Abstract
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 (Gmax) and a half-saturation constant (KG). This empirical application of a Michaelis-Menten like description ignores the multiple underlying feedbacks between physiology contributing to growth, cell size, elemental stoichiometry and cell motion. Here we explore these relationships with reference to the kinetics of the nutrient transporter protein, the transporter rate density at the cell surface (TRD; potential transport rate per unit plasma-membrane area), and diffusion gradients. While the half saturation value for the limiting nutrient increases rapidly with cell size, significant mitigation is afforded by cell motion (swimming or sedimentation), and by decreasing the cellular carbon density. There is thus potential for high vacuolation and high sedimentation rates in diatoms to significantly decrease KG and increase species competitive advantage. Our results also suggest that Gmax for larger non-diatom protists may be constrained by rates of nutrient transport. For a given carbon density, cell size and TRD, the value of Gmax/KG remains constant. This implies that species or strains with a lower Gmax might coincidentally have a competitive advantage under nutrient limited conditions as they also express lower values of KG. The ability of cells to modulate the TRD according to their nutritional status, and hence change the instantaneous maximum transport rate, has a very marked effect upon transport and growth kinetics. Analyses and dynamic models that do not consider such modulation will inevitably fail to properly reflect competitive advantage in nutrient acquisition. This has important implications for the accurate representation and predictive capabilities of model applications, in particular in a changing environment.
Highlights
The relationship between nutrient uptake kinetics and growth rate is seen as a critical determinate in competition for organisms reliant on the transport of dissolved nutrients, and often plays a key role in structuring marine ecosystem models [1,2,3]
We consider interactions between cell size and cellular carbon density, elemental stoichiometry, motion through the water, and growth rate potential with nutrient transport. While facets of such interactions have been considered before [3,4,5] we present a new analysis that explores how traits at the level of nutrient transport work through to better explain how nutrient availability controls organism growth and competitive advantage
These plots clearly show the competitive advantage for nutrient transport of being small, and of motion achieved through either swimming or sedimentation
Summary
The relationship between nutrient uptake kinetics and growth rate is seen as a critical determinate in competition for organisms reliant on the transport of dissolved nutrients, and often plays a key role in structuring marine ecosystem models [1,2,3]. We consider interactions between cell size and cellular carbon density (as linked to vacuolation, for example), elemental stoichiometry, motion through the water, and growth rate potential with nutrient transport While facets of such interactions have been considered before [3,4,5] we present a new analysis that explores how traits at the level of nutrient transport work through to better explain how nutrient availability controls organism growth and competitive advantage. Most obviously there is the difference between the short term relationship between nutrient (substrate) concentration at the cell surface (S0) and nutrient transport rate into the organism, and the longer term relationship between S0 and organism growth rate This difference develops because nutrient transport is controlled by various feedback processes that develop during post-transport assimilation of the nutrient, and are related to the organisms’ physiological history and thence to its growth rate. These factors affect the difference between S0 and the substrate concentration in the bulk water (S1); it is the latter which is determined in chemical analyses of water and features as a variable in models, while the former is the concentration of importance for the organism itself
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