Abundant nitrite-oxidizing metalloenzymes in the mesopelagic zone of the tropical Pacific Ocean

Numerous biogeochemical reactions occur within the oceans’ major oxygen minimum zones, but less attention has been paid to the open ocean extremities of these zones. Here we report measurements on oxygen minimum zone waters from the Eastern to the Central Tropical North Pacific, which we analysed using metaproteomic techniques to discern the microbial functions present and their influence on biogeochemical cycling. We found nitrite oxidoreductase—an iron-rich enzyme from Nitrospina bacteria—to be one of the most abundant microbial proteins present in the mesopelagic zone, with over 60 billion molecules per litre. Estimated reaction rates imply that this enzyme is undersaturated and that its high abundance provides a latent mesopelagic catalytic capacity to rapidly oxidize nitrite derived from episodic fluxes of degrading sinking organic matter. In addition, given the enzyme’s intensive iron demand, its high abundance represents a previously unrecognized microbial reservoir within suboxic mesopelagic zones. Nitrite oxidoreductase may also contribute to other reactions involving nitrogen and redox-sensitive metals. We suggest that the abundance and extent of nitrite oxidoreductase may increase with continued deoxygenation in the oceans, and result in increased mesopelagic demand for iron and other potential changes to marine biogeochemical cycles. Continued deoxygenation of the oceans will probably lead to enhanced demand for iron, as implied by the abundance of an iron-rich enzyme in the mesopelagic waters of the Pacific.