Award: OCE-1030518

The single-celled cyanobacterium Prochlorococcus is the most abundant photosynthetic organism in the open ocean, and as such contributes significantly to the carbon cycle and global climate. However, the influence of seasonal changes on the populations of these important microbes are not well understood, particularly at the high latitudes. In this work we examined how the population size and composition changes from winter to summer in the North Pacific Ocean, and sought to identify environmental parameters (e.g. temperature, water column mixing, virus attack) that might be involved in these changes. We found that in the winter, the abundances of the major genetic types followed that observed in our prior studies in the Atlantic and the South Pacific. That is, one lineage dominated the warmer, lower latitudes, and another lineage dominated the colder, higher latitudes. The transition of dominance in these studies was found at approximately 19C. Surprisingly, however, while the overall trends in terms of zones of dominance were also found in the summer, the transition point was not at 19C, but much warmer. This was a consequence of an absence of northern expansion of the two genetic types: while the temperatures increased in the summer, the populations did not expand into these warmer regions, but rather appeared to maintain the structure we observed in the winter. This result indicates that the population structure, while influenced by temperature to some extent, is likely being influenced by environmental or physiological factors that we do not yet fully appreciate. A more complete understanding of the factors that together with temperature regulate the growth and function of these important photosynthetic microbes will be important as we consider the impacts of climate change on the global processes of photosynthesis and carbon cycling. As part of this study we were also able to resolve that a main predator of marine microbes – viruses – are also influenced in their activity by temperature. This observation has significant implications for researchers that study climate change, as influences of temperature on predation rates have not been well studied. This project supported the training of several graduate students and undergraduates. It also supported hands-on oceanographic experiences for a science journalism student and a high school teacher, both of which participated in the three-week summer cruise. Last Modified: 01/08/2016 Submitted by: Erik Zinser