Project: Amino Acid Molecular-Level Stable Isotopic and Enantiomeric Ratios: A New Approach for Understanding Source and Transformation of Organic Nitrogen in the Sea.

Particulate and dissolved organic nitrogenous material (PON & DON) represent the major active pools of reduced organic N in the ocean. Sinking PON and advected DON from the surface into the ocean's interior represent the main pathways for export of new production, nitrogen flux and remineralization, as well as long-term organic nitrogen storage as DON in the deep sea. The chemical identity of organic nitrogen is key to understanding its sources, roles in ocean food webs, and the biogeochemical mechanisms that regulate its cycles. However, the large majority of PON in the deep ocean, and DON at all depths, cannot be identified at the molecular level. Accumulating evidence now indicates that this material is composed of largely unaltered biomolecules (amide N functions), and is likely dominated in most reservoirs by amino acids (AA). While hydrolyzable AA composition has long been a powerful tool for investigating diagenetic transformations, traditional AA measurements have intrinsic limitations for differentiating specific sources and transformations of ON in the ocean's water column.

In this research, two PIs from University of California Santa Cruz will develop a new set of molecular-level tools based on the recognition of *15N and *13C AA stable isotopic patterns that potentially record both a metabolic signature of their synthetic origin, as well as diagnostic signatures of subsequent heterotrophic transformations. They hypothesize that, together with enantiomeric (D/L) ratios, *15N and *13C AA signatures can be used to determine both ultimate source and heterotrophic processing of organic N at a level of specificity and detail that has not previously been possible. To test these hypotheses, they will conduct a set of lab experiments using multiple prokaryotic and eukaryotic algae in a linked series of feeding experiments with bacterial, protist, and macrozooplankton transformations. Upon completion of this proposal, ocean scientists will be in an excellent position to extend these methods from the laboratory to field studies with confidence.

Among its broader impacts, this proposal will have far-reaching implications for our basic understanding of how organic nitrogen cycling works in the ocean. This in turn would have direct and indirect impacts on our understanding of carbon cycling, as well as how other researchers parameterize regional and global biogeochemical models. Undergraduate, graduate and post-doctoral education will be furthered through active participation in laboratory and data synthesis activities. Two graduate students will be provided a unique educational background in molecular level isotopic tools and biogeochemistry. Undergraduate research will also be involved throughout the project.