Anaerobic Nitrogen Cycling: Tales from the Eastern Tropical Pacific

A variety of incubation experiments were conducted during two cruises in the eastern tropical North and South Pacific oxygen deficient zones (ODZs), targeting the various anaerobic microbial metabolisms – anammox, denitrification, nitrate reduction, and nitrite oxidation – known to occur in these regions. Combined, we show the imbalance of these processes leads to the large nitrite accumulation observed throughout the global ODZs. Nitrogen loss is attributed to both anammox and denitrification, in set ratios that are determined primarily by the organic matter stoichiometry supplied from the euphotic layer overlying the anoxic zone. These ratios, however, are further impacted by oxygen concentrations, whereby anammox rates are less affected than denitrification at equivalent in situ conditions. We observe that anammox represents between 30% and 85% of the fixed nitrogen loss from these two regions, with the higher proportion occurring shallower in the ODZ. We also show there is a rapid cycling between nitrite and nitrate in the absence of oxygen that can help explain the elevated anammox rates.