Isotopic heterogeneity and cycling of organic nitrogen in the oligotrophic ocean

Abstract

We measured the nitrogen (N) isotopic composition (δ15N) of a large set (n = 38) of high‐molecular‐weight (HMW) dissolved organic nitrogen (DON) samples isolated from the tropical and subtropical North Atlantic and Pacific Oceans. The δ15N signature of surface HMW DON is relatively invariable in both oligotrophic basins (4.1 ± 0.6‰ in the Atlantic; 5.4 ± 0.8‰ in the Pacific) and shows little correlation with sources or concentrations of N supporting new production in the euphotic zone. While large variations in δ15N of bulk HMW DON are not apparent, δ15N of proteins isolated from sites with relatively high rates of N2 fixation (>80 µmol N m−2 d−1) were consistently depleted in 15N relative to bulk HMW DON and to proteins isolated from sites where N2 fixation does not routinely occur. This small component of HMW DON appears to be cycling more rapidly than bulk HMW DON and may be indicative of fresh DON contributed by organisms in the surface ocean. Furthermore, δ15N of DNA extracted from the bacterial size fraction (0.2‐0.5 µm) revealed that free‐living bacteria may be an important sink for isotopically depleted N produced during N2 fixation. We suggest that there exists a tight coupling between the production and uptake of DON contributed by diazotrophs (N2 fixers) in regions where N2 fixation provides a major input of new nitrogen.