Denitrification as the dominant nitrogen loss process in the Arabian Sea

Nitrogen lost at sea Fixed nitrogen availability is the factor limiting primary production in more than half of the world's oceans. Recent reports have shown that anammox (anaerobic ammonium oxidation), rather than conventional denitrification producing dinitrogen gas (N 2 ), is responsible for all or most of the fixed nitrogen loss in oxygen limited regions of the ocean. Ward et al . demonstrate that denitrification, rather than anammox, dominates the fixed nitrogen loss in the Arabian Sea. In this region denitrifying bacteria are more abundant than anammox bacteria, even when anammox rates are significant. This work addresses a major uncertainty in our understanding of the global nitrogen cycle and the fixed nitrogen inventory, confirming that denitrification is a major process in the overall marine nitrogen cycle. Fixed nitrogen availability limits primary production in over half of the world's oceans. Oceanic oxygen minimum zones (OMZs) are responsible for about 35% of oceanic dinitrogen gas (N 2 ) production and up to half of that occurs in the Arabian Sea. It has recently been argued that anaerobic ammonia oxidation (anammox) alone is responsible for fixed nitrogen loss in the OMZs; however, here it is shown that denitrification rather than anammox dominates the N 2 loss term in the Arabian Sea. Primary production in over half of the world’s oceans is limited by fixed nitrogen availability. The main loss term from the fixed nitrogen inventory is the production of dinitrogen gas (N 2 ) by heterotrophic denitrification or the more recently discovered autotrophic process, anaerobic ammonia oxidation (anammox). Oceanic oxygen minimum zones (OMZ) are responsible for about 35% of oceanic N 2 production and up to half of that occurs in the Arabian Sea 1 . Although denitrification was long thought to be the only loss term, it has recently been argued that anammox alone is responsible for fixed nitrogen loss in the OMZs 2 , 3 , 4 . Here we measure denitrification and anammox rates and quantify the abundance of denitrifying and anammox bacteria in the OMZ regions of the Eastern Tropical South Pacific and the Arabian Sea. We find that denitrification rather than anammox dominates the N 2 loss term in the Arabian Sea, the largest and most intense OMZ in the world ocean. In seven of eight experiments in the Arabian Sea denitrification is responsible for 87–99% of the total N 2 production. The dominance of denitrification is reproducible using two independent isot