The goal of this project is to study mechanisms that help maintain cooperative iron acquisition in bacterial communities. Cooperative iron acquisition is crucial to the survival of many microbial communities, but it is vulnerable to exploitation by social cheaters that reap the benefits of iron acquisition without paying the cost. Cooperative iron acquisition is widespread in microbial communities, including communities that are important for soil ecology, agriculture, and medicine. This work will provide important information about the conditions and circumstances that stabilize such communities and general information about how more complex communities are stabilized. This project will contribute to the training and career development of students at the graduate, undergraduate and high school levels.
Much has been learned about the molecular mechanisms of microbial cooperative behaviors such as biofilm formation, virulence, and nutrient acquisition. However, only recently have microbiologists also begun to investigate these behaviors from the perspective of social evolution. The focus of this project is on the evolutionary stability of cooperative iron acquisition in pseudomonads. Several Pseudomonas species acquire iron by secreting pyoverdines, a structurally diverse class of non-ribosomal peptide siderophores that are internalized by cognate receptors. The evolutionary stability of pyoverdine cooperation is challenged by intraspecific cheating between P. aeruginosa pyoverdine producers and non-producers, and by interspecific cheating between two Pseudomonas species, P. aeruginosa and P. protegens, in which the latter recognizes the pyoverdine produced by the former but not vice versa. Both cases involve the potential exploitation of cooperative pyoverdine production by cells that reap the benefits without paying the costs. Based on preliminary experimental and modeling data, it is hypothesized that bacterial growth physiology, specifically the nature of the growth-rate limiting nutrient, and asymmetric competition, specifically pyoverdine receptor affinity, are two important factors that determine the costs and benefits of cheating. These hypotheses will be tested with a combination of modeling and experimental work. Modeling will involve whole-genome metabolic models and dynamic population growth models. Experimental work will involve batch and chemostat growth of single and co-cultures under defined nutrient conditions, as well as measurements of pyoverdine receptor binding affinities and expression levels. These studies will provide insights into the mechanisms that contribute to the evolutionary stability of cooperative iron acquisition. Cooperative iron acquisition and other microbial social behaviors are of great relevance to natural ecosystems and have implications for the evolution of sociality in general. Graduate, Undergraduate and high school students will be trained in a wide range of skills in microbial genetics, physiology, biochemistry, evolutionary biology, and mathematical modeling. High school students will be hosted through the Apprenticeship for Science and Engineering, a summer internship program at Oregon State University.
