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
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.