Development of rapid ammonium uptake during starvation of batch and chemostat cultures of the marine diatom Thalassiosira pseudonana

Cell nitrogen quotas and uptake rates following ammonium additions were measured during ammonium-limited growth transients obtained by starving batch and chemostat cultures of Thalassiosira pseudonana (Clone 3 H). During starvation, cell quotas decreased by more than 50% in batch cultures. In chemostat cultures, the drop in cell quota during starvation decreased with dilution rate, from more than 50% at 1.45 d^-1, to less than 10% at 0.22 d^-1. Minimal levels of 3 to 4×10^-2 pg-at. N cell^-1 were reached after 24 h starvation in both batch and chemostat cultures. Uptake rates over the first minute of perturbation experiments were 3 times the long-term (10 to 30 min) rates. In batch cultures, specific uptake rates increased from 4 d^-1 to 20 d^-1 after 24 h starvation. Uptake rates per cell were independent of starvation time and dilution rate in chemostat cultures, but lower in non-starved batch cultures. The implications of these data for models of phytoplankton growth are discussed: the data support models which predict a depression in average growth rates when diatoms encounter microscale patches in oligotrophic environments.