Determining the Ecological and Physiological Factors Contributing to the Competitive Success of Prochlorococcus in the Oligotrophic Ocean

Prochlorococcus is a genus of extremely successful marine cyanobacteria. This success is realized through its pervasive biogeographical range and presence in almost all open ocean environments where it usually it the dominant phytoplankton. Limited capabilities of culturing and genetic manipulation of this organism have resulted in assumptions about this success overwhelmingly based on field observations. These studies have assumed adaptations for resource uptake and utilization in nutrient limited environments to cause dominance of Prochlorococcus over other photosynthetic microbes. In an attempt to definitively explain this through laboratory culture, we developed a culturing system to assay questions of nutrient limitation effects upon Prochlorococcus and its competitive ability based on nitrogen limited populations within the North Pacific Subtropical Gyre. We determined that competition for nitrogen did explain the population dynamics of Prochlorococcus and another cyanobacterium, Synechococcus, and were able to recreate their observed abundances in nitrogen limited laboratory culture. Interestingly this outcome could only be achieved through the inclusion of a heterotrophic bacterium, which facilitated the success of Prochlorococcus through complex crossfeeding interactions. In an effort to further explore the important microbial interactions influencing this dominant cyanobacterium, we developed an additional culturing system to determine if rival phytoplankton (Synechococcus and picoeukaryotes), with whom Prochlorococcus competes for nutrients, could protect Prochlorococcus from oxidative stress from hydrogen peroxide. These rivals successfully protected Prochlorococcus from mortality when cocultured at ecologically relevant abundances. Lastly we determined the affect of seasonal change on Prochlorococcus ecotypes and potential relationships between high-light and low-light adapted ecotypes and between ecotypes and abundant microbial phyla that varied seasonally. Herein we speculate on the overall importance of microbial interactions based on nutrient transformation and environmental conditioning to the ecology and biogeography of Prochlorococcus.