Evolution across environmental nutrient concentrations: How microbes evolve and maintain low half-saturation for growth rate

Microbes grow faster when they have more of a limiting nutrient. The relationship between nutrient concentration and population growth rate is critical to predict how members of microbial communities respond to environmental perturbations. Models of this response, such as the widely used Monod model, are generally characterized by a maximum growth rate and a half-saturation concentration of the resource. What values should we expect for these half-saturation concentrations, and how should they depend on the resource concentration in nature?

We survey growth response data across a wide range of resources and microbial taxa, including many phytoplankton. We find that the half-saturation concentrations vary across orders of magnitude, even for the same organism and resource. To explain this variation, we develop an evolutionary model to show that demographic fluctuations (genetic drift) can constrain the adaptation of half-saturation concentrations. We find that this effect fundamentally differs depending on the type of population dynamics: Populations undergoing periodic bottlenecks of fixed size will adapt their half-saturation concentrations in proportion to the environmental resource concentrations, but populations undergoing periodic dilutions of fixed size will evolve half-saturation concentrations that are largely decoupled from the environmental concentrations.

Our model provides testable predictions on how the ability for low concentration growth is distributed across microbial habitats and along resource gradients, but it also reveals how an evolved half-saturation concentration may not reflect the organism"s environment. In particular, this explains how taxa in resource-rich environments can still evolve fast growth at low resource concentrations.