Synopsis: Microbes Share, But Not Too Much

Microbial cooperation involves the diffusion of nutrients from one species to another, but too high a diffusion rate weakens the cooperative bond.
Synopsis figure
R. Menon and K. Korolev, Phys. Rev. Lett. (2015)

Microbes of different species often share vital biomolecules with each other. These nutrients diffuse out into the environment, so that the cooperation, or “mutualism,” occurs over a wide spatial range. A new model characterizes the effect of this diffusion. Surprisingly, a large diffusion rate weakens the mutualism and can lead to the extinction of one of the species.

In microbial mutualism, one microbe species releases, for example, an amino acid that diffuses out and is later consumed by another microbe, which returns the favor by sharing a different nutrient. Biologists can model this relationship using so-called game theory, in which organisms follow certain strategies to improve their evolutionary survival. However, these models have not so far addressed the role played by diffusion.

In their new model, Rajita Menon and Kirill Korolev from Boston University, Massachusetts, considered a one-dimensional system in which two microbe species each shared a nutrient with multiple subcolonies. The researchers derived an analytical expression for the survival of each species. For a small diffusion rate, the two species intermix, and cooperation remains a stable trait over multiple generations. But as the diffusion rate increases, mixing decreases because the organisms no longer need to be near each other to receive the shared nutrients. If, however, the diffusion rate is different for two shared nutrients, then the species with the slower-diffusing molecule dominates the system, forcing the other species towards extinction. Menon and Korolev claim that their model may explain why some shared molecules have chemical units that appear to have no function except to slow down their diffusion.

This research is published in Physical Review Letters.

–Michael Schirber


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