Probing nitrogen metabolism in the redox gradient of the Black Sea.

T he Black Sea, earth’s largest anoxic water body, serves as a model system for studies of marine chemistry and biology. The article by Lam et al. (1) in this issue of PNAS describes a new approach to assessing the activity of Black Sea nitrifying bacteria and archaea that are globally important in nitrogen metabolism. The Black Sea was formed after the retreat of the glaciers in the most recent warming period of the Pleistocene, 10,000 years ago. As water levels rose, water flowed from the Mediterranean Sea through the Sea of Marmara into the isthmus between what is now termed the Eastern and Western worlds, thereby forming the Bosporus Strait. The seawater flowed into a large freshwater lake that received runoff from the Danube and other rivers of Eastern Europe and Asia. The more dense seawater from the Bosporus sank to the lowest depths of this prehistoric lake, resulting in the formation of a salinity gradient and density stratification. The aerobic, fresh surface waters of the Black Sea sustain a fishery that extends to a depth of 85 m. Beneath that depth, oxygen is depleted to a concentration of 5 M in a redox zone that is referred to as the suboxic zone (2). The suboxic zone, which is 30 m in depth, harbors important nitrogen cycling metabolisms including nitrification, denitrification, and anammox (anaerobic ammonia oxidation). Below the suboxic zone lies the sulfidic zone, dominated by sulfate reduction, which extends to the profundal depth of 2,200 m. Methanotrophic bacteria and archaea responsible for the anaerobic oxidation of methane are sulfate-reducers that reside in the sulfidic zone (3) and completely use the methane released in bubbles from gas seeps from the Black Sea’s sediments (4).