Relationship between nitrate reductase and nitrate uptake in phytoplankton in the Peru upwelling region1,2

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Abstract
Nitrate reductase (NR) activity and 15NO3‒ uptake in phytoplankton were compared under different environmental conditions on two cruises in the upwelling region off Peru. The NR activity and NO3‒ uptake rates responded differently to light and nutrients and the differences led to variations in the uptake; reductase ratio. Analysis of these variations suggests that the re‐equilibration time of the two processes in response to environmental perturbation is an important source of variability. The nitrate uptake system responds faster than the nitrate reductase system. Considering these differences in response time, the basic differences in the two processes, and the differences in their measurement, we conclude that the NR activity measures the current nitrate‐reducing potential, which reflects NO3‒ assimilation before the sampling time, while 15NO3‒ uptake measures NO3‒ assimilation in the 6‐h period following sampling. Thus, considering the sampling time as a point of reference, the former is a measure of the past and the latter is a measure of the future.