Inherent and apparent traits in microbial nutrient uptake

Abstract

Nearly 50 yr ago, the Michaelis-Menten (MM) model, originally derived for enzyme kinetics, was adapted to characterize microbial nutrient uptake and has become a framework for defining microbial traits in competition theory, evolutionary dynamics, and ocean ecosystem models. We provide theoretical evidence that microbial traits and environmental properties are not appropriately distinguished in current ecological modeling that makes use of MM models, and we propose a framework where inherent microbial traits are explicitly distinguished from environmental variables. This provides novel expectations on how nutrient uptake is affected by cell size, porter density, temperature, and nutrient regimes, and we show that uptake kinetics and tradeoffs likely differ between oligotrophic and eutrophic regimes. We present mechanistic expressions for the affinity and the half-saturation (K) coefficients, and our results suggest that K might behave opposite to that commonly assumed in ecological modeling and should be abandoned as an index of uptake efficiency. Our results further suggest that the effect of organism size, considered a master trait in modeling, on specific uptake and growth rates is effectively modified by porter density, although differently in oligotrophic and eutrophic regimes. While nutrient uptake studies have commonly been carried out at a bulk scale, which is much larger than that experienced by the organisms, our study emphasize the need for observations at the scale of the individual.