Revisiting nitrification in the Eastern Tropical South Pacific: A focus on controls

Abstract

Nitrification, the oxidation of ammonium ( NH4+) to nitrite ( NO2−) and to nitrate ( NO3−), is a component of the nitrogen (N) cycle internal to the fixed N pool. In oxygen minimum zones (OMZs), which are hotspots for oceanic fixed N loss, nitrification plays a key role because it directly supplies substrates for denitrification and anaerobic ammonia oxidation (anammox), and may compete for substrates with these same processes. However, the control of oxygen and substrate concentrations on nitrification are not well understood. We performed onboard incubations with 15N‐labeled substrates to measure rates of NH4+ and NO2− oxidation in the eastern tropical South Pacific (ETSP). The spatial and depth distributions of NH4+ and NO2− oxidation rates were primarily controlled by NH4+ and NO2− availability, oxygen concentration, and light. In the euphotic zone, nitrification was partially photoinhibited. In the anoxic layer, NH4+ oxidation was negligible or below detection, but high rates of NO2− oxidation were observed. NH4+ oxidation displayed extremely high affinity for both NH4+ and oxygen. The positive linear correlations between NH4+ oxidation rates and in situ NH4+ concentrations and ammonia monooxygenase subunit A (amoA) gene abundances in the upper oxycline indicate that the natural assemblage of ammonia oxidizers responds to in situ NH4+ concentrations or supply by adjusting their population size, which determines the NH4+ oxidation potential. The depth distribution of archaeal and bacterial amoA gene abundances and N2O concentration, along with independently reported simultaneous direct N2O production rate measurements, suggests that AOA were predominantly responsible for NH4+ oxidation, which was a major source of N2O production at oxygen concentrations > 5 µM.

Relative impacts of light, temperature, and reactive oxygen on thaumarchaeal ammonia oxidation in the North Pacific Ocean

“…The high substrate affinities of marine AOA reflect their remarkable adaptation to growth under conditions of constantly low energy flux. This is thought to provide a competitive advantage relative to AOB, and is consistent with the high activity and abundance of marine AOA observed in subsurface waters of nutrient‐limited marine environments, such as the oligotrophic gyre (Mincer et al ; Sintes et al ) and in oxygen deficient zones (Peng et al ). Thus, photosensitivity is possibly a more relevant controlling variable in the open ocean.…”

Oxidation kinetics and inverse isotope effect of marine nitrite-oxidizing isolates

“…So far, the kinetics of nitrite oxidation have not been investigated for marine NOB pure cultures. Recent investigations of natural assemblages in OMZs focused on O 2 dependence during nitrite oxidation and found a very high O 2 affinity, which supports the relevance of nitrification even in the core of OMZs (Bristow et al 2016, Peng et al 2016). …”

Nitrite isotopes as tracers of marine N cycle processes

“…Most NOB are believed to require O 2 for respiration, although the utilization of alternative electron acceptors has been shown in some NOB [29][30][31]. Even though NOB have a high affinity for O 2 [21,24,32], O 2 supply most likely controls the rate of nitrite oxidation in the SNM.…”