Understanding microbial control of dissolved organic nitrogen (DON) in the ocean: New amino acid tracers for bacterial source and cycling of refractory DON

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This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).One of the most mysterious components of the global nitrogen cycle is the vast amount of dissolved nitrogen-containing organic molecules (dissolved organic nitrogen, or “DON”) which exist in the ocean. The identities of these molecules are mostly unknown, but they are important in the ocean’s biological and geochemical cycles, because they persist for many thousands of years and in many cases these molecules are too complex for simple organisms at the base of the food web to use. This project will develop new chemical tools to understand this material, including stable isotopes, radioisotopes, and the chirality of amino acids. Together, this new set of tracers will focus on the roles of marine bacteria as sources and creators of persistent dissolved organic nitrogen. The project will begin in the laboratory, where the team will first grow large cultures of ocean algae. Natural ocean bacteria will then be added, and investigators will measure the proposed new tracers in dissolved nitrogen materials created. These data will reveal how the proposed tracers work: how and when they are produced, if they map to bacterial sources, and how bacterial degradation may affect them. The investigators will then conduct a series of field expeditions at contrasting sites on the California coast and in the central Pacific Ocean, isolating large amounts of natural dissolved organic nitrogen. The team will apply what has been learned in the lab together with the first ever radiocarbon measurements on individual tracer molecules. Together these activities will open a new window into sources and cycling of the critical marine dissolved organic nitrogen material. The project will support the education, training, and career development of a graduate student, a postdoctoral researcher, and undergraduate students from underrepresented groups, who will be recruited to receive real hands-on laboratory and field experience. This project will develop three new amino acid–based proxies for bacterial source and alteration of dissolved organic nitrogen: 1) compound-specific stable carbon isotope fingerprinting of essential amino acids, 2) a recently discovered suite of new D-amino acids found to be concentrated in radiocarbon-old, low molecular weight DON material, and 3) individual amino acid radiocarbon values for key hypothesized tracer amino acids. The investigators hypothesize that refractory ocean DON is essentially completely microbial, however that it is also far more diverse than has previously been understood in terms of its sources, molecular composition, and especially cycling rates. In particular, the project will test the idea of completely different DON origin, molecular composition, and cycling rates in high vs. low molecular weight DON fractions throughout the water column. Autotrophic and heterotrophic bacterial growth / degradation experiments will test assumptions about the sources and degradation- related changes to D/L and stable carbon isotope signatures. Synoptically making these measurements together with individual amino acid radiocarbon values in high productivity coastal vs. oligotrophic Pacific gyre DON in both surface and deep ocean will allow understanding the diversity of DON bacterial sources, degradation impacts, and N-specific cycling rates for the first time.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.