Preprints
https://doi.org/10.5194/bg-2021-280
https://doi.org/10.5194/bg-2021-280

  29 Oct 2021

29 Oct 2021

Review status: this preprint is currently under review for the journal BG.

Controls on nitrite oxidation in the upper Southern Ocean: insights from winter kinetics experiments in the Indian sector

Mhlangabezi Mdutyana1,2, Tanya Marshall1, Xin Sun3,4, Jessica M. Burger1, Sandy J. Thomalla2,5, Bess B. Ward3, and Sarah E. Fawcett1,5 Mhlangabezi Mdutyana et al.
  • 1Department of Oceanography, University of Cape Town, Rondebosch, South Africa
  • 2Southern Ocean Carbon and Climate Observatory (SOCCO), CSIR, Rosebank, South Africa
  • 3Department of Geosciences, Princeton University, Princeton, New Jersey, USA
  • 4Department of Ecology and Evolutionary Biology, Yale University, New Haven, Connecticut, USA
  • 5Marine and Antarctic Research centre for Innovation and Sustainability (MARIS), University of Cape Town, Cape Town, South Africa

Abstract. Across the Southern Ocean in winter, nitrification is the dominant mixed-layer nitrogen cycle process, with some of the nitrate produced therefrom persisting to fuel productivity during the subsequent growing season, potentially weakening the spring/summer biological CO2 sink. To better understand the controls on Southern Ocean nitrification, we conducted nitrite oxidation kinetics experiments in surface waters across the western Indian sector in winter. While all experiments (seven in total) yielded a Michaelis-Menten relationship with substrate concentration, the nitrite oxidation rates only increased substantially once the nitrite concentration exceeded 115±2.3 to 245±18 nM, suggesting that nitrite oxidizing bacteria (NOB) require a minimum (i.e., "threshold") nitrite concentration to produce nitrate. The half-saturation constant ranged from 134±8 to 403±24 nM, indicating a relatively high affinity of Southern Ocean NOB for nitrite, in contrast to results from culture experiments. Despite the high affinity of NOB for nitrite, its concentration rarely declines below 150 nM in the Southern Ocean's mixed layer, regardless of season. In the upper mixed layer, we measured ammonium oxidation rates that were two- to seven-fold higher than the coincident rates of nitrite oxidation, indicating that nitrite oxidation is the rate-limiting step for nitrification in the winter Southern Ocean. The decoupling of ammonium and nitrite oxidation, combined with a possible nitrite concentration threshold for NOB, may explain the non-zero nitrite that persists throughout the Southern Ocean's mixed layer year-round. We hypothesize that the apparent threshold nitrite requirement of NOB indicates nitrite undersaturation of the heme-rich nitrite oxidoreductase enzyme, perhaps driven by the limited availability of iron in surface waters.

Mhlangabezi Mdutyana et al.

Status: open (until 26 Dec 2021)

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  • RC1: 'Comment on bg-2021-280', Anonymous Referee #1, 02 Dec 2021 reply

Mhlangabezi Mdutyana et al.

Mhlangabezi Mdutyana et al.

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Short summary
Nitrite oxidizing bacteria in the winter Southern Ocean show a high affinity for nitrite but require a minimum (i.e., "threshold") concentration before they increase their rates of nitrite oxidation significantly. The classic Michaelis-Menten model thus cannot be used to derive the kinetic parameters, so a modified equation was employed that also yields the threshold nitrite concentration. Substrate limitation of the enzyme that oxidizes nitrite to nitrate may be due to low iron availability.
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