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Abstract
Labile organic matter released by phytoplankton in the surface ocean supports diverse communities of heterotrophic bacteria. Interactions between bacteria within the region directly surrounding a phytoplankton known as the phycosphere are largely mediated by metabolites they consume and those they excrete. This dissertation addresses how bacterial communities assemble and interact in ecological hot-spots such as phycospheres. To untangle the complexity of interactions between marine heterotrophic bacteria, we used novel fitness and chemotaxis assays featuring organic matter either provided by living co-cultured phytoplankton, or as single metabolites previously identified to be components of phytoplankton exometabolomes. In the first and second study, we measured the fitness of genes advantageous to the success of focal bacterium Ruegeria pomeroyi when sharing resources in phycospheres generated by the diatom Thalassiosira pseudonana. Bacteria were found to compete for phytoplankton metabolites and inorganic nutrients. We found that flexibility of a bacterium to utilize a wide variety of available metabolites not only supports coexistence but allows for plasticity as the landscape of competition changes with varying bacterial membership. In addition to competition, bacteria exchanged metabolites at concentrations sufficient to alleviate auxotrophies. Finally, phycosphere bacteria were found to alter the physiochemical properties of their local environment through respiration and detoxification of reactive oxygen species. In the third study, we determined the roles of metabolites as signals directing bacterial foraging strategies, and as substrates enabling growth after arrival at an ecological hot-spot. We quantitatively separated the effects that these roles play in community assembly using pairs of bacterial strains competing in a model hotspot environment. We found that a single metabolite can vary in its role as a signal or substrate for a bacterial strain, and that each bacterial strain can respond differently to the same metabolite, underscoring the complex nature of microbial foraging strategies and interactions in simple communities. Collectively, these studies elucidated interactions driving the assembly of bacterial communities within ecological hot-spots in the surface ocean.