Massive nitrogen loss from the Benguela upwelling system through anaerobic ammonium oxidation

TLDR

Phytoplankton growth in many oceanic regions is limited by nitrogen because fixation of N₂ cannot offset losses of fixed inorganic nitrogen, with 30–50 % of global nitrogen loss occurring in oxygen‑minimum zones and traditionally attributed to denitrification. The study hypothesizes that anammox drives significant nitrogen loss in other oceanic oxygen‑minimum zones. In the Benguela upwelling system, anammox bacteria—not denitrification—are the primary drivers of massive fixed‑nitrogen loss in the oxygen‑minimum zone, as evidenced by nutrient profiles, high anammox rates, cell abundances, and specific biomarker lipids.

Abstract

In many oceanic regions, growth of phytoplankton is nitrogen-limited because fixation of N 2 cannot make up for the removal of fixed inorganic nitrogen (NH + 4 , NO - 2 , and NO - 3 ) by anaerobic microbial processes. Globally, 30-50% of the total nitrogen loss occurs in oxygen-minimum zones (OMZs) and is commonly attributed to denitrification (reduction of nitrate to N 2 by heterotrophic bacteria). Here, we show that instead, the anammox process (the anaerobic oxidation of ammonium by nitrite to yield N 2 ) is mainly responsible for nitrogen loss in the OMZ waters of one of the most productive regions of the world ocean, the Benguela upwelling system. Our in situ experiments indicate that nitrate is not directly converted to N 2 by heterotrophic denitrification in the suboxic zone. In the Benguela system, nutrient profiles, anammox rates, abundances of anammox cells, and specific biomarker lipids indicate that anammox bacteria are responsible for massive losses of fixed nitrogen. We have identified and directly linked anammox bacteria to the removal of fixed inorganic nitrogen in the OMZ waters of an open-ocean setting. We hypothesize that anammox could also be responsible for substantial nitrogen loss from other OMZ waters of the ocean.