Nitrate uptake in marine phytoplankton: Comparison of half‐saturation constants from seven species1

Seven species of marine phytoplankton were assayed for the presence of a NO3−,CI−‐activated ATPase, thought to be associated with the cell membrane. Only the single dinoflagellate tested, Amphiclinium carterae, failed to show ATPase activity, while the specific activity of the enzyme ranged over one order of magnitude in the other species examined. It is suggested that the enzyme is primarily responsible for the active transport of NO3− across the plasmalemma and furthermore the Km for NO3− activation of the enzyme is physiologically manifested by Ks values for NO3− uptake by whole cells. The enzymatic mechanism may explain the observed Michaelis‐Menten kinetics for NO3− uptake.