One of the biggest challenges in microbial ecology is determining how responsive microbial communities are to altered environments. Most studies examine microbial communities as constrained only by their habitat, but are not simultaneously examining the impacts of environment and newly introduced microbial communities. The intermingling of previously distinct communities, along with respective environments, termed ‘community coalescence’, occurs frequently with microbes, yet little is known about how assembly mechanisms regulate these novel communities. Consequently, the goal of this project was to understand how historically distinct microbial communities interact in the context of seawater intrusion. Seawater movement into relatively eutrophic freshwater occurs periodically in North Carolina coastal wetlands, and is forecasted to increase. Aquatic systems are ideal for examining microbial community coalescence: feasible isolation of microbial communities from their environment allows for full reciprocal transplanting of communities into new environments. We implemented a one week laboratory incubation by pairing pure and mixed microbial inocula and sterilized environments from both fresh- and seawater into a full factorial experiment to simulate coalescence under controlled conditions. Our experimental design enabled the parsing of the key assembly mechanisms of community coalescence: environmental filtering and competition.
Using 16S rRNA-based gene sequencing to determine microbial community structure, we found increased diversity among coalescing communities relative to starting communities. The mixed community was created by combining microbes from each environment, so the full genetic diversity was present for species sorting during the merging. Equally mixed communities exposed to freshwater conditions closely resembled the structure of intact freshwater inoculum, and vice versa when mixed communities were exposed to seawater. However, under mixed environments (i.e. true coalescent conditions), the merged communities all converged on that of the seawater community structure. Seawater therefore imposed a stronger environmental filter on structuring of microbial communities. Environmental filtering was a stronger driver of assembly, and coalescent-sensitive microbial clades were phylogenetically anti-correlated between sea- and freshwater microbes. In contrast, the clades that survived both environmental filtering and novel competition in the coalescent communities exhibited phylogenetic overlap among originally fresh- and seawater taxa. These results demonstrate that seawater intrusion may drive the merged microbial community towards a seawater-like community, indicating that environmental filtering is the stronger assembly mechanism on coalescing microbial communities. Our study enhances our understanding of how communities merge, during a time when the environment is undergoing rapid change and more frequent species mixing.