Sources and transformation of dissolved and particulate organic nitrogen in the North Pacific Subtropical Gyre indicated by compound-specific delta N-15 analysis of amino acids

This study explores the use of compound-specific nitrogen isotopes of amino acids (delta N-15(AA)) of coupled dissolved and particulate organic nitrogen (DON, PON) samples as a new approach to examine relative sources, transformation processes, and the potential coupling of these two major forms of N cycle in the ocean water column. We measured delta N-15(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 delta N-15(AA) measurement precision for DON than earlier work, allowing us to resolve previously obscured differences in delta N-15(AA) signatures, both with depth and between ON pools. We propose that delta N-15 values of total hydrolysable amino acids (THAA) represents a proxy for proteinaceous ON delta N-15 values in DON and PON. Together with bulk delta N-15 values, this allows delta N-15 values and changes in bulk, proteinaceous, and other-N to be directly evaluated. These novel measurements suggest three main conclusions. First, the delta N-15(AA) signatures of both surface and mesopelagic HMW DON suggest mainly heterotrophic bacterial sources, with mesopelagic HMW DON bearing signatures of far more degraded material compared to surface material. These results contrast with a previous proposal that HMW DON delta N-15(AA) patterns are essentially pre-formed by cyanobacteria in the surface ocean, undergo little change with depth. Second, different delta N-15(AA) values and patterns of HMW DON vs. suspended PON in the surface NPSG suggest that sources and cycling of these two N reservoirs are surpisingly decoupled. Based on molecular delta N-15 signatures, we propose a new hypothesis that production of surface HMW DON is ultimately derived from subsurface nitrate, while PON in the mixed layer is strongly linked to N-2 fixation and N recycling. In contrast, the comparative delta N-15(AA) signatures of HMW DON vs. suspended PON in the mesopelagic also suggest a possible PON source for some HMW DON in the mid-water column. Together, these results suggest that conversion of relatively labile ON to less labile DON by heterotrophic bacteria (a microbial nitrogen pump) may be the key pathway for production and alteration of DON in both the surface and the mesopelagic oligotrophic ocean. Finally, in contrast to THAA, delta N-15 values of the other-N were substantially less affected by heterotrophic alteration, which may be consistent with a larger than expected contribution of amino sugars, or other less labile nitrogenous organic molecules. (C) 2017 Elsevier Ltd. All rights reserved.