Geophysical Research Letters (May 2024)

Top‐Down Control of Ammonia Oxidizers by Grazing in the North Pacific

  • Silin Ni,
  • Min Nina Xu,
  • Moge Du,
  • Edward A. Laws,
  • Shuh‐Ji Kao

DOI
https://doi.org/10.1029/2024GL108484
Journal volume & issue
Vol. 51, no. 10
pp. n/a – n/a

Abstract

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Abstract The impact of grazing pressure on ammonia oxidizers (AO) has never been quantified in the field. Here we develop a new method to quantify the grazing rates on AO in aquatic systems. We introduce 15NH4+ tracer into the traditional dilution experiments to measure AO's apparent growth rates by using the end‐product of 15NH4+ oxidation, that is, 15NOx−. Field studies in the North Pacific revealed that 15NOx− in the end‐product was sensitive enough to detect AO's grazing rates. Experiments from the lower euphotic zone showed NH4+ replete growth rates of 0.40 d−1 and 0.77 d−1 of AO and in situ grazing rates on AO of 0.41 d−1 and 0.45 d−1, respectively, indicating a strong top‐down control by grazing on AO. Compiled data show a vertical decoupling between ammonia oxidation rates and AO abundance within the euphotic zone, indicating that strong grazing may have affected the distribution of AO in the global ocean.

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