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

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Abstract
  • The uptake of methylammonium (MA) by Chesapeake Bay phytoplankton was measured in short term experiments by using the 14C radioisotope. This uptake was inversely related to ambient NH4+ concentrations, and the specificity for uptake of both substrates was demonstrated by competitive inhibition. At stations with low ambient NH4+, MA uptake followed saturation kinetics, but the Vmax values decreased over time during 10–60-min incubations. Uptake rates of MA were generally lower than those of NH4+, but in short incubations (10– 15 min) the Vmax values were similar. Although MA and NH4+ are taken up by the same transport system, the utility of MA as an NH4+ analogue is restricted by the strong inhibition of MA uptake by low concentrations of NH4+ and by the fact that in some cases MA uptake primarily reflects membrane transport, whereas NH4+ 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 NH4+ uptake overestimate K8 and underestimate Vmax for the membrane transport system.
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  • Wheeler, Patricia A., McCarthy, James J., ( 1982), Methylammonium uptake by Chesapeake Bay phytoplankton: Evaluation of the use of the ammonium analogue for field uptake measurements, Limnology and Oceanography, 27, doi: 10.4319/lo.1982.27.6.1120.
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  • 27
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  • 6
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  • This work was supported by grants OCE 78-26011 and OCE 80-22990 from the National Science Foundation.
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  • 0024-3590

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