Sources and Transformation of Dissolved and Particulate Organic Nitrogen in the North Pacific Subtropical Gyre Indicated by Compound-Specific Nitrogen Isotope Analysis of Amino Acids

Dissolved and particulate organic nitrogen (DON, PON) together play the roles in N cycling in the upper ocean. This study explores the use of compound-specific nitrogen isotope of amino acids (δ¹⁵N-AA) of coupled DON and PON samples as a new approach to examine relative sources, transformation processes, and potential coupling of the main detrital organic nitrogen form in the ocean water column. We measured δ¹⁵N-AA distributions in high-molecular-weight dissolved organic nitrogen (HMW DON) and suspended PON in the North Pacific Subtropical Gyre (NPSG) from surface to mesopelagic depths. A new analytical approach achieved far greater δ¹⁵N-AA measurement precision for DON than earlier work, allowing us to resolve previously obscured differences in δ¹⁵N-AA signatures, both with depth and between ON pools.

First, the δ¹⁵N-AA signatures of both surface and mesopelagic HMW DON suggest mainly heterotrophic bacterial sources, with the mesopelagic HMW DON bearing signatures of far more degraded material. These results contrast with a previous proposal that DON δ¹⁵N-AA patterns are essentially “pre-formed” in the surface ocean, undergoing little further change with depth. Second, different δ¹⁵N-AA values and patterns of HMW DON vs. suspended PON in the surface NPSG suggest that their sources and cycling are largely decoupled. We propose a new idea for production of surface HMW DON, in which ON is ultimately derived from subsurface nitrate, while PON in the mixed layer is strongly linked to N₂ fixation and N recycling. In contrast, the comparative δ¹⁵N-AA values of HMW DON and suspended PON in the mesopelagic depth suggest a potentially important role of PON in the production of HMW DON at mesopelagic depths. Overall, these new δ¹⁵N-AA data have provided an unprecedented window into the sources, cycling, and likely biogeochemical linkage of DON and PON pools in the oligotrophic ocean.