Glutamine Synthetase Ase Actyvity in in Chaetoceros Affinis (Bacillariophyceae): Comparison With Other Estimates of Nitrogen Utilization During Nutrient Perturbation1

Glutamine synthftase (GS) activity was investigated in a nitratt limited continuous culture of the marine diatom Chaeloccros afTinis (Lauder) Hustedt before and after the perturbation of the culture medium with 10 μM of 15 N labelled nitrate. Parallel studies were carried out on nitrate reductase(NR). nitrate uptake and assimilation, and Ievels of cellular nitrogen containing compounds with the objective to determine the validity of the GS assay as a measure of nitrate utilization. Activities in N‐deficient cells, grown at steady state, correlated well with uptake and assimilation rates. In N‐sufftcient celts, however, during the nitrate pertirbation period, they accounted only for about 10% of the two latter rates, when ambient nitrate concentrations were high (0. 7‐10 μ). It is proposed that under these growth conditions an alternative pathway via glutamate dehydrogenase (GDH) was operative. At low ambient nitrate concentrations (0.1‐0.7 μM), GS activities, uptake and assimilation rates again balanced rather well. Thus, the data support the view that GDH activity is associated with high levels and GS with low levels of external or internal nitrogen.