CHARACTERIZATION OF NITROGEN UPTAKE BY NATURAL-POPULATIONS OF AUREOCOCCUS ANOPHAGEFFERENS (CHRYSOPHYCEAE) AS A FUNCTION OF INCUBATION DURATION, SUBSTRATE CONCENTRATION, LIGHT, AND TEMPERATURE

Nitrogen uptake studies were conducted during an aestival ''brown tide '' bloom in Shinnecock Bay, Long Island, New York. The same station wa s sampled in late July and mid-August 1995 when Aureococcus anophageff erens composed >90% and 30-40% of the total cell density, respectively . Experiments were designed to examine the effect of incubation durati on on the uptake kinetics, and the effect of light and temperature dep endencies of NH4+, urea, and NO3- uptake. Maximum specific uptake rate s (V-max') decreased in the order NH4+, urea, NO3- and were nonlinear with time for NH4+ and urea, both of which exhibited an exponential de cline between 1 and 10 min and then did not significantly change for 6 0 min. Nitrogen uptake kinetic experiments exhibited a typical hyperbo lic response for urea and NO3-. Half-saturation constants (K-s) were c alculated to be 0.03 and 0.12 mu mol . L(-1) for urea and NO3-; respec tively, but could not be calculated for NH4+ under these experimental conditions. Nutrient uptake rate versus irradiance (NI) experiments sh owed that maximum uptake rates occurred at less than or equal to 1% of incident irradiance on both sampling dates and that values of V-max-c ell'(NH4+) were on average 30% greater than V-max-cell'(urea), and a 3 -4-fold decrease in calculated K-lt values for both NH4+ and urea. The results from these experiments demonstrate that A. anophagefferens ha s a higher affinity for NH4+ and urea than for NO3- and that this part icular species is adapted to use these substrates at low irradiances a nd concentrations. The data presented in this study are also consisten t with the hypothesis that A. anophagefferens may be an oceanic clone that was displaced by an anomalous oceanographic event.