The ammonium:nitrate delivery ratio to cells of Dunaliella tertiolecta in continuous culture had no effect on the ambient nitrate concentrations at steady state, indicating that the assimilation of these radicals is competitive, not suppressive, under nitrogen limiting conditions. The physiological parameters, C: cell and Chl: cell, were invariant over the range of delivery ratios. Cellular growth potential and yield coefficients (N:cell and N:C) showed a linear decline with increasing ammonium:total nitrogen input ratios, supporting the hypothesis that NH, ’ is not stored intracellularly. At low NH&+ contributions the yield coefficients displayed characteristics not predictable from considerations of solely ammonium or nitrate limited growth. The N:cell ratio is an excellent measure of the cellular growth potential resulting from intracellular reserves. Ammonium and nitrate are important nitrogen sources for phytoplankton. Dcscription of the nitrate-ammonium dynamics of marine phytoplankton is complex due, in part, to the differing capacities for incorporation and utilization of these radicals. Though the specific uptake rates of NIL+ and NOa- are a function of their ambient concentrations, according to Michaelis-Menten kinetics, their coassimilation does not occur at identical rates. When available together, ammonium is assimilated preferentially to nitrate (e.g. Grant et al
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