Investigating Microbial Community Ecology Using Synthetic Bacterial Communities

The functions of microbial communities are crucial to sustaining life on Earth and have a pervasive impact on human society. Thus, there is great interest in developing a nuanced understanding of the processes that control microbial communities, with the hope that we can apply such knowledge to manipulate and potentially engineer microbial communities to carry out specific functions. However, a complex set of interacting biotic and abiotic factors influence the structure and function of microbial communities. The rise of 'omics technologies has offered excellent tools to obtain observational data from microbial communities and generate hypotheses regarding the ecology of these systems. However, experimental approaches are required to evaluate such hypotheses and ultimately unravel the complexity of microbial communities. Artificially constructed communities of bacteria, referred to as "synthetic communities", offer a powerful approach with which we can investigate ecological hypotheses in a controlled environment.Here, I present work using synthetic bacterial communities to study interspecific interactions, coexistence, and ecological invasion. In the first chapter, I evaluated the assumption that the interaction between two members of a community is unaffected by the surrounding community context and found that changes in community richness and density were strong predictors of how interaction effects varied across contexts. In the second chapter, I decomposed a set of bacterial isolates into all pairwise and n-2 communities to compare coexistence between these "bottom-up" and "top-down" contexts and found that pairwise observations of coexistence and exclusion were useful but incomplete predictors of the composition of complex assemblages. In the third chapter, I investigated how the timing of an ecological invasion affected the success of the invader and the impact on the resident community. I found evidence that the effect of timing on invasion outcome was associated with changes in resource use efficiency over the course of community assembly.