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

The uptake of methylammonium (MA) by Chesapeake Bay phytoplankton was measured in short term experiments by using the ‘4c radioisotope. This uptake was inversely related to ambient NH4+ concentrations, and the specificity for uptake of both substrates was dem-onstrated by competitive inhibition. At stations with low ambient NH4+, MA uptake followed saturation kinetics, but the V,,, vnlucs decreased over time during 10-60-min incubations. Uptake rates of MA were generally lower than those of NH4+, but in short incubations (lo-15 min) the V,,, values were similar. Although lMA and NH4+ are taken up by the same transport system, the utility of MA as an NHdk analogue is restricted by the strong inhibition of MA uptake by low concentrations of NH,+ and by the fact that in some cases MA uptake primarily reflects membrane transport, whereas N11,+ uptake can also reflect metabolic assimilation. Our results show that in typical uptake experiments, NH4+ 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 NIL+ uptake overestimate K, and underestimate V,,