Nitrate reductase and glutamate dehydrogenase activities in Skeletonema costatum as measures of nitrogen assimilation rates

Abstract

Nitrate reductase (NADH-NR) and glutamate dehydrogenase (NADPH-GDH) activities were measured in Skeletonema costatum (Grev.) Cleve in ammonium and nitrate limited continuous cultures before and after additions of nitrate and/or ammonium. Comparisons of enzymic activity with nitrogen uptake and assimilation rates, external and internal nitrate concentrations, and external ammonium concentrations were made in order to assess the roles of NR and GDH in nitrogen assimilation and to determine their suitability as measures of nitrogen assimilation rates. NR activity appeared to be induced by internal rather than external nitrate concentrations. Ammonium in the medium reduced NR activity under some environmental conditions, but not others. However, ammonium acted indirectly, perhaps by causing the accumulation of an internal pool of an intermediate of ammonium assimilation. NR activity was found to approximate nitrate assimilation rates during growth limited by the nitrate supply and undeT some conditions in the presence of high nitrate and ammonium concentrations in the medium. Under other environmental conditions, NR activity did not agree with nitrate assimilation rates; a second nitrate reducing mechanism may operate when these conditions prevail. GDH activities were consistently low, representing less than 5% of the ammonium uptake and assimilation rates. Consequently, it is proposed that ODH is not the primary ammonium assimilating enzyme under most environmental conditions and cannot be used as a measure of ammonium assimilation.