Enhancement of anammox by the excretion of diel vertical migrators

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 111; no. 44; pp. 15653 - 15658
• Main Authors: Bianchi, Daniele, Babbin, Andrew R., Galbraith, Eric D.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 04.11.2014; National Acad Sciences
• Series: From the Cover
• Subjects: Acoustic data; Ammonium Compounds - metabolism; Anaerobiosis - physiology; Animal Migration - physiology; Animal migration behavior; Animals; Bacteria; Biogeochemistry; Biological Sciences; Denitrification; Excretion; Floods; Models, Biological; Nitrites - metabolism; Nitrogen; Nutrient removal; Oceans; Oceans and Seas; Oxidation; Oxidation-Reduction; Oxygen; Physical Sciences; Plankton; Quaternary ammonium compounds; Sea water; Seas; Zooplankton; Zooplankton - physiology; diel vertical migration; denitrification; anammox; oxygen minimum zone
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1410790111

Measurements show that anaerobic ammonium oxidation with nitrite (anammox) is a major pathway of fixed nitrogen removal in the anoxic zones of the open ocean. Anammox requires a source of ammonium, which under anoxic conditions could be supplied by the breakdown of sinking organic matter via heterotrophic denitrification. However, at many locations where anammox is measured, denitrification rates are small or undetectable. Alternative sources of ammonium have been proposed to explain this paradox, for example through dissimilatory reduction of nitrate to ammonium and transport from anoxic sediments. However, the relevance of these sources in open-ocean anoxic zones is debated. Here, we bring to attention an additional source of ammonium, namely, the daytime excretion by zooplankton and micronekton migrating from the surface to anoxic waters. We use a synthesis of acoustic data to show that, where anoxic waters occur within the water column, most migrators spend the daytime within them. Although migrators export only a small fraction of primary production from the surface, they focus excretion within a confined depth range of anoxic water where particle input is small. Using a simple biogeochemical model, we suggest that, at those depths, the source of ammonium from organisms undergoing diel vertical migrations could exceed the release from particle remineralization, enhancing in situ anammox rates. The contribution of this previously overlooked process, and the numerous uncertainties surrounding it, call for further efforts to evaluate the role of animals in oxygen minimum zone biogeochemistry. Significance Nitrogen, the limiting nutrient for primary production across much of the ocean, is converted to biologically inactive N ₂ by denitrification and anaerobic ammonium oxidation (anammox) in anoxic waters. Anammox requires an active source of ammonium, which can be provided by concurrent denitrification. However, anammox has been observed at high rates in the absence of denitrification, and the source of ammonium has remained cryptic. Using a combination of observations and models, we suggest that zooplankton and micronekton provide a missing source of ammonium to anoxic waters through diel vertical migrations, fueling anammox and decoupling it from denitrification. This previously overlooked mechanism can help to reconcile observations with theory and highlights the role of animals on ocean biogeochemistry.