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 was sampled in late July and mid-August 1995 when Aureococcus anophagefferens composed >90% and 30-40% of the total cell density, respectively. Experiments were designed to examine the effect of incubation duration on the uptake kinetics, and the effect of light and temperature dependencies of NH4+, urea, and NO3- uptake. Maximum specific uptake rates (V-max') decreased in the order NH4+, urea, NO3- and were nonlinear with time for NH4+ and urea, both of which exhibited an exponential decline between 1 and 10 min and then did not significantly change for 60 min. Nitrogen uptake kinetic experiments exhibited a typical hyperbolic response for urea and NO3-. Half-saturation constants (K-s) were calculated to be 0.03 and 0.12 mu mol . L(-1) for urea and NO3-; respectively, but could not be calculated for NH4+ under these experimental conditions. Nutrient uptake rate versus irradiance (NI) experiments showed 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-cell'(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 has a higher affinity for NH4+ and urea than for NO3- and that this particular species is adapted to use these substrates at low irradiances and concentrations. The data presented in this study are also consistent with the hypothesis that A. anophagefferens may be an oceanic clone that was displaced by an anomalous oceanographic event.