FIG 5 .

Model predictions at various HCO₃ conditions. With increasing HCO₃ concentrations, NO₃ is limiting and is completely required for biomass production; subsequently, NO₃ storage declines. Excess carbon is released as DMSP, which is the only reaction in the model allowing carbon secretion (A). Panel B shows biomass production and NO₃ storage when NO₃ uptake is increased linearly shortly before NO₃ becomes limiting (at an HCO₃ uptake level of 5 mM). Simulation results when carbon can be stored as chrysolaminarin or TAGs are depicted in panel C. Here, NO₃ uptake is constrained and constant as in panel A. The model predicts that excess carbon is stored in the form of TAGs. (D) Effect of the carbon/nitrogen (C/N) ratio on biomass. Within this simulation, carbon uptake was varied from 0 to 10 mM and nitrogen uptake was kept constant at 0.535 mM. Biomass production increases at C/N ratios under 10.28 and stagnates at ratios over 10.28 because of limiting nitrogen availability. Note that for all of the simulations shown, the available CO₂ was forced to be taken up.