Transient situations in nitrate assimilation by marine diatoms. 2. Changes in nitrate and nitrite following a nitrate perturbation1

Abstract

Nitrate uptake and the excretion arid subsequent uptake of nitrite by nitrogen‐limited cells of Skeletonema costatun and Phaeodactylum tricornutum following the addition of nitrate were measured under low light intensity. Nitrate uptake kinetics in the light period were markedly different from those in the dark period. The influence of the previous nitrogen source on initial nitrate uptake rates was species‐ and light‐dependent. Nitrite excretion was related to external nitrate in a hyperbolic way. The relationship between nitrite uptake and external nitrate was not affected by light, but by temperature. When nitrate concentrations decreased below 1 µM, nitrite was taken up, except when a temperature change was imposed. These results are discussed in relation to the estimation of kinetic parameters of nitrate uptake and the origin of the primary nitrite maximum in oligotrophic oceanic areas.