of kinetics, E, huxleyi is expected to perform well in P-controll ed ecosystems.