23 Apr 2021

23 Apr 2021

Review status: this preprint is currently under review for the journal BG.

Insights into nitrogen fixation below the euphotic zone: trials in an oligotrophic marginal sea and global compilation

Siqi Wu1,2, Moge Du1,2, Xianhui Sean Wan1,3, Corday Selden4, Mar Benavides5, Sophie Bonnet5, Robert Hamersley6, Carolin R. Löscher7,8, Margaret R. Mulholland4, Xiuli Yan9, and Shuh-Ji Kao1,2,10 Siqi Wu et al.
  • 1State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, 361102, China
  • 2College of Ocean and Earth Sciences, Xiamen University, Xiamen, 361102, China
  • 3Department of Geosciences, Princeton University, Princeton, NJ 08544, USA
  • 4Department of Ocean & Earth Sciences, Old Dominion University, Norfolk, VA 23529, USA
  • 5Aix Marseille Univ., Université de Toulon, CNRS, IRD, MIO UM 110, Marseille, 13288, France
  • 6Environmental Studies, Soka University of America, Aliso Viejo, 92656, USA
  • 7Nordcee, Department of Biology, University of Southern Denmark, Odense, DK-5230, Denmark
  • 8D-IAS, University of Southern Denmark, Odense, DK-5230, Denmark
  • 9Marine Science Institute and Guangdong Provincial Key Laboratory of Marine Biotechnology College of Science, Shantou University, Shantou, 515063, China
  • 10State Key Laboratory of Marine Resources Utilization in South China Sea, Hainan University, Haikou, 570228, China

Abstract. Nitrogen (N2) fixation, the energetically expensive conversion of N2 to ammonia, plays an important role in balancing the global nitrogen budget. Defying historic paradigms, recent studies have detected non-cyanobacterial N2 fixation in deep, dark oceanic waters. Even low volumetric rates can be significant considering the large volume of these waters. However, measuring aphotic N2 fixation is an analytical challenge due to the low particulate nitrogen (PN) concentrations. Here, we investigated N2 fixation rates in aphotic waters in the South China Sea (SCS). To increase the sensitivity of N2 fixation rate measurements, we applied a novel approach requiring only 0.28 μg N for measuring the isotopic composition of particulate nitrogen. We conducted parallel 15N2-enriched incubations in ambient seawater, seawater amended with amino acids and poisoned (HgCl2) controls, along with incubations that received no tracer additions to distinguish biological N2 fixation. Experimental treatments differed significantly from our two types of controls, those receiving no additions and killed controls. Amino acid additions masked N2 fixation signals due to the uptake of added 14N-amino acid. Results show that the maximum dark N2 fixation rates (1.28 ± 0.85 nmol N L−1 d−1) occurred within upper 200 m, while rates below 200 m were mostly lower than 0.1 nmol N L−1 d−1. Nevertheless, N2 fixation rates between 200 and 1000 m accounted for 39 ± 32 % of depth-integrated dark N2 fixation rates in the upper 1000 m, which is comparable to the areal nitrogen inputs via atmospheric deposition. Globally, we found that aphotic N2 fixation studies conducted in oxygenated environments yielded rates similar to those from the SCS (< 1 nmol N L−1 d−1), regardless of methods, while higher rates were occasionally observed in low-oxygen (< 62 µM) regions. Regression analysis suggests that particulate nitrogen concentrations could be a predictive proxy for detectable aphotic N2 fixation in the SCS and eastern tropical south Pacific. Our results provide the first insight into aphotic N2 fixation in SCS and support the importance of the aphotic zone as a globally-important source of new nitrogen to the ocean.

Siqi Wu et al.

Status: open (until 13 Jun 2021)

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Siqi Wu et al.


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Short summary
Nitrogen (N2) fixation is one of the most important nutrient sources to the ocean. Here, we report N2 fixation in the deep, dark ocean in the South China Sea via a highly sensitive new method and elaborate controls, showing the overlooked importance of N2 fixation in the deep ocean. By global data compilation, we also provide an easy measured basic parameter to estimate deep N2 fixation. Our study may help to expand the area limit of N2 fixation studies and better constrain global N2 fixation.