Microbial communities impact the biological and chemical processes occurring in nearly every habitat on earth and are increasingly being called upon to solve some of the most pressing problems facing our society. These communities are often tremendously diverse, with a single gram of soil estimated to contain thousands to millions of different microbial species. These levels of diversity appear to conflict with the predictions of the competitive exclusion principle, which states that two species that utilize the same resource cannot coexist in a constant environment. This raises two of the most challenging questions in microbial ecology. First, how can so many microbial species coexist in the environment? In other words, what maintains these levels of diversity in the face of competitive exclusion? Second, how are all these species assembled within a community? Thus far, we lack general principles that can be used to address these types of questions.Cross-feeding is a widely observed assembly phenomenon that, at the surface, appears to conflict with the competitive exclusion principle. Cross-feeding occurs when the complete degradation of a single energy-yielding substrate is partitioned between two or more microorganisms, where one microorganism partially degrades a substrate to intermediates and other microorganisms then further degrade the intermediates. Why would a microorganism only partially degrade a substrate? Why don't completely degrading microorganisms always outcompete cross-feeding microorganisms? Under what biological and environmental conditions is cross-feeding a likely evolutionary outcome? These questions have yet to be fully answered. The objective of this research is to understand why cross-feeding sometimes emerges in microbial communities. To address these questions, this work proposes to investigate cross-feeding using respiratory denitrification in the bacterium Pseudomonas stutzeri. Mutants of P. stutzeri with different denitrification capabilities will be constructed, assembled into cross-feeding consortia, and experimentally investigated to accomplish the following specific goals. o Derive and experimentally test general principles that predict when cross-feeding is likely to emerge from a single population of complete denitrifiers. o Test if coevolutionary changes occur during the evolution of syntrophic cross-feeding microorganisms and determine if coevolutionary changes act as barriers to gene flow. Achieving these two goals will help us to better understand how microorganisms are assembled within communities and how diversity is maintained in the face of competitive exclusion.