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; expr essed as C-specific N transport) into Emiliania huxleyi and Heterosigma car terae were related to N status (cellular N:C). There was considerable varia tion in the magnitude of T-max relative to N:C. For nitrate transport in bo th 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 decre ased. 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 decreas ed. 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 tr ansport 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. Rela tionships far ammonium and nitrate T-max have been computed for simulations of the growth of E.huxleyi and H.carterae. The implications of these resul ts for modelling ammonium-nitrate interactions are discussed. It is shown t hat attempts to obtain a relationship between T-max and N:C from steady-sta te chemostat data may result in significant deviations from relationships d etermined using incubation techniques. Further, it is more important to obt ain a good estimate for ammonium T-max because of its dominant role in affe cting the ammonium-nitrate interaction.