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

Nitrate reductase (NADH-NR) and glutamate dehydrogenase (NADPH-GDH)activities were measured in Skeletonema costatum (Grev.) Clevein ammonium and nitrate limited continuous cultures before andafter additions of nitrate and/or ammonium. Comparisons of enzymicactivity with nitrogen uptake and assimilation rates, externaland internal nitrate concentrations, and external ammonium concentrationswere made in order to assess the roles of NR and GDH in nitrogenassimilation and to determine their suitability as measuresof nitrogen assimilation rates. NR activity appeared to be inducedby internal rather than external nitrate concentrations. Ammoniumin the medium reduced NR activity under some environmental conditions,but not others. However, ammonium acted indirectly, perhapsby causing the accumulation of an internal pool of an intermediateof ammonium assimilation. NR activity was found to approximatenitrate assimilation rates during growth limited by the nitratesupply and undeT some conditions in the presence of high nitrateand ammonium concentrations in the medium. Under other environmentalconditions, NR activity did not agree with nitrate assimilationrates; a second nitrate reducing mechanism may operate whenthese conditions prevail. GDH activities were consistently low,representing less than 5% of the ammonium uptake and assimilationrates. Consequently, it is proposed that ODH is not the primaryammonium assimilating enzyme under most environmental conditionsand cannot be used as a measure of ammonium assimilation. ¹ Contribution number 1095 from the Department of Oceanography,University of Washington