Transient situations in nitrate assimilation by marine diatoms. 4. Non-linear phenomena and the estimation of the maximum uptake rate

Abstract

Under conditions of substrate saturation and otherwise constant environmental parameters, the uptake rate (V'max) of nitrate (and other nitrogen compounds) can vary widely following the addition of nitrate (or other nitrogen compounds) to medium containing unicellular algae under various physiological states. Cases are presented in which V'max changes in different directions during the incubation, over a time scale shorter than the generation time or the day-night periodicity. The influence of each pattern is discussed in relation to the Michaelis-Menten equation. Examples are given in which the experimental protocol will yield data reflecting a relationship between uptake rate and time rather than uptake rate and substrate concentration. It is shown that the two methods now in use for estimating V'max will not give similar results unless the uptake rate remains constant over the duration of the measurement. In nature as well as in culture, this condition seems to be seldom met. For short-term incubations, physical phenomena such as adsorption and diffusion are difficult to separate from biological ones. As the incubation duration increases, biological processes such as enzyme induction, feedback regulation and product excretion will lead to deformations of classical enzymatic kinetics.