The Uptake of Methylammonium and Dimethylammonium by the Diatom, Phaeodactylum Tricornutum

SUMMARY In agreement with previous work with several different organisms, methylammonium uptake by Phaeodactylum is shown to take place by a mediated transport process with Michaelis–Menten kinetics with a Ks of about 35 μM There was no evidence for more than one mediated uptake system. Uptake of methylammonium shows a strong dependence on metabolism. In contrast, uptake of dimethylammonium is by passive diffusion. Uptake of methylammonium, but not that of dimethylammonium, was Na* dependent although the rate of uptake of both compounds was depressed by increasing K+ concentration. Rates of uptake of both compounds increased as a consequence of nitrogen deprivation of the cells and were depressed by addition of low (ca. 2 to 10 μM) concentrations of ammonium, uptake of methylammonium being the more sensitive to ammonium inhibition. It is concluded that methylammonium is probably taken up by an ammonium transport system and that its use allows more detailed study of the characteristics of that system.