Missing Pieces to the Puzzle of Nitrogen Cycling in Ocean Oxygen Deficient Zones (ODZs)

Recent advances have profoundly changed and complicated our view of N-cycling in the ocean. It is now generally recognized that fixed N loss to N₂ in the major Oxygen Deficient Zones (ODZs) occurs through a combination of heterotrophic denitrification (the sequential reduction of NO₃^- to N₂ ) and chemosynthetic anammox (reacts NH₄⁺ with NO₂^- to form N₂). However, disagreement persists regarding the relative importance of these microbial pathways and their controlling factors. In addition, pathways such as NO₂^- oxidation have been identified to operate in ODZ’s which, while potentially important for controlling N₂ production, should not be energetically feasible in an anaerobic environment.

In synthesizing the results of studies using molecular biological, ¹⁵N tracer rate, and biogeochemical techniques, a number inconsistencies arise regarding 1) the relative importance of denitrification and anammox, 2) the role of NO₂^- oxidation, and 3) the role of the NO₂^- pool that builds up in ODZs. Part of these differences may be ascribed to previously unrecognized temporal and spatial variability, though datasets collected in the same ocean region at the same time have lead to divergent stories. Along these lines a case is building for mesoscale eddies as hotspots for N-loss.

But other evidence suggests as of yet unknown mechanisms or pathways for N-loss. For example, an “excess ²⁹N” signal has been observed for ¹⁵N tracer rate experiments in ODZs as well as other marine environments that cannot conventionally be attributed to either denitrification or anammox. Lastly, natural abundance isotope data from the Peru ODZ cannot be reproduced by models incorporating current views of dominant pathways. In this talk, we will review the inconsistencies between datasets, propose new pathways and mechanisms for N-loss that could help reconcile these discrepancies, and suggest future lines of research.