Revising the nitrogen cycle in the Peruvian oxygen minimum zone

<jats:p> The oxygen minimum zone (OMZ) of the Eastern Tropical South Pacific (ETSP) is 1 of the 3 major regions in the world where oceanic nitrogen is lost in the pelagic realm. The recent identification of anammox, instead of denitrification, as the likely prevalent pathway for nitrogen loss in this OMZ raises strong questions about our understanding of nitrogen cycling and organic matter remineralization in these waters. Without detectable denitrification, it is unclear how NH <jats:sub arrange="stack">4</jats:sub> <jats:sup arrange="stack">+</jats:sup> is remineralized from organic matter and sustains anammox or how secondary NO <jats:sub arrange="stack">2</jats:sub> <jats:sup arrange="stack">−</jats:sup> maxima arise within the OMZ. Here we show that in the ETSP-OMZ, anammox obtains 67% or more of NO <jats:sub arrange="stack">2</jats:sub> <jats:sup arrange="stack">−</jats:sup> from nitrate reduction, and 33% or less from aerobic ammonia oxidation, based on stable-isotope pairing experiments corroborated by functional gene expression analyses. Dissimilatory nitrate reduction to ammonium was detected in an open-ocean setting. It occurred throughout the OMZ and could satisfy a substantial part of the NH <jats:sub arrange="stack">4</jats:sub> <jats:sup arrange="stack">+</jats:sup> requirement for anammox. The remaining NH <jats:sub arrange="stack">4</jats:sub> <jats:sup arrange="stack">+</jats:sup> came from remineralization via nitrate reduction and probably from microaerobic respiration. Altogether, deep-sea NO <jats:sub arrange="stack">3</jats:sub> <jats:sup arrange="stack">−</jats:sup> accounted for only ≈50% of the nitrogen loss in the ETSP, rather than 100% as commonly assumed. Because oceanic OMZs seem to be expanding because of global climate change, it is increasingly imperative to incorporate the correct nitrogen-loss pathways in global biogeochemical models to predict more accurately how the nitrogen cycle in our future ocean may respond. </jats:p>