Scaling nutrient transport kinetics of marine microbes

Nutrient acquisition is a critical determinant for the competitive advantage for auto- and osmohetero- trophs alike. Nutrient limited growth is commonly described on a whole cell basis through reference to a maximum growth rate (G_max) and a half-saturation constant (K_G). This empirical application of a Michaelis-Menten like description ignores the multiple underlying feedbacks and trade-offs between physiology and the physical environment. By applying mechanistic trait-based scaling of nutrient uptake, considering growth, cell size, elemental stoichiometry, cell motion and diffusion gradients the results identify fundamental shortcomings in the interpretation of empirically derived relationships used to describe nutrient uptake in microbes. The ability of cells to modulate the potential transport rate per unit plasma-membrane area (the Transporter Rate Density, TRD) according to their nutritional status, and hence change the instantaneous maximum transport rate, has a very marked effect upon transport and growth kinetics. For example, for a given carbon density, cell size and TRD, the nutrient affinity (here defined as G_max/K_G) can remain constant. This implies that species or strains with a lower G_max might coincidentally have a competitive advantage under nutrient limited conditions as they also express lower values of K_G. Our results highlight the importance of accounting for organism physiology and related feedbacks in ecological applications and climate change studies.