Date of Award


Degree Type


Degree Name

Doctor of Philosophy



Major Professor

Erik R. Zinser

Committee Members

Jeff Becker, Alison Buchan, Shawn Campagna, George O'Toole


In the oligotrophic waters of the world’s open oceans physical factors such as pH, salinity, and temperature are generally stable. The nutrient limited conditions as well as the low environmental variability endemic to these ecosystems select for specialists that gain fitness advantages through minimalism, efficiency, and thrift. These physical characteristics are thought to reduce nutrient demand while allowing for constant metabolic activity and growth, but the mechanisms that promote these fitness advantages are currently unknown. To better understand how these physiologies improve selective fitness for the dominant phytoplankton, we observed metabolic parameters under environmental conditions typical to these waters. In an environment characterized by low physical variation, the cellular processes of our model organisms – the genomically streamlined phytoplankton Prochlorococcus spp. follow the trends of their ultimate energy source: the predictable daily oscillations of solar energy. Through observations on metabolic biomarkers associated with carbon, energy, and nitrogen metabolism over diel photic cycles, we sought to gain insights into the coordination of these intersecting metabolisms as a function of the time of day. We found that although cellular carbon and energy stores vary over the photic period, incorporation of ammonium into glutamate can occur throughout the diel cycle. Efficiency and thrift in nitrogen metabolisms are thought to be under strong selective pressure as inorganic nitrogen is the growth limiting nutrient for Prochlorococcus in the North Pacific Subtropical Gyre. To develop a deeper understanding of metabolic processes utilized by nitrogen limited phytoplankton, we performed laboratory and field experiments, investigating metabolisms of nitrogen limited and nutrient replete pure cultures as well as the effect of a nitrogen pulse on metabolisms of natural populations in the North Pacific. Low rates of carbon fixation and flow of fixed carbon through large metabolic pools were observed in our nitrogen limited experiments and may suggest reduced growth under limiting conditions is not caused by lack of metabolic precursors, but through the polymerization of biological macromolecules such as proteins and nucleic acids. These data provide primary observations of the metabolic balance of carbon, energy and nitrogen metabolisms in the most abundant photosynthetic organism on Earth.

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