ALBERT

Description: N2O production rates from ammonium, nitrite and nitrate and nitrate reduction rates and ammonium oxidation rates from the top 400 m water depth off the coast of Peru sampled from R/V Meteor during M138 in June 2017.

Description: In March/April 2018 during a cruise on R/V Sally Ride, SR1805, 15N-NH4+ incubations in 60mL glass serum bottles were performed to measure ammonium oxidation rates to nitrite and nitrous oxide in different depth at 3 different stations in the oxygen deficient zone (ODZ) of the Eastern Tropical North Pacific off the coast of Mexico. Water samples were collected from 30L Niskin bottles deployed with a conductivity-temperature-depth profiler (CTD, Seabird Electronics). The goal was to get a better understanding on the controls of nitrous oxide (N2O) production. The N2O production rate experiments were performed according to Bourbonnais et al. 2021 (https://doi.org/10.3389/fmars.2021.611937). Furthermore, ammonium (NH4+), nitrite (NO2-) and nitrate (NO3-) as well as N2O concentrations were determined using standard fluorometric (Holmes et al. 1999, https://doi.org/10.1139/f99-128), photometric (Strickland and Parsons 1972, hdl:10013/epic.46454.d001), chemiluminescent (Braman and Hendrix 1989, doi:10.1021/ac00199a007) and mass spectrometric techniques (McIlvin and Casciotti 2010, https://doi.org/10.4319/lom.2010.8.54), respectively. The N2O yield per nitrite produced was calculated. The archaeal ammonia monooxygenase gene subunit A (amoA) copy numbers/mL were determined using qPCR as described previously (Peng et al. 2015, https://doi.org/10.1002/2015GB005278).

Description: The Eastern Tropical South Pacific oxygen minimum zone (ETSP-OMZ) is a site of intense nitrous oxide (N2O) flux to the atmosphere. This flux results from production of N2O by nitrification and denitrification, but the contribution of the two processes is unknown. The rates of these pathways and their distributions were measured directly using 15N tracers. The highest N2O production rates occurred at the depth of peak N2O concentrations at the oxic-anoxic interface above the oxygen deficient zone (ODZ) because slightly oxygenated waters allowed (1) N2O production from both nitrification and denitrification and (2) higher nitrous oxide production yields from nitrification. Within the ODZ proper (i.e., anoxia), the only source of N2O was denitrification (i.e., nitrite and nitrate reduction), the rates of which were reflected in the abundance of nirS genes (encoding nitrite reductase). Overall, denitrification was the dominant pathway contributing the N2O production in the ETSP-OMZ.

Description: Salt marshes provide numerous valuable ecological services. In particular, nitrogen (N) removal in salt marsh sediments alleviates N loading to the coastal ocean. N removal reduces the threat of eutrophication caused by increased N inputs from anthropogenic sources. It is unclear, however, whether chronic nutrient over-enrichment alters the capacity of salt marshes to remove anthropogenic N. To assess the effect of nutrient enrichment on N cycling in salt marsh sediments, we examined important N cycle pathways in experimental fertilization plots in a New England salt marsh. We determined rates of nitrification, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) using sediment slurry incubations with 15 N labeled ammonium or nitrate tracers under oxic headspace (20% oxygen / 80% helium). Nitrification and denitrification rates were more than ten-fold higher in fertilized plots compared to control plots. By contrast, DNRA, which retains N in the system, was high in control plots but not detected in fertilized plots. The relative contribution of DNRA to total nitrate reduction largely depends on the carbon/nitrate ratio in the sediment. These results suggest that long-term fertilization shifts N cycling in salt marsh sediments from predominantly retention to removal. Long-term fertilization alters the relative importance of nitrate reduction pathways in salt marsh sediments: NO 3 - reduction in salt marsh sediments (PDF Download Available). Available from: https://www.researchgate.net/publication/305480944_Long-term_fertilization_alters_the_relative_importance_of_nitrate_reduction_pathways_in_salt_marsh_sediments_NO_3_-_reduction_in_salt_marsh_sediments [accessed Jun 6, 2017].

Description: The ocean is estimated to contribute up to ~20% of global fluxes of atmospheric nitrous oxide (N2O), an important greenhouse gas and ozone depletion agent. Marine oxygen minimum zones contribute disproportionately to this flux. To further understand the partition of nitrification and denitrification and their environmental controls on marine N2O fluxes, we report new relationships between oxygen concentration and rates of N2O production from nitrification and denitrification directly measured with 15N tracers in the Eastern Tropical Pacific. Highest N2O production rates occurred near the oxic‐anoxic interface, where there is strong potential for N2O efflux to the atmosphere. The dominant N2O source in oxygen minimum zones was nitrate reduction, the rates of which were 1 to 2 orders of magnitude higher than those of ammonium oxidation. The presence of oxygen significantly inhibited the production of N2O from both nitrification and denitrification. These experimental data provide new constraints to a multicomponent global ocean biogeochemical model, which yielded annual oceanic N2O efflux of 1.7–4.4 Tg‐N (median 2.8 Tg‐N, 1 Tg = 1012 g), with denitrification contributing 20% to the oceanic flux. Thus, denitrification should be viewed as a net N2O production pathway in the marine environment.

Description: Nitrite oxidation is an essential step in transformations of fixed nitrogen. The physiology of nitrite oxidizing bacteria (NOB) implies that the rates of nitrite oxidation should be controlled by concentration of their substrate, nitrite, and the terminal electron acceptor, oxygen. The sensitivities of nitrite oxidation to oxygen and nitrite concentrations were investigated using 15N tracer incubations in the Eastern Tropical North Pacific. Nitrite stimulated nitrite oxidation under low in situ nitrite conditions, following Michaelis-Menten kinetics, indicating that nitrite was the limiting substrate. The nitrite half-saturation constant (Ks = 0.254 ± 0.161 μM) was 1–3 orders of magnitude lower than in cultivated NOB, indicating higher affinity of marine NOB for nitrite. The highest rates of nitrite oxidation were measured in the oxygen depleted zone (ODZ), and were partially inhibited by additions of oxygen. This oxygen sensitivity suggests that ODZ specialist NOB, adapted to low-oxygen conditions, are responsible for apparently anaerobic nitrite oxidation. Key Points: • Nitrite addition stimulated nitrite oxidation in both oxic and anoxic waters • Natural assemblages of marine nitrite-oxidizing bacteria have high affinity for nitrite • Addition of oxygen at μM-level inhibited nitrite oxidation in oxygen depleted waters