UPTAKE OF NITRATE, AMMONIUM, AND UREA BY NITROGEN‐STARVED CULTURES OF MICROMONAS PUSILLA (PRASINOPHYCEAE): TRANSIENT RESPONSES 1

Nitrogen uptake rates were measured as a function of time following saturating additions (15 μMg‐at N· −1 ) of 15 N‐labelid ammonium, urea, and nitrate to N‐starved cultures of the picoflagellate Micromonas pusilla Butcher. Uptake rates were estimated from both the accumulation of 15 N into the cells and the disappearance of nitrogen from the medium. Transient elevated (surge) uptake rates of NH 4 + and urea were observed after enrichment. During the first 5 min the initial urea and NH 4 + uptake rates were 2‐ and 4‐fold greater than the maximum growth rate (μM max )observed prior to No 3 − depletion in the cultures. The elevated urea uptake rates declined quickly to a relatively constant value, whereas the initial rates of NH 4 + uptake declined rapidly but were followed by a subsequent increase prior to remaining roughly constant. Nitrate was not taken up as readily by N‐starved M. pusilla as the reduced N forms. Although NO 3 + uptake commenced immediately after enrichment (i.e. no lag period) the N‐Specific rate over the next 6 h averaged half the μM max observed during NO 3 − replete conditions.