Coral reef health depends on an intricate relationship among the coral animal, photosynthetic algae, and a complex microbial community. The holobiont can impact the nutrient balance of their hosts amid an otherwise oligotrophic environment, including by cycling physiologically important nitrogen compounds. Here we use 15N-tracer experiments to produce the first simultaneous measurements of ammonium oxidation, nitrate reduction, and nitrous oxide (N2O) production among five iconic species of reef-building corals (Acropora palmata, Diploria labyrinthiformis, Orbicella faveolata, Porites astreoides, and Porites porites) in the highly protected Jardines de la Reina reefs of Cuba. Nitrate reduction is present in most species, but ammonium oxidation is low potentially due to photoinhibition and assimilatory competition. Coral-associated rates of N2O production indicate a widespread potential for denitrification, especially among D. labyrinthiformis, at rates of ~1 nmol cm−2 d−1. In contrast, A. palmata displays minimal active nitrogen metabolism. Enhanced rates of nitrate reduction and N2O production are observed coincident with dark net respiration periods. Genomes of bacterial cultures isolated from multiple coral species confirm that microorganisms with the ability to respire nitrate anaerobically to either dinitrogen gas or ammonium exist within the holobiont. This confirmation of anaerobic nitrogen metabolisms by coral-associated microorganisms sheds new light on coral and reef productivity.

The Eastern Tropical North Pacific (ETNP), like the other marine oxygen deficient zones (ODZs), is characterized by an anoxic water column, nitrite accumulation at the anoxic core, and fixed nitrogen loss via nitrite reduction to N2O and N2 gases. Here, we constrain the relative contribution of biogeochemical processes to observable features such as the secondary nitrite maximum (SNM) and local pH maximum by simultaneous measurement of inorganic nitrogen and carbon species. High‐resolution sampling within the top 1 km of the water column reveals consistent chemical features previously unobserved in the region, including a tertiary nitrite maximum. Dissolved inorganic carbon measurements show that pH increases with depth at the top of the ODZ, peaking at the potential density of the SNM at σθ = 26.15 ± 0.06 (1 s.d.). We developed a novel method to determine the relative contributions of anaerobic ammonium oxidation (anammox), denitrification, nitrite oxidation, dissimilatory nitrate reduction to nitrite, and calcium carbonate dissolution to the nitrite cycling in the anoxic ODZ core. The calculated relative contributions of each reaction are slightly sensitive to the assumed C:N:P ratio and the carbon oxidation state of the organic matter sinking through the ODZ. Furthermore, we identify the source of the pH increase at the top of ODZ as the net consumption of protons via nitrite reduction to N2 by the denitrification process. The increase in pH due to denitrification impacts the buffering effect of calcite and aragonite dissolving in the ETNP.

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.

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.