New microbial communities often arise through the mixing of two or more separately assembled parent communities, a phenomenon that has been termed “community coalescence”. Understanding how the interaction structures of complex parent communities determine the outcomes of coalescence events is an important challenge. While recent work has begun to elucidate the role of competition in coalescence, that of cooperation, a key interaction type commonly seen in microbial communities, is still largely unknown. Here, using a general consumer-resource model, we study the combined effects of competitive and cooperative interactions on the outcomes of coalescence events. To do so, we simulate coalescence events between pairs of communities with different degrees of competition for shared carbon resources and cooperation through cross-feeding on leaked metabolic by-products (facilitation). We also study how structural and functional properties of post-coalescence communities evolve when they are subjected to repeated coalescence events. We find that in coalescence events, the less competitive and more cooperative parent communities contribute a higher proportion of species to the new community because of their superior ability to deplete resources and resist invasions. Consequently, when a community is subjected to repeated coalescence events, it gradually evolves towards being less competitive and more cooperative, as well as more speciose, robust and efficient in resource use. Encounters between microbial communities are becoming increasingly frequent as a result of anthropogenic environmental change, and there is great interest in how the coalescence of microbial communities affects environmental and human health. Our study provides new insights into the mechanisms behind microbial community coalescence, and a framework to predict outcomes based on the interaction structures of parent communities.

Microbial communities are ubiquitous in nature. Although processes driving the assembly of these consortia are not yet well understood, new communities frequently emerge when two or more microbial ensembles encounter each other and mix to yield a new functioning aggregation, termed "community coalescence". Despite recent advances in our understanding of coalescence, theoretical work has focused mainly on competition, and more work is necessary to determine role of other common microbial interactions, such as cooperation. In this work, we study the combined effects that competitive and cooperative interactions have in the outcome of coalescence events. We simulate communities with varying levels of each type of interaction using a consumer-resource model with cross-feeding on metabolic by-products. We then perform coalescence simulations and measure interaction levels on the pre- and post-coalescence communities using new metrics of competition and cooperation that we present previously. We find that when both interactions are present, the less competitive community tends to succeed in community coalescence, regardless of its cooperativity, suggesting that minimizing competition is the main driving force of this process. When competition is weak however, simulations show that highly cooperative communities are at a disadvantage in coalescence events, indicating that multi-species invasions tend to intercept cooperative links. Microbial community coalescence is gaining popularity in applied and basic research due to its multiple advantages. In the absence of theory that supports real life observations, here we develop a framework to understand and predict the result of community coalescence events from the perspective of biotic interactions present in the community.

Multistable nonequilibrium systems are abundant outcomes of nonlinear dynamics with feedback but still relatively little is known about what determines the stability of the steady states and their switching rates in terms of entropy and entropy production. Here, we will link fluctuation theorems for the entropy production along trajectories with the action obtainable from the Freidlin-Wentzell theorem to elucidate the thermodynamics of switching between states in the large volume limit of multistable systems. We find that the entropy production at steady state plays no role, but the entropy production during switching is key. Additional stabilising and destabilising effects arise from the steady-state entropy and diffusive noise, respectively. The relevance to biology, ecological, and climate models is apparent.

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