Microbes frequently migrate en masse as whole communities and in tandem with their environment, often merging with other microbial communities along with their respective environments in a process called ‘community coalescence’. Aquatic confluences represent clear examples of community coalescence, where two initially separated water bodies merge along with their microbiomes. To survive these merges, it is assumed that individual success is primarily constrained by environmental filtering but key associations between microorganisms (e.g. syntrophy and symbiosis) could be main drivers of these novel assemblages. The retention of a given community after coalescence may be due to higher initial connectivity among its microorganisms, or ‘community cohesion’, which may be an underappreciated driver of microbiome structure. The fundamental goal here was to explore the outcomes of aquatic community coalescence and the role of environmental filtering and biotic interactions, including cohesion, in determining how microbial communities originating from distinct aquatic habitats assemble. We tested this along a salinity/nutrient gradient where seawater intrusion of freshwater wetlands seasonally occurs. We are also extending this to two other aquatic environmental gradients (nutrients and temperature), where we characterized the community and chemistry of each gradient and conducted experimentally induced community coalescence.
Results/Conclusions
After experimentally-implemented seawater intrusion, we found clear convergence of both community structure and microbial function to that of the seawater microbiome. This was despite a potential advantage of the freshwater microbiome, characterized by substantially higher initial biomass. Overall, this coalescence scenario led to a substantial decrease in both alpha diversity and enzymatic activity. Our study demonstrates how environmental filtering plays a pivotal role in community coalescence, while novel biotic interactions and mass effects may have limited influence on structuring the resultant microbial community. These results document a strong asymmetric community response to environmental mixing and provides a model for future study of microbial community coalescence. Using phylofactorization, we identified subsets of clades from each starting microbiome that consistently occurred in tandem after the community coalescence, due either to species sorting or cohesion. Therefore, by measuring the degree of community cohesion, we can differentiate between true species sorting at the individual level and tight microbial associations operating at the community-scale. This study presents novel insights on the assembly mechanisms and on the fundamental trade-off between community cohesion and stability during massive microbial dispersal events.