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.
Highlights
The marine fixed nitrogen (N) inventory is primarily determined by the input of new N via atmospheric deposition, biological dinitrogen (N2) fixation, riverine flux and its loss from the ocean via denitrification and anammox (Wang et al, 40 2019)
We found that aphotic N2 fixation studies conducted in oxygenated environments yielded rates similar to those from the South China Sea (SCS) (< 1 nmol N L-1 d-1), regardless of methods, while higher rates were occasionally observed in low-oxygen (< 62 μM) regions
dissolved organic matter (DOM) addition experiments remain sparse, and further experiments in different regions would be helpful to elucidate the effect of DOM on ANF. 95 Here, we present the first reported N2 fixation measurements from the mesopelagic SCS, the largest oxygenated marginal sea in the western Pacific Ocean
Summary
The marine fixed nitrogen (N) inventory is primarily determined by the input of new N via atmospheric deposition, biological dinitrogen (N2) fixation, riverine flux and its loss from the ocean via denitrification and anammox (Wang et al, 40 2019). DFAA additions resulted in one to seven-fold enhancement of ANF in most studies (Benavides et al, 2015; Bonnet et al, 2013; Rahav et al, 2013, 2015; 90 Selden et al, 2019), some studies reported no enhancement (Benavides et al, 2015; Selden et al, 2019), or even an inhibitory effect (Selden et al, 2019) These inconsistencies could be due to population-specific substrate preferences and metabolic diversity, variability in energy or carbon limitation, decreased DOM utilization under low temperature (Selden et al, 2019), or incubation artifacts (e.g., pressure, bottle effects, etc). To better constrain low N2 fixation rates below the euphotic zone, we first utilized alkaline persulfate digestion (Knapp et al, 2005) coupled with a denitrifier method (persulfate-denitrifier method hereafter) to measure PN concentration and isotopic composition (Casciotti et al, 2002; McIlvin and Casciotti, 2011; Sigman et al, 2001) This method requires only 0.28 μg N (or 20 nmol N) rather than the roughly 10 μg N required when using standard 100 EA-IRMS analysis (White et al, 2020). Water samples for incubations were obtained from a rosette of 24 × 12 L Niskin bottles
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