Ecological Controls on Prochlorococcus sp. Diversity, Composition, and Activity at High Taxonomic Resolution

<p>Although there are many examples of microbial biogeography, few microbes have been studied at high taxonomic resolution over large spatial scales. As a result, the environmental and ecological processes that drive niche partitioning, diversity, composition, and activity of microbial taxa are often poorly understood. To address this gap, I examine the most abundant phytoplankton in the global ocean, Prochlorococcus sp., a marine cyanobacterium. Using amplicon libraries of the Prochlorococcus internal transcribed spacer (ITS) region and 23S rRNA gene as markers, I demonstrate several key differences between the two major high light (HL) clades of Prochlorococcus. First, by examining ITS amplicon libraries at high taxonomic resolution it is revealed that “sub-ecotype” clades have unique, cohesive responses to environmental variables and distinct biogeographies, suggesting that presently defined ecotypes can be further partitioned into ecologically meaningful units. Whereas unique combinations of environmental traits drive the distribution of the HL-I sub-ecotype clades, the HL-II sub-ecotype clades appear ecologically coherent. Second, using 23S rRNA and rDNA libraries I show that activity (rRNA) and abundance (rDNA) are highly correlated for Prochlorococcus across all sites and operational taxonomic units (OTUs) in the surface ocean, demonstrating a tight coupling between activity and abundance. Finally, I investigate the associations between Prochlorococcus and the rest of the microbial community in the North Pacific and find region-specific trends in both strength and sign. Associations with other microbes are strongest for HL-I in the temperate region and strongest for HL-II in the sub-tropical gyre. This dissertation clarifies the relative importance of the environment, geography, community, and taxonomy in terms of their role in creating complex assemblages of Prochlorococcus and helps improve our understanding of how marine microbial communities are assembled in situ.</p>Dissertatio