Anaerobic oxidation of ammonium (anammox) in oxygen minimum zones (OMZs) is a major pathway of oceanic nitrogen loss. Ammonium released from sinking particles has been suggested to fuel this process. During cruises to the Peruvian OMZ in April–June 2017 we found that anammox rates are strongly correlated with the volume of small particles (128–512 µm), even though anammox bacteria were not directly associated with particles. This suggests that the relationship between anammox rates and particles is related to the ammonium released from particles by remineralization. To investigate this, ammonium release from particles was modelled and theoretical encounters of free-living anammox bacteria with ammonium in the particle boundary layer were calculated. These results indicated that small sinking particles could be responsible for ~75% of ammonium release in anoxic waters and that free-living anammox bacteria frequently encounter ammonium in the vicinity of smaller particles. This indicates a so far underestimated role of abundant, slow-sinking small particles in controlling oceanic nutrient budgets, and furthermore implies that observations of the volume of small particles could be used to estimate N-loss across large areas.
Bibliographical noteFunding Information:
We thank Gabriele Klockgether and Philipp Hach for mass spectrometry and elemental analysis, Matija Lagator for help with C content measurements, Angelina Klett for help with particle size determination, Gerd Krahmann and Hermann Bange for CTD and nutrient data, Markus Dengler and Hermann Bange for their support during M136 and M138 respectively, and Simon Ramondenc for discussion of the multiple linear regression. We thank the captain and crew of R/V Meteor for their support during cruises M136 and M138. Further, we thank Bennett Lambert for discussions on encounter rates. This work was supported by the Max Planck Society and the DFG-funded collaborative research center 754 “Climate-biogeochemistry interactions in the tropical Ocean” (Work Packages B4, B3 and B8). R.K. and H.H. were furthermore supported by the “CUSCO— Coastal Upwelling System in a Changing Ocean” project (WP5) funded by the Federal Ministry of Education and Research (Germany). R.K. also acknowledges support via a “Make Our Planet Great Again” grant of the French National Research Agency within the “Programme d’Investissements d’Avenir”; reference “ANR-19-MPGA-0012”. S.A. and J.M. acknowledge funding from the Max Planck Society (MPG) for the “Multiscale Approach on the Role of Marine Aggregates” (MARMA) project. H.K.M. and M.H. I. received funding from the DFG under Germany’s Excellence Strategy (no. EXC-2077-390741603). M.H.I. was supported by the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research and the HGF Young Investigator Group SeaPump “Seasonal and regional food web interactions with the biological pump”: VH-NG-1000.
© 2021, The Author(s).