INHIBITION OF NITRATE UPTAKE BY AMMONIUM AND UREA IN THE EUKARYOTIC PICOFLAGELLATE MICROMONAS-PUSILLA (BUTCHER) MANTON-ET-PARKE

Experiments were carried out with nitrate-replete cultures of the picoflagellate Micromonas pusilla (Butcher) Manton et Parke to investigate the effects of ammonium and urea additions on nitrate uptake. By employing both N-15 methodology and by measuring the ambient nutrient concentrations over time, the uptake rates of the three substrata could be determined. Nitrate uptake was completely inhibited by NH4+ concentrations of 1-10-mu-g-at N . l-1, whereas a 10-mu-g-at N . l-1 perturbation of urea resulted in a 28% reduction of NO3- uptake and both N substrata were taken up simultaneously. This is the first study of the uptake interaction(s) of multiple N substrata for a picoplankter.