KINETICS OF NITROGEN-15 LABELLED AMMONIUM UPTAKE BY CAULERPA CUPRESSOIDES (CHLOROPHYTA)1,2,3

The incorporation of ¹⁵NH₄⁺ as a function of time and concentration was used to estimate ammonium uptake by Caulerpa cupressoides (West) C. Agardh, taken from its habitat on the sediments of Tague Bay lagoon, St. Croix, U.S. Virgin Islands. ¹⁵N-based uptake rates followed Michaelis-Menten type saturation kinetics; the maximum uptake rate was 8.7 ± 3.0 μmol N/g dry wt·h at 26° C and the half-saturation constant was 48 ± 10 μM (X?± SE). The high half-saturatiaon constant reflects the dependence of Caulerpa on sediment pore waters as a nitrogen source. Calculations of uptake from isotope time course data were compared to estimates made from ammonium depletion. More ammonium disappeared than could be accounted for by the incorporation of ¹⁵N in Caulerpa, and isotope dilution of the ammonium pool is shown not to be responsible for underestimates of uptake, primarily because large ¹⁵N additions (40–300 μM) were used. It is suggested that either (1) a secondary ammonium sink such as wall sorption or bacterial uptake significantly influenced ammonium concentrations, or (2) ¹⁵N was lost as labelled dissolved organic nitrogen or volatilized during ¹⁵N sample preparation.