Transient situations in nitrate assimilation by marine diatoms. III. short-term uncoupling of nitrate uptake and reduction

Abstract

The response of nitrate uptake, storage, and reduction by nitrogen-limited cultures of three marine diatoms has been investigated following the addition of nitrate. During perturbation experiments, uncoupling between uptake and reduction was extensive in Skeletonema costatum (Grev.) Cleve and Phaeodactylum tricomutum Bohlin, as internal nitrate could reach 15% of cell nitrogen compared with 1 to 2% under steady-state conditions. This capacity for nitrate storage allowed these species to maintain high rates of nitrate reduction at low external nitrate levels. In contrast, Chaetoceros affinis Lauder accumulated much less nitrate during a perturbation, indicating a closer coupling between uptake and reduction, a phenomenon which was also observed in the field for nitrogen sufficient and nitrogen deficient phytoplankton. Nitrate reduction was related to external nitrate by a hyperbola when data was obtained by either the graded addition or the perturbation method. The relationship to internal nitrate clearly showed saturation kinetics only when data were obtained by the latter method. The sudden decrease in nitrate reduction upon external nitrate depletion, however, made it necessary to separate the data into two groups and to fit them to two hyperbolas with different kinetic characteristics.