[B-CG07] [EE] Earth and Planetary Science Frontiers for Life and Global Environment

[BCG07-09] The microbial nitrogen pump in the ocean: Role of heterotrophic bacteria in the ocean DON pool indicated by compound-specific δ¹⁵N analysis of amino acids

*Yasuhiko T. Yamaguchi^1,2, Matthew D. McCarthy² (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.University of California, Santa Cruz)

Keywords:Dissolved Organic Matter, Marine Nitrogen Cycle, Amino Acids, Heterotrophic Bacteria, Compound-Specific Isotope Analysis, Deep Ocean

Dissolved organic nitrogen (DON) is one of the most important - but perhaps least understood - components of the modern ocean nitrogen cycle. We explored the use of compound-specific nitrogen isotope analysis of amino acids (δ¹⁵N_AA) of DON as a new approach to examine relative sources and transformation processes of the main detrital organic nitrogen form in the ocean water column. We measured δ¹⁵N_AA distributions in high-molecular-weight DON (HMW DON) and suspended particulate organic nitrogen (PON) samples from various oceanic regions including the oligotrophic open ocean (the North Pacific Subtropical Gyre) and a coastal upwelling system (the California Current System). A new analytical approach using HPLC purification of amino acids 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. The δ¹⁵N_AA values of DON in the various oceanic regions and depths (surface to mesopelagic) suggest mainly heterotrophic sources, with the mesopelagic and upwelled DON bearing signatures of far more degraded material. These results contrast with a previous proposal that DON are essentially "pre-formed" in the surface ocean, undergoing little further change with depth. Together with the results of amino acid enantiomers (D/L), these results suggest that transformation by heterotrophic bacteria is the main source for DON in the ocean.