Exploring the mechanism of ammonium uptake in phytoplankton with an ammonium analogue,methylamine

Uptake of the ammonium analogue methylamine hydrochloride was used to examine the mechanism of ammonium transport in several species of marine phytoplankton (mainly Skeletonema costatum, but also Thalassiosira weissflogii, Emiliania huxleyi and Gonyaulax polyedra). Ambient pH had no effect on the retention of methylamine between pH 3.6 to 9.2 HgCl₂ markedly inhibited methylamine transport. Transport was temperature-dependent and the Michaelis-Menten equation described methylamine transport as a function of concentration in experiments of less then 20 to 30 min duration. Pulse-chase experiments showed that only 5 to 15% of intracellular methylamine was readily exchanged with external methylamine or ammonium, and the remaining 85 to 95% exchanged more slowly. These results suggest that methylamine transport, hence ammonium uptake, probably was an active metabolic process as it occurred against typical values of algal electrochemical gradients. The acid-trap model of ammonium transport was not consistent with these data. A two-compartment model where ammonium is transported into the cytoplasm and vacuole is proposed.