Variations in the maximum transport rates for ammonium and nitrate in the prymnesiophyte Emiliania huxleyi and the raphidophyte Heterosigma carterae

Changes in the maximum rates of ammonium and nitrate transport (T-max; expressed as C-specific N transport) into Emiliania huxleyi and Heterosigma carterae were related to N status (cellular N:C). There was considerable variation in the magnitude of T-max relative to N:C. For nitrate transport in both species and for ammonium in H.carterae, the curve linking N:C and T-max was bell shaped; T-max increased initially as N:C fell and then T-max decreased. In contrast, T-max for ammonium in E.huxleyi did not decrease to such low levels and N-specific N transport increased continually as N:C decreased. While T-max for ammonium into E.huxleyi could be an order of magnitude higher than the N-transport rate required to support growth, for nitrate transport into both species T-max was coupled more closely to growth rate. In H.carterae, T-max for ammonium was up to four times that for nitrate. Relationships far ammonium and nitrate T-max have been computed for simulations of the growth of E.huxleyi and H.carterae. The implications of these results for modelling ammonium-nitrate interactions are discussed. It is shown that attempts to obtain a relationship between T-max and N:C from steady-state chemostat data may result in significant deviations from relationships determined using incubation techniques. Further, it is more important to obtain a good estimate for ammonium T-max because of its dominant role in affecting the ammonium-nitrate interaction.