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- Title
Top‐Down Control of Ammonia Oxidizers by Grazing in the North Pacific.
- Authors
Ni, Silin; Xu, Min Nina; Du, Moge; Laws, Edward A.; Kao, Shuh‐Ji
- 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. Plain Language Summary: Grazing by zooplankton exerts important control on microbes in the aquatic systems. Ammonia oxidizers (AO) are chemoautotrophic microorganisms that utilize most of their substrate as a source of energy rather than biomass (low biomass yield but high transformation efficiency) and mediate nitrification, a critical process in biogeochemical cycles. Due to the technical difficulty of differentiating AO from other microbes and calculating their change in abundance, the influence of grazing on the AO population in the field has never been quantified. Here we develop a new method that introduces a highly sensitive isotope‐tracing technique into traditional dilution experiments to focus on functional chemoautotrophic AO and quantify their NH4+‐replete growth rates and rates of grazing on them. We found our method to be effective, and for the first time, we revealed that grazing induced a strong top‐down control on AO at the base of the euphotic zone in the North Pacific Ocean. This influence could also be the reason for the vertical decoupling of the ammonia oxidation rate and AO abundance globally. We therefore suggest a wider exploration of grazing influence on AO in field experiments for a deeper understanding of the role of grazing in the marine nitrogen cycle. Key Points: A method that introduced 15NH4+ isotope tracers into dilution experiments is developed to quantify the grazing rate on ammonia oxidizersField experiments showed that grazing consumed more than half of ammonia oxidizer daily growth at the base of the euphotic zone in the North PacificThe strong top‐down control may be widespread to shape the distributions of ammonia oxidizers in the global ocean
- Subjects
GRAZING; OXIDIZING agents; EUPHOTIC zone; AMMONIA; BIOGEOCHEMICAL cycles
- Publication
Geophysical Research Letters, 2024, Vol 51, Issue 10, p1
- ISSN
0094-8276
- Publication type
Article
- DOI
10.1029/2024GL108484