Combined effects of physics and physiology explain the observed pattern of nitrate uptake kinetics in the ocean

Recent trait-based modeling of nutrient uptake by microorganisms (Aksnes & Cao. Marine Ecology Progress Series 440, p. 41-51, 2011; Fiksen et al. Limnology and Oceanography 58, p. 193-202, 2013) has advanced our understanding of how the nutrient uptake kinetics should depend on cell size and extracellular diffusion of nutrient molecules. This has provided a basis for better understanding observed patterns in terms of traits and fundamental physical processes, and for formulating more realistic models of plankton ecosystems. Here we extend the trait-based models using the principle of optimality subject to a physiological trade-off between the maximum uptake rate vs. the number of uptake sites. Then we test the predictions of each model, with and without the trade-off, against observed patterns for kinetic parameters describing the rate of nitrate uptake by natural assemblages of oceanic plankton as measured by ship-board experiments. The new model is able to reproduce: 1) the tendency of half-saturation constants to increase with nitrate concentration in the ocean, in terms of the trade-off, and 2) the wide variability in measured half-saturation constants, in terms of a realistic range of cell sizes for oceanic phytoplankton. We finally present a coherent explanation for the observed pattern in terms of both adaptation of physiology to environmental nutrient concentrations, which results in greater half- saturation constants for cells of any size adapted to higher vs. lower nutrient concentrations, and cell size, which tends to increase with ambient nutrient concentration. This provides a basis for modeling size as an adaptive trait in planktonic ecosystem models.