Nitrous oxide turnover in the suboxic zone of the Black Sea

Abstract 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 minor N 2 O emitter. We measured persistent potential N 2 O formation from ammonia oxidation (up to 0.2 nmol N L −1 d −1 ) and sporadic rates of high N 2 O production from denitrification (up to 20 nmol N L −1 d −1 ) in the suboxic zone of the western Black Sea. Molecular analysis identified the Nitrososphaerales as the dominant ammonia oxidizers, while the genetic potential to form N 2 O from nitrite via denitrification was mainly encoded by Alpha‐ and Gammaproteobacteria . The measured potential N 2 O reduction rates (up to 24 nmol N L −1 d −1 ) could exceed N 2 O formation rates from both nitrite and ammonia, with the capacity for N 2 O reduction mainly encoded and expressed by presumably chemolithotrophic Gammaproteobacteria and Marinisomatia . Our results suggest that, despite the diverse pathways of N 2 O production operating in the Black Sea, the low emissions likely result from tight coupling of N 2 O production and consumption, facilitated by the presence and activity of organisms that seem to be specialized in N 2 O reduction.