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Optimal nitrogen-to-phosphorus stoichiometry of phytoplankton


Redfield noted the similarity between the average nitrogen-to-phosphorus ratio in plankton (N:P = 16 by atoms) and in deep oceanic waters (N:P = 15; refs 1, 2). He argued that this was neither a coincidence, nor the result of the plankton adapting to the oceanic stoichiometry, but rather that phytoplankton adjust the N:P stoichiometry of the ocean to meet their requirements through nitrogen fixation, an idea supported by recent modelling studies3,4. But what determines the N:P requirements of phytoplankton? Here we use a stoichiometrically explicit model of phytoplankton physiology and resource competition to derive from first principles the optimal phytoplankton stoichiometry under diverse ecological scenarios. Competitive equilibrium favours greater allocation to P-poor resource-acquisition machinery and therefore a higher N:P ratio; exponential growth favours greater allocation to P-rich assembly machinery and therefore a lower N:P ratio. P-limited environments favour slightly less allocation to assembly than N-limited or light-limited environments. The model predicts that optimal N:P ratios will vary from 8.2 to 45.0, depending on the ecological conditions. Our results show that the canonical Redfield N:P ratio of 16 is not a universal biochemical optimum, but instead represents an average of species-specific N:P ratios.

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Figure 1: Structural N:P ratio of 29 species of freshwater and marine phytoplankton.
Figure 2: Fitness measures as a function of allocation to assembly machinery.

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We thank P. Falkowski, I. Loladze, S. Pacala and D. Tilman for comments and discussion. We acknowledge support from the Andrew W. Mellon Foundation and the National Science Foundation.

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Correspondence to Christopher A. Klausmeier.

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Structural N:P ratio of 29 species of freshwater and marine phytoplankton. (PDF 39 kb)

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Klausmeier, C., Litchman, E., Daufresne, T. et al. Optimal nitrogen-to-phosphorus stoichiometry of phytoplankton. Nature 429, 171–174 (2004).

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