KINETICS OF NITROGEN (NITRATE, AMMONIUM AND UREA) UPTAKE BY THE PICOFLAGELLATE MICROMONAS-PUSILLA (PRASINOPHYCEAE)

The kinetics of N (nitrate, ammonium and urea) uptake by the eucaryotic picoflagellate Micromonas pusilla (Butcher) Manton et Parke were investigated using N-replete batch cultures and the N-15 tracer technique. Maximum specific uptake rate (V(max)) of NH4+ was 0.13 . h-1, > 2 times the V(max) of NO3- or urea (almost-equal-to 0.05 . h-1). The half-saturation constants (K(s)) for urea, NH4+ and NO3- were similar with an average value of 0.4-mu-g-at N . l-1 which is within the range reported for small oceanic diatoms. The greater initial slope of the Michaelis-Menten plot for NH4+ uptake suggests that M. pusilla can utilize low concentrations of NH4+ more effectively than equivalent concentrations of urea or NO3-. The results of this first study of N uptake kinetics by an eucaryotic picoplankter demonstrate the preference for NH4+ over urea and NO3- as a nitrogenous nutrient source, which may contribute to the relative success of picoplankton in oligotrophic oceanic regions.