The Black Sea is an inland body of water with a surface area of roughly 422,000 km2 and a maximum depth of 2,210 metres . Located in southeastern Europe, it is bound by the borders of Russia, Ukraine, Georgia, Turkey, Bulgaria and Romania . It is connected to the Atlantic Ocean through the Marmara, Aegean and Mediterranean Seas via the Bosphorus and Dardanelles Straits. The Black Sea is classified as a meromictic body of water. The McGraw Hill Science Encyclopedia  defines a meromictic lake as having two distinct layers, the top mixolimnion and the lower monimolimnion, that do not mix. The top layer of the Black Sea is approximately 50 to 100 metres deep and is exposed to oxygen in the atmosphere; the bottom layer is 2000 metres deep and has no dissolved oxygen . What makes the Black Sea unique is that it is not a lake, but a large basin in which two distinct layers exist, forming the largest anoxic body of water on Earth . The sea formed thousands of years ago after glaciers from the ice age retreated and left a large freshwater lake in which saltwater from the Mediterranean Sea flowed in. The saltwater sank to the bottom of the lake and led to stratification based on density differences . This separation between the oxic and anoxic zones allows for various microorganisms to inhabit the different environments . Aerobic organisms can be found in upper layer, whereas anaerobic organisms are found at greater depths below the surface. The interface between the oxic and anoxic zones, referred to as the suboxic, holds a high density of microbes, with aerobic and anaerobic bacteria living within metres of each other . The Black Sea serves as a model ecosystem for studying microbial life, as it functions as a redox gradient in which microorganisms respire through various redox reactions .
Chemical and Physical Characteristics
The physical environment of the Black Sea contributes to the non-mixing layers of water. The temperature at the surface varies during the seasons, falling as low as -0.5⁰C during the winter and rising as high as 26⁰C during the summer . At depths of 50 -100 metres the temperature ranges from 6 - 8⁰C and at depths over 400 metres the temperature does not fluctuate during the seasons, remaining around 8⁰C . The rapidly changing temperatures at the surface of the sea helps prevent the mixolimnion from mixing with the monimolimnion and being exposed to the atmosphere .
Salinity plays a role in the density gradient that results from the dissolved salt. The surface waters hold approximately 17 parts per thousand of salt. At depths below 400 metres, the salt concentration reaches over 30 parts per thousand . This further separates the lower mixolimnion from the upper monimolimnion waters with similar temperature ranges .
The dissolved oxygen content is what differentiates the mixolimnion from the monimolimnion layer. The upper mixolimnion holds over 200 µM of dissolved oxygen, while the interface suboxic zone has 75 µM of dissolved oxygen as a result of aerobic respiration being performed by microorganisms above . Below 130 metres, no oxygen is present . These oxic and anoxic zones allow for a redox gradient to form. A redox gradient shows depletion of terminal electron acceptors in order of decreasing redox potential for the respiratory processes occurring. Oxygen, the best electron acceptor, is preferentially used first and is depleted by aerobic organisms found in the mixolimnion . This affects the microbial activity and rates of organic matter decomposition at greater depths . Redox processes are normally found in sediments of centimetre intervals, but the Black Sea’s chemical processes occur over hundreds of metres .
Schubotz et al. used membrane lipid analysis to determine the presence of various aerobic bacteria in the Black Sea . The oxic zone includes various cyanobacteria, methanotrophs, and nitrifying bacteria. Some specific classes include Actinobacteria, Gammaproteobacteria and Alphaproteobacteria .
This interface holds both aerobic and anaerobic microbes. These include cyanobacteria, denitrifying bacteria, methanotrophs, methanogens, iron-reducing bacteria, manganese-reducing bacteria, anammox bacteria, and purple sulfur bacteria . Specific bacteria include Planctomycetes , Deltaproteobacteria  and the genera of Arcobacter and Pelobacter .
All these organisms respire by anaerobic processes and cannot tolerate oxygen. Microbes include sulfate-reducing bacteria (upper part of anoxic zone), purple and green sulfur bacteria, fermenting bacteria, methanotrophs, methanogens, manganese-reducing bacteria, anammox bacteria and various Archaea .
Cyanobacteria in the oxic zone derive their energy through the process of photosynthesis . Photosynthesis involves energy from photons used to fix carbon dioxide into sugars and oxygen . The oxygen and sugars produced can be used by aerobic bacteria to respire (use oxygen as a terminal electron acceptor), and in return, these aerobic bacteria provide cyanobacteria with carbon dioxide from the oxidation of sugars.
Denitrification is performed by nitrate and nitrite reducing bacteria found in the suboxic zone . Denitrification uses nitrate (NO3-) or nitrite (NO2-) as terminal electron acceptors, and converts them into the gaseous products nitric oxide (NO), nitrous oxide (N2O) or dinitrogen (N2), oxidizing organic matter in the process . Nitrate is used after oxygen is depleted, indicating it is the best electron acceptor remaining. This explains why the majority of denitrifying bacteria are located just below the oxic zone . Denitrification can have greater effects on the environment, including production of greenhouse gases (N2O), and the degradation of dead organic matter in the sea.
The anammox process primarily occurs in the lower suboxic zone and the anoxic zone of the Black Sea . The process involves the reduction of nitrite and oxidation of ammonium into dinitrogen: NH4+ + NO2− → N2 + 2H2O. It is performed by the Planctomycetes  and is used to release nitrogen into the atmosphere.
A high concentration of carbon dioxide is found in the deepest parts of The Black Sea due to gas seeps in the seafloor and is used by methanogens that respire anaerobically and produce methane. Carbon dioxide has the lowest redox potential of the electron acceptors (NO3-, O2, Fe3+, SO42-) in the Black Sea and thus is limited to microbes at the bottom of the sea .
The Black Sea has over 3000 gas seeps, releasing methane (CH4) from deep-sea mud volcanoes . Methanotrophs can oxidize methane aerobically and anaerobically , yet the majority are found in the anoxic zone where the concentration of methane is the highest . Aerobic methanotrophs oxidize methane using oxygen as the electron acceptor producing formaldehyde . Anaerobic methanotrophs are sulfate-reducing bacteria . Oxygen and nitrate are depleted at the anoxic zone, thus sulfate is the next best terminal electron acceptor that microbes use to respire and oxidize methane into other organic compounds .
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