THE ROLE OF GLUTAMINE SYNTHETASE IN THE INCORPORATION OF AMMONIUM IN SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)1

The K_m for ammonia for glutamine synthetase and glutamate dehydrogenase was measured in enzyme extracts from Skeletonema costatum (Grev.) Cleve. At similar physiological pH and temperature the half-saturation constant for glutamine synthetase was 29 μM, whereas for GDH it was 28mM. On the basis of relative enzymic activity, as well as substrate affinity, it is suggested that glutamine synthetase is the enzyme primarily responsible for the incorporation of ammonium into the amino acid pool, when extracellular nitrogen is at ecological concentrations.