Insights into the Biotic Factors Driving the Outcome of Coalescence Events Between Soil bacterial Communities

Coalescence events, which consist in the mixing of previously separated communities, are frequent in nature or as a result of human activities. Despite recently gaining attention as a tool to test ecological theories and engineer microbial communities, little is known about the factors that influence the outcome of such coalescence events. Here, we evaluated the relative importance of three community properties - namely, diversity, composition, and density - in determining coalescence outcome and biotic interactions among members of the coalescing bacterial communities. We combined enrichment and dilution approaches to successfully manipulate the diversity, composition, and density of a soil bacterial community. Subsequently, we mixed the manipulated and the native soil bacterial communities into microcosms containing their native, albeit sterilized, soil. We found that manipulation of the density and composition resulted in the largest shifts in the structure of the resulting coalesced communities, explaining 24.7% and 6.8% of the variance in the beta-diversity of the coalesced communities, respectively. Coalescence events impacted up to 35% of the dominant OTUs in the native community, with a predominance of negative effects. Thus, the number of OTUs exhibiting significant decreases in relative abundances was more than twice as high as those exhibiting increased relative abundances. Our results also revealed that community density had the greatest explanatory power for the variance in the relative abundance of the OTUs negatively affected by coalescence events. In particular, all significantly affected OTUs that belonged to the Bacillales exhibited a decrease in relative abundance in several of the coalesced communities, which was related to the density of some members of the AlphaProteobacteria and GammaProteobacteria in the manipulated community suspensions. Overall, our data indicate that community density and composition were the main properties determining the outcome of coalescence events. We also showed that coalescence experiments can offer insights into multi-species interactions in complex environments, which is invaluable for eventually engineering complex microbial communities for our benefit.