Methylammonium uptake by Chesapeake Bay phytoplankton: Evaluation of the use of the ammonium analogue for field uptake measurements 1

The uptake of methylammonium (MA) by Chesapeake Bay phytoplankton was measured in short term experiments by using the 14 C radioisotope. This uptake was inversely related to ambient NH 4 + concentrations, and the specificity for uptake of both substrates was demonstrated by competitive inhibition. At stations with low ambient NH 4 + , MA uptake followed saturation kinetics, but the V max values decreased over time during 10–60‐min incubations. Uptake rates of MA were generally lower than those of NH 4 + , but in short incubations (10–15 min) the V max values were similar. Although MA and NH 4 + are taken up by the same transport system, the utility of MA as an NH 4 + analogue is restricted by the strong inhibition of MA uptake by low concentrations of NH 4 + and by the fact that in some cases MA uptake primarily reflects membrane transport, whereas NH 4 + uptake can also reflect metabolic assimilation. Our results show that in typical uptake experiments, NH 4 + uptake is rate‐limited by either internal pool filling or metabolic assimilation rather than by the membrane transport system. As a consequence, the kinetic parameters reported in the literature for NH 4 + uptake overestimate K s and underestimate V max for the membrane transport system.