Postdoctoral Fellowship: OCE-PRF: Isolation and Assessment of Synechococcus with Novel nitrate Assimilation Loss Phenotype

The bacterial phytoplankton Synechococcus is a key driver of the carbon cycle, and the carbon they produce can be removed from the atmosphere and exported to the deep ocean, so it is important to understand their role as we seek to limit the impacts of climate change. Natural limitations in the resources necessary for phytoplankton growth, such as the micronutrient iron, can limit the amount of carbon removed from the atmosphere by these organisms. Likely as an adaptation to iron limitation, the newly discovered Synechococcus in the iron-limited northeast subarctic Pacific (NESAP) has lost the genes for converting nitrate into ammonium, which is needed for protein and DNA synthesis. These are the first members of this group known to have lost this pathway, and it effectively restricts these strains into a “recycling” state, in which carbon can only be fixed along with ammonium that is released from organic matter degradation and carbon emission, resulting in no net gain of fixed carbon. Recent global marine genetic data analysis suggests this adaptation by Synechococcus is occurring in all major iron-limited ocean regions, and because climate change will alter iron availability in the ocean, it is critical to understand the impact this adaptation will have on the marine carbon cycle. This project aims to study this nitrate utilization loss adaptation by culturing these new NESAP strains, studying their physiology, and using genetic techniques across multiple sites in the NESAP to determine their range and abundance in this part of the ocean. This project will support the training of a postdoctoral scholar, train two undergraduate STEM students, and include public scientific outreach via the North Carolina Poetry Society. This project’s goals are: (1) collect the NESAP nitrate-utilization loss Synechococcus ecotype, (2) determine the seasonal and spatial dynamics of different Synechococcus nitrogen-utilization ecotypes in the NESAP, and (3) calculate and compare the metabolic states of the nitrate-utilization gene loss ecotype to that of other Synechococcus nitrogen-utilization ecotypes. Field work will be done in collaboration with the Canadian Line P program, in which seawater will be collected from each Line P station and incubated under various nutrient regimes (high vs low iron, and nitrate, nitrite, or ammonium as the nitrogen source) in order to cultivate the new Synechococcus ecotypes. DNA samples will be collected seasonally at each Line P Station, which transition from nitrogen limited coastal stations to increasingly iron limited open ocean stations, and metagenomes generated from these samples will be used to determine the range of the nutrient utilization gene loss phenotype and temporal population dynamics of each ecotype due to seasonal nutrient fluxes. This data can then be extrapolated to map out the range of other strains exhibiting this phenotype in other iron-limited regions. Finally, Synechococcus strains collected from Line P will be compared to Synechococcus reference strains with complete and partial nitrate utilization pathways under replete and limiting iron conditions, with growth rates, proteomic profiles, and iron quotas per cell being measured in each treatment. Overall, this project’s results will provide new Synechococcus isolates for study by the oceanographic science community, expand our understanding of carbon cycling in iron-limited ocean regions, and contribute vital data to inform climate change marine carbon cycle models. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.