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...

Summary Anaerobic ammonium‐oxidizing (anammox) bacteria are responsible for a significant portion of the loss of fixed nitrogen from the oceans, making them important players in the global nitrogen cycle. To date, marine anammox bacteria found in marine water columns and sediments worldwide belong almost exclusively to the ‘ Candidatus Scalindua’ species, but the molecular basis of their metabolism and competitive fitness is p...