The role of competition versus cooperation in microbial community coalescence

Microbial communities are ubiquitous in nature, and rapid advances have been made recently in our understanding of how they assemble and persist. A relatively poorly understood phenomenon is that of "community coalescence", where a new microbial community arises through the encounter and mixing of two or more separately assembled communities. In particular, while recent theoretical work has begun to elucidate the role of competition in coalescence, that of the other key interaction type commonly seen in microbial communities, cooperation, remains unclear. Here, we study the combined effects of competitive and cooperative interactions on the outcome of coalescence events. We simulate communities with varying levels of each type of interaction using a general consumer-resource model where microbial populations can both compete, and cooperate through cross-feeding on metabolic by-products. We perform extensive coalescence simulations of pairs of pre-coalescence communities lying on the spectrum from competition to cooperation, to gain general insights into their relative roles in determining coalescence success. We find that when both interactions are present in two parent communities, the less competitive one tends to dominate during coalescence, regardless of its cooperativity, suggesting that minimizing competition is the main driving force of this process. When competition in parent communities is weak however, counter-intuitively, more cooperative ones are at a disadvantage in coalescence events because multi-species invasions tend to disrupt established cooperative links. Our results provide new, empirically-relevant predictions about key factors determining microbial community coalescence, and a framework to make new predictions about coalescence based on the interaction structures of coalescing communities.