Mutualistic cross-feeding in microbial systems generates bistability via an Allee effect

Abstract

In microbial ecosystems, species not only compete for common resources but may also display mutualistic interactions as a result from metabolic cross-feeding. Such mutualism can lead to bistability. Depending on the initial population sizes, species will either survive or go extinct. Various phenomenological models have been suggested to describe bistability in mutualistic systems. However, these models do not account for interaction mediators such as nutrients. In contrast, nutrient-explicit models do not provide an intuitive understanding of what causes bistability. Here, we reduce a theoretical nutrient-explicit model of two mutualistic cross-feeders in a chemostat, uncovering an explicit relation to a growth model with an Allee effect. We show that the dilution rate in the chemostat leads to bistability by turning a weak Allee effect into a strong Allee effect. This happens as long as there is more production than consumption of cross-fed nutrients. Thanks to the explicit relationship of the reduced model with the underlying experimental parameters, these results allow to predict the biological conditions that sustain or prevent the survival of mutualistic species.

Complex Ecotype Dynamics Evolve in Response to Fluctuating Resources

“…One major caveat to the black queen hypothesis is an innate vulnerability of mutualistic cross-feeding relationships to invasion by cheaters that benefit from the public goods without contributing ( 68 ). As such, mutualistic cross-feeding is expected to be (i) the most stable in structured environments ( 69 ), where access to the public goods can be limited; (ii) subject to Allee effects ( 70 ), where an optimal ratio of each cooperator type exists, maximizing metabolic efficiency and population size while also reducing an excess availability of public goods that may otherwise encourage the evolution of cheaters; and (iii) beneficial under resource-limited conditions ( 15 ), where cooperation can help maintain genetic variation. All of these conditions appear to be true for our experimental populations, as 10-day feast/famine cycles result in increased biofilm formation and an optimum 1:2 ratio exists between ecotypes for maximum population fitness.…”

Spawner weight and ocean temperature drive Allee effect dynamics in Atlantic cod, Gadus Morhua: inherent and emergent density regulation

“…With this study we contribute to the emerging recognition of dynamic Allee effects (Tirronen et al, 2022) that can interact with the environment (e.g. (Berec, 2019;Vet et al, 2020;Winter et al, 2020)). We find that Allee effects are common in Atlantic cod and are highly dynamic under different spawner weight and SST conditions.…”

Mathematical analysis of a competition model with mutualism

“…For instance, in [3], a discrete model of mutualism was studied with the aim of investigating the persistent property of the system. S. Vent et al in [28], have studied a model of two mutualistic species via cross-feeding in a chemostat. This model was reduced from a six-dimensional system to a two-dimensional one.…”