Date of Award


Degree Type


Degree Name

Doctor of Philosophy



Major Professor

Gary S. Sayler

Committee Members

Dr. James Drake, Dr. Jay Garland, Dr. Alice Layton, Dr. Beth Mullin


A fundamental goal of ecological research is to better understand the relationship between structural diversity, functional diversity and ecosystem stability. Insight into the mechanisms that regulate microbial ecosystem function should provide important new information to improve system control for ecological, agricultural and biotechnological applications that depend on microbial processes. For example, defined microbial consortia have been employed to improve plant growth through enhanced nutrient uptake, production of plant growth promoting hormones or control of pathogens. However, a lack of consistency of results in field application has limited the widespread use of microbial inoculants. A major goal in rhizosphere studies, therefore, has been to evaluate strategies to introduce or manipulate rhizosphere flora to improve survival of beneficial organisms or suppress deleterious ones. This is of direct importance for application to closed environmental systems employed by the National Aeronautics and Space Administration (NASA), since microbial inocula will likely be used to initiate any biological waste processing system used on long-term and deep space missions. To assure safe, effective, reliable functioning of inoculated systems, it will be important to manage and potentially manipulate microbial populations within these systems to obviate any negative interactions and to optimize the positive functions of plant associated communities.

This research focuses on the use of a model plant-based graywater waste processing system to evaluate the effect of species richness on community phenotype and community invasibility. The principal hypothesis of this research is that a community of rhizosphere organisms, constructed using parameters described in this research, will be as effective as an undefined industrial activated sludge community in terms of its ability to: establish in the rhizosphere, persist over time, resist invasion, and degrade organic compounds. To address this hypothesis, the microbial inocula of different structure were evaluated based on the ability of the populations to establish and persist in situ, resist invasion by outside species, and withstand changes in nutritional or environmental conditions. A combination of PCR-based phylogenetic analysis and quantification, community level physiological profiling (CLPP) and microarray analysis was employed to evaluate the effect of microbial community structure on community dynamics, community stability and ecosystem processes.

Differences in species richness corresponded to differences in phenotypic potential (surfactant degradation and CLPP) and differences in resistance to invasion. All of the communities were phenotypically diverse, however, only communities with higher species richness were able to degrade surfactant and to resist invasion by a competitor. The constructed community was phenotypically diverse, and even degraded surfactant to a greater extent than any of other the other treatments. However, this community was invaded to a greater extent than any of the other communities, the invader persisted in the population, and one member of the community was completely displaced upon invasion. These results suggest that species richness improved system performance by increasing community persistence, resistance to invasion and ability to degrade surfactant.

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Microbiology Commons