MODELING THE INTERACTIONS BETWEEN AMMONIUM AND NITRATE UPTAKE IN MARINE-PHYTOPLANKTON

An empirically based mathematical model is presented which can simulat e the major features of the interactions between ammonium and nitrate transport and assimilation in phytoplankton. The model (ammonium-nitra te interaction model), which is configured to simulate a generic micro alga rather than a specified species, is constructed on simplified bio chemical bases. A major requirement for parametrization is that the N: C ratio of the algae must be known and that transport and internal poo l sizes need to be expressed per unit of cell C. The model uses the si ze of an internal pool of an early organic product of N assimilation ( glutamine) to regulate rapid responses in ammonium-nitrate interaction s. The synthesis of enzymes for the reduction of nitrate through to am monium is induced by the size of the internal nitrate pool and repress ed by the size of the glutamine pool. The assimilation of intracellula r ammonium (into glutamine) is considered to be a constitutive process subjected to regulation by the size of the glutamine pool. Longer ter m responses have been linked to the nutrient history of the cell using the N:C cell quota. N assimilation in darkness is made a function of the amount of surplus C present and thus only occurs at low values of N:C. The model can simulate both qualitative and quantitative temporal shifts in the ammonium-nitrate interaction, while inclusion of a deri vation of the standard quota model enables a concurrent simulation of cell growth and changes in nutrient status.