Comparison of Half‐saturation Constants for Growth and Nitrate Uptake of Marine Phytoplankton 2

Growth rates and rates of nitrate uptake by N-depleted cells were measured for an oceanic diatom, Chaetoceros gracilis, and a neritic diatom, Asterionella japonica, as functions of nitrate concentration of the medium. Both growth and N-uptake rates appeared to be hyperbolic with nitrate concentration and could be fit to an equation of Michaelis-Menten form: where v is rate, V(m) . is the maximum rate, S is nitrate concentration, and Ks is the half-saturation constant. Ks values for uptake and growth were similar if not identical for each species. Uptake experiments can provide a presumptive measure of Ks for growth, thought to be an ecologically significant characteristic of a species.