Transient situations in nitrate assimilation by marine diatoms. 1. Changes in uptake parameters during nitrogen starvation1

The parameters of nitrate uptake by nitrogen‐limited and nonlimited cells of Phaeodactylum tricornutum change extensively during nitrate starvation. The saturated uptake rate, affinity constant, and nitrogen cell content decrease by half to three‐fourths in 3 days, while the cells remain metabolically active. Under continuous light, the absolute uptake rate goes through a maximum after 3–5 h of starvation, then decreases significantly. Upon re‐exposure to nitrate, nonlinear uptake is observed if the cells have been deprived of nitrate for more than 24 h. After a lag time which is related to the duration of the previous starvation phase, a shift‐up in saturated uptake rate occurs, leading to recovery of the initial rate within hours. The direction of change in the specific uptake rate depends on the physiological state of the cells before the starvation phase. The relationship between specific uptake rate and nitrogen cell content of nitrogen‐starved cultures, although in contrast to that found with nitrogen‐limited cultures, has the advantage of better describing results from field studies: low uptake rates and affinity constants in oligotrophic waters and high values in eutrophic waters. These findings have implications within the context of the theory of nutrient limitation and, more generally, in relation to the use of the Michaelis‐Menten equation in modeling nutrient uptake by marine phytoplankton.