Evidence for the Participation of Glutamate Dehydrogenase in Ammonium Assimilation by Stichococcus Bacillaris

Summary Cells of the green alga, Chlorella vulgaris (strain 21l/8k) have high NADPH‐glutamate de‐hydrogenase (GDH) activity when grown on ammonium (NH4+); activity is low in nitrate (NO3−)‐grown or nitrogen‐depleted cells. In contrast, cells of Stichococcus bacillaris (strain 379/5) contain little NADPH‐GDH activity when grown on NH4+ but much when grown on NO3− or depleted of nitrogen. Despite the high NADPH‐GDH activity in NH4+‐grown cells of C. vulgaris, NH4+ assimilation by this organism is strongly inhibited by methionine sulphoximine (MSO), an inhibitor of glutamine synthetase. NH4+ assimilation by NH4+‐grown S. bacillaris is similarly almost completely inhibited by MSO. However, NH4+ assimilation by NQ3‐grown S. bacillaris is much less inhibited by MSO and it is suggested that the high NADPH‐GDH activity of these cells is partly responsible for the remaining NH4+ assimilation.