Interactions between nitrate and ammonium in Emiliania huxleyi

The coccolithophorid Emiliania huxleyi was grown with ammonium and nitrate in "stretched-batch" culture. This provided a continuous input of fresh med ia at a dilution rate of 0.1 d(-1) and maintained the derepression of N-ass imilatory systems during increasing N-stress, while exhibiting batch cultur e growth dynamics. Estimates of Kt (half-saturation constant for transport) for both N nutrients were less than 0.4 mu M, With a median of 0.2 mu M Du ring the development of N-stress, the maximum rate of N-specific transport (Vt(max)) for ammonium increased while that for nitrate more closely matche d the concurrent rate of N-specific growth as N-stress developed. When both N nutrients were added together, respective Kt values showed no significan t change, suggesting the operation of discrete nitrate and ammonium porters . In contrast, there were changes in Vt(max) when both nutrients were assim ilated concurrently. However, higher concentrations of nitrate (4 mu M) wer e required to achieve a 50% depression of ammonium Vt(max) compared to the concentration of ammonium (0.5 mu M) required to bring about a similar depr ession of nitrate Vt(max) This is consistent with the regulation of transpo rt of both nutrients by an organic product of N-assimilation, which accumul ates more rapidly following ammonium addition to N-stressed cells. Ammonium -grown cells contained higher concentrations of internal free amino acids, such as glutamine, than nitrate-grown cells. Interactions between ammonium and nitrate transport and assimilations could be simulated using the Ammoni um-Nitrate Interaction Model, which contains constants for Kt and transport s regulated by the level of the early product of N-assimilation, glutamine. There were no indications of a significant loss of N from the [dissolved i norganic N plus particulate organic N] fraction for exponentially growing E miliania huxleyi in experiments, and there was no need to include such a pr ocess in the model for it to simulate the experiments. (C) 1999 Elsevier Sc ience B.V. All rights reserved.