Differences in abilities to assimilate different nutrients

Morel etal. (1991) give an equation for growth rate in a steady state at low external concentration of a trace metal such as iron. Slightly modified, it is  

S is the ambient concentration of available metal, kL is the rate constant for the uptake reaction in which the metal combines with a transport ligand, whose abundance per cell is given by LT. Having drawn attention to the likelihood that the Fe-transport molecules of eukaryotic algae are membrane bound siderophores, like those of cyanobacteria, Morel etal. conclude that there is  

Whatever constraints exist on the biogeochemical composition of phyto-plankters, uptake abilities ought to be potentially more variable. It should be possible for the transport sites for one nutrient in the cell wall to be increased at the expense of sites for other nutrients. However, this does not seem to occur in the case of iron (Sunda Huntsman, 1995), perhaps because of 

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