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Updated: Feb 25, 2026

The Benthic Exchange of O2, N2 and Dissolved Nutrients Using Small Core Incubations
Published on: August 3, 2016
Biological nitrogen fixation in the oxygen-minimum region of the eastern tropical North Pacific ocean
Amal Jayakumar1, Bonnie X Chang2,3, Brittany Widner4
1Department of Geosciences, Princeton University, Princeton, NJ, USA.
Abstract:
Biological nitrogen fixation (BNF) was investigated above and within the oxygen-depleted waters of the oxygen-minimum zone of the Eastern Tropical North Pacific Ocean. BNF rates were estimated using an isotope tracer method that overcame the uncertainty of the conventional bubble method by directly measuring the tracer enrichment during the incubations. Highest rates of BNF (~4 nM day-1) occurred in coastal surface waters and lowest detectable rates (~0.2 nM day-1) were found in the anoxic region of offshore stations. BNF was not detectable in most samples from oxygen-depleted waters. The composition of the N2-fixing assemblage was investigated by sequencing of nifH genes. The diazotrophic assemblage in surface waters contained mainly Proteobacterial sequences (Cluster I nifH), while both Proteobacterial sequences and sequences with high identities to those of anaerobic microbes characterized as Clusters III and IV type nifH sequences were found in the anoxic waters. Our results indicate modest input of N through BNF in oxygen-depleted zones mainly due to the activity of proteobacterial diazotrophs.
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