We grew Thalassiosira weissflogii to steady state over a range of Fe-limiting conditions with nitrate or ammonium as the N source. Nitrate-dependent cells had faster Fe-uptake rates, contained significantly higher intracellular Fe quotas, and grew faster than cells cultivated with NH4+ when Fe was most limiting. Under these conditions, carbon (C): chlorophyll a ratios and the minimum fluorescence yield per chlorophyll a increased, but N source had no effect on either parameter. The ratio of variable to maximum fluorescence (FvFm-1) declined little with Fe limitation even when T weissflogii was grown at 25% of its maximum rate (mu(max)). C: N ratios were higher in nitrate than in ammonium-grown cells and were constant at all Fe levels. Protein was independent of Fe and N, and amino acids were lowest in cells using NO;. The P content of T. weissflogii (mol P L-1 cell volume) increased by 1.5 times as Fe became most limiting to growth, causing N: P and C: P ratios to decline significantly. The elemental stoichiometry for Fe-limited new production of T. weissflogii (0.25 mu(max)) was thus 70C: 10N: 5.9Si: 1P: 0.00074Fe (by mols) compared with 97C:14N:4.7Si:1P:0.029Fe for nutrient-replete conditions. Uptake rate ratios of NO3-:PO43- showed the same dependence on Fe as the cellular N: P quotas, decreasing as [Fe] decreased. Iron limitation influenced the elemental composition of this marine diatom and will alter the assimilation ratios of C, N, and P in the high nitrate, low chlorophyll regions of the sea.
