The kinetics of ammonium uptake and oxidation across the Southern Ocean
International audience Central to the Southern Ocean's role in setting atmospheric CO2 is the seasonal alternation between upward mixing of nutrients and their subsequent consumption by phytoplankton. Active nutrient cycling within the mixed layer, including the release of ammonium (NH4+) and i...
Published in: | Limnology and Oceanography |
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ftinsu:oai:HAL:hal-03632080v1 2024-02-11T10:08:47+01:00 The kinetics of ammonium uptake and oxidation across the Southern Ocean Mdutyana, Mhlangabezi Sun, Xin Burger, Jessica Flynn, Raquel Smith, Shantelle Horsten, Natasha Roychoudhury, Alakendra Planquette, Hélène Bucciarelli, Eva Thomalla, Sandy Ward, Bess Fawcett, Sarah Department of Oceanography Cape Town University of Cape Town Department of Geosciences Princeton Princeton University Laboratoire des Sciences de l'Environnement Marin (LEMAR) (LEMAR) Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Institut Universitaire Européen de la Mer (IUEM) Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS) Department of Earth Sciences Stellenbosch Stellenbosch University ANR-17-EURE-0015,ISBlue,Interdisciplinary Graduate School for the Blue planet(2017) 2022-03-17 https://hal.univ-brest.fr/hal-03632080 https://doi.org/10.1002/lno.12050 en eng HAL CCSD Association for the Sciences of Limnology and Oceanography info:eu-repo/semantics/altIdentifier/doi/10.1002/lno.12050 hal-03632080 https://hal.univ-brest.fr/hal-03632080 doi:10.1002/lno.12050 ISSN: 0024-3590 EISSN: 1939-5590 Limnology and Oceanography https://hal.univ-brest.fr/hal-03632080 Limnology and Oceanography, 2022, ⟨10.1002/lno.12050⟩ [SDU.OCEAN]Sciences of the Universe [physics]/Ocean Atmosphere info:eu-repo/semantics/article Journal articles 2022 ftinsu https://doi.org/10.1002/lno.12050 2024-01-24T17:29:47Z International audience Central to the Southern Ocean's role in setting atmospheric CO2 is the seasonal alternation between upward mixing of nutrients and their subsequent consumption by phytoplankton. Active nutrient cycling within the mixed layer, including the release of ammonium (NH4+) and its removal by phytoplankton and nitrifiers, also affects Southern Ocean CO2 drawdown, yet remains poorly understood. We conducted kinetics experiments across the Southern Ocean south of Africa to investigate the dependence of NH4+ uptake (summer, winter) and oxidation (winter) on NH4+ concentration. NH4+ uptake followed a Michaelis–Menten function in both seasons, with the maximum rate (Vmax) decreasing poleward, apparently controlled mainly by light in winter and temperature in summer. The half-saturation constant (Km) increased poleward with increasing ambient NH4+ ([NH4+]amb) and was threefold higher in winter (150–405 nM) than in summer (41–115 nM), suggesting that summertime phytoplankton are adapted to low-NH4+ conditions while winter communities typically receive a higher NH4+ supply. NH4+ oxidation showed a high affinity for NH4+ (Km = 28–137 nM), suggesting a dominant role for ammonia-oxidizing archaea, and followed a Michaelis–Menten curve only when [NH4+]amb was ≤ 90 nM. Vmax was near-constant across the region regardless of [NH4+]amb, temperature, or light. From coincident mixed-layer NH4+ oxidation and iron measurements, we hypothesize that iron availability may (co-)limit the Vmax of NH4+ oxidation. If verified, this suggestion has implications for models that parameterize nitrification as a linear function of [NH4+]amb. Additionally, iron depletion may limit the role of mixed-layer nitrification, which is dominant in the winter Southern Ocean, in offsetting phytoplankton CO2 drawdown annually. Article in Journal/Newspaper Southern Ocean Institut national des sciences de l'Univers: HAL-INSU Southern Ocean Limnology and Oceanography 67 4 973 991 |
institution |
Open Polar |
collection |
Institut national des sciences de l'Univers: HAL-INSU |
op_collection_id |
ftinsu |
language |
English |
topic |
[SDU.OCEAN]Sciences of the Universe [physics]/Ocean Atmosphere |
spellingShingle |
[SDU.OCEAN]Sciences of the Universe [physics]/Ocean Atmosphere Mdutyana, Mhlangabezi Sun, Xin Burger, Jessica Flynn, Raquel Smith, Shantelle Horsten, Natasha Roychoudhury, Alakendra Planquette, Hélène Bucciarelli, Eva Thomalla, Sandy Ward, Bess Fawcett, Sarah The kinetics of ammonium uptake and oxidation across the Southern Ocean |
