The elemental stoichiometry and composition of an iron-limited diatom

Abstract

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 (Fv Fm-1) declined little with Fe limitation even when T. weissflogii was grown at 25% of its maximum rate (μmax). 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 NO3-. 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 (0.25μmax) 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. © 2005, by the American Society of Limnology and Oceanography, Inc.