Nitrate transport and ammonium-nitrate interactions at high nitrate concentrations and low temperature

The implications of the operation of biphasic nitrate transport and the effects of low temperature incubations on phytoplankton N-physiology are discussed. The inclusion of 2 nitrate transport systems (low rate, high affinity; high rate, low affinity) with different regulatory thresholds and temperature-dependent steps can be included readily in the structure of existing mechanistic mode ls of N-assimilation. These models can then reproduce the lack of complete repression of nitrate transport by ammonium at high nitrate concentrations documented for some species, and the enhancement of nitrate assimilation in low temperature incubations. Further research is required to clarify the temporal dynamics of these interactions in order to demonstrate their ecological significance and to validate the performance of the models.