The Black Sea is the largest meromictic marine basin characterized by well‐oxygenated surface waters and a deep sulfidic anoxic core, separated by a ca. 50‐m‐thick suboxic zone. Nitrogen cycling at oxic‐anoxic boundaries of oxygen‐depleted marine environments has been identified as a major source of nitrous oxide (N 2 O), a potent greenhouse gas. Contrary to this, previous work suggests that the Black Sea is considered only a...

Summary Fixed nitrogen is released by anaerobic ammonium oxidation (anammox) and/or denitrification from (marine) ecosystems. Nitrite, the terminal electron acceptor of the anammox process, occurs in nature at very low concentrations and is produced via (micro)aerobic oxidation of ammonium or nitrate reduction. The coupling of sulfide‐dependent denitrification to anammox is particularly interesting because besides hydrogen, su...