M.:The elemental stoichiometry and composition of an iron-limited diatom

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 NH41 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 (Fv F) declined little with Fe limitation21m even when T. weissflogii was grown at 25 % of its maximum rate (mmax). 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 L21 cell volume) increased by 1.5 times as23 Fe became most limiting to growth, causing N: P and C: P ratios to decline significantly. The elemental stoichi-ometry for Fe-limited new production of T. weissflogii (0.25mmax) 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 NO: PO showed the same dependence on Fe as the cellular N: P quotas, decreasing as [Fe] decreased. Iron2 323 4 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