Abstract. Oxygen minimum zones (OMZs), due to their large volumes of perennially deoxygenated waters, are critical regions for understanding how the interplay between anaerobic and aerobic nitrogen (N) cycling microbial pathways affects the marine N budget. Here we present a suite of measurements of the most significant OMZ N cycling rates, which all involve nitrite (NO2–) as a product, reactant, or intermediate, in the Eastern Tropical North Pacific (ETNP) OMZ. These measurements and comparisons to data from previously published OMZ cruises present additional evidence that NO3– reduction is the predominant OMZ N flux, followed by NO2– oxidation back to NO3–. The combined rates of both of these N recycling processes were observed to be much greater (up to nearly 200x) than the combined rates of the N loss processes of anammox and denitrification, especially in waters near the anoxic / oxic interface. We also show that NO2– oxidation can occur in functionally anoxic incubations, measurements that further strengthen the case for truly anaerobic NO2– oxidation. We also evaluate the possibility that NO2– dismutation provides the oxidative power for anaerobic NO2– oxidation. Although almost all treatments returned little evidence for dismutation (as based on product inhibition, substrate stimulation, and stoichiometric hypotheses), results from one treatment under conditions closest to in situ NO2– values may support the occurrence of NO2– dismutation. The partitioning of N loss between anammox and denitrification differed widely from stoichiometric predictions of at most 29 % anammox; in fact, N loss rates at many depths consisted entirely of anammox. Through investigating the magnitudes of NO3– reduction and NO2– oxidation, testing for anaerobic NO2– oxidation, examining the possibility of NO2– dismutation, and further documenting the balance of N loss processes, these new data shed light on many open questions in OMZ N cycling research.
Abstract. Oxygen minimum zones (OMZs), due to their large volumes of perennially deoxygenated waters, are critical regions for understanding how the interplay between anaerobic and aerobic nitrogen (N) cycling microbial pathways affects the marine N budget. Here we present a suite of measurements of the most significant OMZ N cycling rates, which all involve nitrite (NO2–) as a product, reactant, or intermediate, in the Eastern Tropical North Pacific (ETNP) OMZ. These measurements and comparisons to data from previously published OMZ cruises present additional evidence that NO3– reduction is the predominant OMZ N flux, followed by NO2– oxidation back to NO3–. The combined rates of both of these N recycling processes were observed to be much greater (up to nearly 200x) than the combined rates of the N loss processes of anammox and denitrification, especially in waters near the anoxic / oxic interface. We also show that NO2– oxidation can occur in functionally anoxic incubations, measurements that further strengthen the case for truly anaerobic NO2– oxidation. We also evaluate the possibility that NO2– dismutation provides the oxidative power for anaerobic NO2– oxidation. Although almost all treatments returned little evidence for dismutation (as based on product inhibition, substrate stimulation, and stoichiometric hypotheses), results from one treatment under conditions closest to in situ NO2– values may support the occurrence of NO2– dismutation. The partitioning of N loss between anammox and denitrification differed widely from stoichiometric predictions of at most 29 % anammox; in fact, N loss rates at many depths consisted entirely of anammox. Through investigating the magnitudes of NO3– reduction and NO2– oxidation, testing for anaerobic NO2– oxidation, examining the possibility of NO2– dismutation, and further documenting the balance of N loss processes, these new data shed light on many open questions in OMZ N cycling research.
Abstract. Oxygen minimum zones (OMZs), due to their large volumes of perennially deoxygenated waters, are critical regions for understanding how the interplay between anaerobic and aerobic nitrogen (N) cycling microbial pathways affects the marine N budget. Here, we present a suite of measurements of the most significant OMZ N cycling rates, which all involve nitrite (NO2-) as a product, reactant, or intermediate, in the eastern tropical North Pacific (ETNP) OMZ. These measurements and comparisons to data from previously published OMZ cruises present additional evidence that NO3- reduction is the predominant OMZ N flux, followed by NO2- oxidation back to NO3-. The combined rates of both of these N recycling processes were observed to be much greater (up to nearly 200 times) than the combined rates of the N loss processes of anammox and denitrification, especially in waters near the anoxic–oxic interface. We also show that NO2- oxidation can occur when O2 is maintained near 1 nM by a continuous-purge system, NO2- oxidation and O2 measurements that further strengthen the case for truly anaerobic NO2- oxidation. We also evaluate the possibility that NO2- dismutation provides the oxidative power for anaerobic NO2- oxidation. The partitioning of N loss between anammox and denitrification differed widely from stoichiometric predictions of at most 29 % anammox; in fact, N loss rates at many depths were entirely due to anammox. Our new NO3- reduction, NO2- oxidation, dismutation, and N loss data shed light on many open questions in OMZ N cycling research, especially the possibility of truly anaerobic NO2- oxidation.
Additional Information on Anammox and Denitrification Profiles (SR1805 and FK180624)In order to calculate the denitrification and anammox rates using Eq.(7 -9) , the amount of 29 and 30 N2 in nmol for each vial must first be calculated using the equations below: ) * 29 28 * 28 ̂2 (S1) 30 2 = ( ) * 30 28 * 28 ̂2 (S2) Where: 30 28 29 28 is the measured ratio in peak area units of the specific sample, 28 ̂2 is the average 28 peak area across the time course (without outliers), and
