Nitrogen (N2) fixation is an important component of the nitrogen cycle, supports primary production by supplying biologically available nitrogen, particularly in oligotrophic regions of the world’s oceans where nitrogen availability often limits growth. This critical process is mediated by diazotrophs, a diverse group of organisms including both Bacteria and Archaea.

This dissertation focuses on the distribution, activity, and physiology of diazotrophs. It places a particular emphasis on the globally distributed UCYN-A/haptophyte symbiosis, which has been observed in a wide range of environments from the oligotrophic open ocean to coastal regions and colder high-latitude waters. The UCYN-A symbiosis is comprised of the N2-fixing cyanobacterial symbiont, UCYN-A, and a photosynthetic alga (haptophyte) host, Braarudosphaera bigelowii. In the first chapter, the autotrophic and phagotrophic modes of nutrition of the UCYN-A/B. bigelowii symbiosis were investigated. The second chapter investigates the range of temperatures in which the UCYN-A/B. bigelowii symbiosis can grow and fix carbon and N, with a particular focus on lower temperatures representative of high-latitude waters. Chapter three reports on a study conducted during a winter cruise through the undersampled North Pacific subtropical gyre (NPSG), specifically in the often-overlooked NPSG, which analyzes the environmental determinants influencing diazotroph distributions and abundances along a transect from Guam to the west coast of California.

In summary, this dissertation illuminates aspects of the physiology of the UCYN-A/B. bigelowii symbiosis, specifically its mixotrophic survival strategy and temperature responses. It also provides valuable insights into the biogeographic distributions of a variety of diazotrophs across an undersampled region of the vast Pacific Ocean. Taken together, this dissertation provides new information on the function and role of diazotrophs in open ocean N cycling.