Compound specific δ15N analysis of amino acids reveals unique sources and differential cycling of high and low molecular weight marine dissolved organic nitrogen

Abstract

Marine dissolved organic nitrogen is the largest pool of reduced nitrogen (N) in the ocean. Yet, despite indications of a bacterial origin, few N-specific approaches have been developed to trace individual microbial N production and degradation mechanisms. Stable nitrogen isotope ratios (δ15N) are one of the most powerful tools to understand sources and cycling of organic N. However, δ15N measurements of total DON can only be made in the surface ocean and interpretations are complicated by the many factors influencing bulk δ15N values. Compound-specific isotope analysis of amino acids (CSI-AA) on isolated DON fractions represents a new approach for DON study throughout the water column with significantly greater information potential. Here, we compare for the first time δ15N amino acid (δ15N-AA) data on baseline inorganic N source and microbial loop processing in coupled ultrafiltered high molecular weight (HMW) and solid-phase extracted low molecular weight (LMW SPE-) DON from the surface to 2500 m deep in the Atlantic and Pacific Subtropical Gyres.δ15N-AA patterns were very different in HMW versus LMW SPE-DON, indicating unique formation and bacterial alteration mechanisms. HMW DON data supported a prior hypothesis that AA in surface HMW DON originate from heterotrophic bacterial production near the nitracline, but also suggests protozoan heterotrophy as an additional major process. In the mesopelagic, distinct HMW DON δ15N-AA patterns suggest heterotrophic bacterial resynthesis of suspended PON. In contrast, the first LMW DON δ15N-AA patterns were surprisingly similar to those of autotrophs, indicating limited resynthesis at any depth. Taken together with prior biomarker data, we hypothesize a direct bacterial surface source of LMW AA-containing molecules which are inherently refractory. Finally, within each size fraction, δ15N-AA patterns were remarkably similar between Station ALOHA and the BATS site, suggesting our findings can be extended to marine DON at least throughout global subtropical gyres.These findings demonstrate CSI-AA as a novel δ15N tool to understand sources and cycling of marine DON at a new level of detail and suggest a significant shift in our understanding of DON source and cycling. Specifically, we hypothesize that the production and degradation mechanisms of LMW AA-containing DON is fundamentally distinct from HMW DON, and that the majority of LMW refractory DON in the sea derives not from degradation of HMW material, but rather from surface production of intrinsically more refractory LMW molecules.