Determining Rates of Hybrid Archaeal N2O Production in the Eastern Tropical North Pacific Ocean with Intramolecular Isotope Measurements

Colette LaMonica Kelly, Stanford University, Stanford, United States, Nicole Mayu Travis, Stanford University, Earth Systems Science, Stanford, CA, United States, Pascale Anabelle Baya, Géosciences Environnement Toulouse (GET), UMR CNRS / IRD / Université Paul Sabatier, Toulouse, France, Claudia Frey, University of Basel, Basel, Switzerland, Bess B Ward, Princeton University, Department of Geosciences, Princeton, NJ, United States and Karen L Casciotti, Stanford University, Oceans Department, Stanford, United States
Abstract:
Nitrous oxide (N2O) is a powerful greenhouse gas, but the rates and mechanisms of its marine production remain poorly constrained. Recent work demonstrates that ammonia-oxidizing archaea may produce N2O from a “hybrid” pathway involving the oxidation of ammonia and reduction of nitrite, and that this pathway of N2O production may be important in oxygen deficient zones (ODZs). To quantify the rates and mechanism of N2O production in the eastern tropical North Pacific (ETNP) ODZ, we applied site-specific N2O isotopomer measurements to traditional 15N tracer experiments, performed in the ETNP ODZ. Implementing isotopomer measurements from tracer experiments in a simple binomial probability model, we found that hybrid N2O production comprised a consistently high percentage of total N2O production at both the shallow oxic-anoxic interface (0.105±0.031 nM/day, 80±31% of total N2O production) and deep oxycline (0.063±0.009 nM/day, 78±15% of total N2O production). In the core anoxic depths of the ODZ, hybrid (as well as total) N2O production from ammonium and nitrite was negligible; instead, N2O production was dominated by reduction from nitrate. We propose a null hypothesis that the nitrogen atoms from ammonium and nitrite should go into different positions in the N2O molecule during hybrid production, but find instead that production of labeled 15N2O-alpha (with 15N in the inner N position) and 15N2O-beta (with 15N in the outer N position) from NH4+ and NO2- are statistically indistinguishable. Averaged across oxic-anoxic transition depths, hybrid production of 15N2O-alpha comprised 52±13% of the total hybrid production from ammonium, and 58±16% of the total hybrid production from nitrite. To test further this surprising result, we implement the binomial probability model at additional stations and depths; further, we expand upon the model to account for 15N exchange between substrate pools to better distinguish between oxidative and reductive sources.
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