Quantifying the importance of urea as a nitrogen source to phytoplankton in the North Pacific Ocean

The productivity of ocean regions near Hawai’i and US territories in the Pacific Ocean is controlled by the amount of essential nutrients in seawater. Nutrients are consumed by phytoplankton that form the base of the food web, including fisheries that deliver economic and nutritional benefit to the United States. The nitrogen-containing molecule urea is an example of a nutrient that is present at very low concentrations and cannot be measured confidently in open ocean waters. This project improves methods used to measure the amount and turnover of urea within surface waters. Advancing understanding of this potentially large pool of bioavailable nitrogen will provide an important step toward predicting the health and productivity of open ocean ecosystems. The project will also provide training in laboratory and oceanographic methods to a postdoctoral researcher and multiple undergraduate students. The science team will participate in research cruises funded by the NSF-supported Hawaii Ocean Timeseries (HOT) program. Numerous lines of evidence collected over several decades have implicated the urea molecule as a source of recycled nitrogen to oligotrophic gyre ecosystems, but this importance has escaped direct quantification at open ocean observatories like the Hawaii Ocean Timeseries site Station ALOHA. This project will rigorously investigate the accuracy of low-level urea analyses by optimizing and comparing several alternative methods and then conduct a series of measurements and experiments onboard Hawaii Ocean Timeseries cruises to quantify phytoplankton utilization of urea. Focusing on Prochlorococcus, the dominant phytoplankton in oligotrophic gyres, this project will determine rates of urea uptake by Prochlorococcus and estimate the extent of Prochlorococcus nitrogen deficiency by comparing growth rates and biomarkers in nitrogen-fertilized and control incubations. Completion of these aims will reconcile paradoxically large reservoirs of urea reported within nitrogen-limited oligotrophic gyres, which, if confirmed by improved analytical methods, may represent an unrecognized limit to the efficiency of the biological pump in oligotrophic gyre ecosystems. 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. NSF Award ID: 2546978 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Nicholas Hawco | Institution: University of Hawaii, HONOLULU, HI | Award Amount: $661,926 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2546978 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2546978.html