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Oxygen Minimum Zones
''' An Oxygen minimum zone (OMZ) is characterized as oxygen deficient layers in the world oceans. Typically found between 200m to 1500m deep below regions of high productivity, such as the western coasts of continents. OMZs can be seasonal following the spring-summer upwelling season. Upwelling of nutrient-rich water leads to high productivity and labile organic matter, that is respired by heterotrophs as it sinks down the water column. High respiration rates deplete the oxygen in the water column to concentrations of 2mg/l or less forming the OMZ. Under these oxygen-starved conditions, energy is diverted from higher trophic levels to microbial communities that have evolved to use other biogeochemical species instead of oxygen, these species include Nitrate, Nitrite, Sulphate etc. Several Bacteria and Archea have adapted to live in these environments by using these alternate chemical species and thrive. The most abundant phyla in OMZs are Proteobacteria, Bacteroidetes, Actinobacteria, and Planctomycetes.

Denitrification
In the absence of oxygen, other biogeochemical species are used in order of the electrochemical series with Nitrate and Nitrite respiration (Denitrification) yielding as much energy as oxygen respiration. Manganese and iodate respiration closely follow nitrate (denitrification), while sulfate and iron reducers are at the bottom of the series yielding the least amount of energy.

Denitrification is the reduction of nitrate and nitrite to nitrous oxide or nitrogen gas, that is released back into the atmosphere. Denitrification closes the nitrogen cycle by reducing the nitrogen available in organic matter fixed by phytoplankton at the surface ocean. Denitrification from OMZ leads to a significant loss of inorganic nitrogen from the oceans and thus limiting growth/productivity in many regions around the world.