Microorganisms encompass most of the diversity on Earth, and are responsible for maintaining the function of various ecosystems, such as in the guts of animals, in soil, and in water. This research is focused on understanding processes that control where microbes live and how they respond to changes in local conditions. A tree exists in a given location if it can survive both the environmental conditions and interactions with its neighbors at that location, and its seedlings may disperse to new locations with new environmental conditions. Unlike trees, microbes frequently migrate as whole communities and in tandem with their environment, often merging with other microbial communities along with their respective environments. For instance, wind movement of soil particles across the landscape or human handshakes result in novel contact of previously separated microbial communities. This process is termed community coalescence. The confluence of two water bodies is a clear example of community coalescence, resulting in a new, united environment with novel interactions among the previously separate microbial communities. This research will experimentally identify how aquatic microbial communities interact and respond to the merger under different environmental conditions. <br/><br/>The goal of this project is to understand how microbial communities from distinct habitats interact under community coalescence, with the following specific objectives: (1) Characterize the membership of microbial communities in the field and their activity at three distinct aquatic environments where coalescence occurs. (2) In the laboratory, track microbial community membership and activity in response to experimentally-imposed coalescence. (3) Analyze experimental data in the context of the patterns observed in the field. Unraveling mechanisms that determine community membership following a coalescence event is difficult, since the abiotic and biotic factors are intertwined. In order to understand the true community ecology of microbes, it is necessary to separate the environmental and biological factors that ultimately determine which organisms persist and which do not. To address these fundamental ecological questions, the research team developed a new approach to directly measure the effects of environmental factors and biotic interactions, independently and in combination, on coalesced communities. This project addresses fundamental concepts in community ecology, dispersal of a massive number of microorganisms in the context of aquatic systems, and directly disentangles biotic and abiotic factors.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.