topic_facet |
[SDU.OCEAN]Sciences of the Universe [physics]/Ocean Atmosphere |
description |
International audience Central to the Southern Ocean's role in setting atmospheric CO2 is the seasonal alternation between upward mixing of nutrients and their subsequent consumption by phytoplankton. Active nutrient cycling within the mixed layer, including the release of ammonium (NH4+) and its removal by phytoplankton and nitrifiers, also affects Southern Ocean CO2 drawdown, yet remains poorly understood. We conducted kinetics experiments across the Southern Ocean south of Africa to investigate the dependence of NH4+ uptake (summer, winter) and oxidation (winter) on NH4+ concentration. NH4+ uptake followed a Michaelis–Menten function in both seasons, with the maximum rate (Vmax) decreasing poleward, apparently controlled mainly by light in winter and temperature in summer. The half-saturation constant (Km) increased poleward with increasing ambient NH4+ ([NH4+]amb) and was threefold higher in winter (150–405 nM) than in summer (41–115 nM), suggesting that summertime phytoplankton are adapted to low-NH4+ conditions while winter communities typically receive a higher NH4+ supply. NH4+ oxidation showed a high affinity for NH4+ (Km = 28–137 nM), suggesting a dominant role for ammonia-oxidizing archaea, and followed a Michaelis–Menten curve only when [NH4+]amb was ≤ 90 nM. Vmax was near-constant across the region regardless of [NH4+]amb, temperature, or light. From coincident mixed-layer NH4+ oxidation and iron measurements, we hypothesize that iron availability may (co-)limit the Vmax of NH4+ oxidation. If verified, this suggestion has implications for models that parameterize nitrification as a linear function of [NH4+]amb. Additionally, iron depletion may limit the role of mixed-layer nitrification, which is dominant in the winter Southern Ocean, in offsetting phytoplankton CO2 drawdown annually. |
author2 |
Department of Oceanography Cape Town University of Cape Town Department of Geosciences Princeton Princeton University Laboratoire des Sciences de l'Environnement Marin (LEMAR) (LEMAR) Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Institut Universitaire Européen de la Mer (IUEM) Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS) Department of Earth Sciences Stellenbosch Stellenbosch University ANR-17-EURE-0015,ISBlue,Interdisciplinary Graduate School for the Blue planet(2017) |
format |
Article in Journal/Newspaper |
author |
Mdutyana, Mhlangabezi Sun, Xin Burger, Jessica Flynn, Raquel Smith, Shantelle Horsten, Natasha Roychoudhury, Alakendra Planquette, Hélène Bucciarelli, Eva Thomalla, Sandy Ward, Bess Fawcett, Sarah |
author_facet |
Mdutyana, Mhlangabezi Sun, Xin Burger, Jessica Flynn, Raquel Smith, Shantelle Horsten, Natasha Roychoudhury, Alakendra Planquette, Hélène Bucciarelli, Eva Thomalla, Sandy Ward, Bess Fawcett, Sarah |
author_sort |
Mdutyana, Mhlangabezi |
title |
The kinetics of ammonium uptake and oxidation across the Southern Ocean |
title_short |
The kinetics of ammonium uptake and oxidation across the Southern Ocean |
title_full |
The kinetics of ammonium uptake and oxidation across the Southern Ocean |
title_fullStr |
The kinetics of ammonium uptake and oxidation across the Southern Ocean |
title_full_unstemmed |
The kinetics of ammonium uptake and oxidation across the Southern Ocean |
title_sort |
kinetics of ammonium uptake and oxidation across the southern ocean |
publisher |
HAL CCSD |
publishDate |
2022 |
url |
https://hal.univ-brest.fr/hal-03632080 https://doi.org/10.1002/lno.12050 |
geographic |
Southern Ocean |
geographic_facet |
Southern Ocean |
genre |
Southern Ocean |
genre_facet |
Southern Ocean |
op_source |
ISSN: 0024-3590 EISSN: 1939-5590 Limnology and Oceanography https://hal.univ-brest.fr/hal-03632080 Limnology and Oceanography, 2022, ⟨10.1002/lno.12050⟩ |
op_relation |
info:eu-repo/semantics/altIdentifier/doi/10.1002/lno.12050 hal-03632080 https://hal.univ-brest.fr/hal-03632080 doi:10.1002/lno.12050 |
op_doi |
https://doi.org/10.1002/lno.12050 |
container_title |
Limnology and Oceanography |
container_volume |
67 |
container_issue |
4 |
container_start_page |
973 |
op_container_end_page |
991 |
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1790608384259522560 |