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Assignment 3: Denitrification
Original- "Denitrification"

Conditions required
Denitrification takes place under special conditions in both terrestrial and marine ecosystems. In general, it occurs where oxygen, a more energetically favourable electron acceptor, is depleted, and bacteria respire nitrate as a substitute terminal electron acceptor. Due to the high concentration of oxygen in our atmosphere, denitrification only takes place in anoxic environments where oxygen consumption exceeds the oxygen supply and where sufficient quantities of nitrate are present. These environments may include certain soils and groundwater, wetlands, oil reservoirs, poorly ventilated corners of the ocean, and in seafloor sediments.

Denitrification generally proceeds through some combination of the following intermediate forms:
 * − → − → NO + →  (g)

The complete denitrification process can be expressed as a redox reaction:
 * 2 NO3− + 10 e− + 12 H+ → N2 + 6 H2O

This reaction shows a fractionation in isotope composition. Lighter isotopes of nitrogen are preferred in the reaction, leaving the heavier nitrogen isotopes in the residual matter. The process can cause delta-values of up to −40, where delta is a representation of the difference in isotopic composition. This can be used to identify denitrification processes in nature.

Edits- "Denitrification"

Half Reactions
Denitrification generally proceeds through some combination of the following half reactions, with the enzyme catalyzing the reaction in parentheses: The complete process can be expressed as a net balanced redox reaction, where nitrate (NO3−) gets fully reduced to dinitrogen (N2):
 * NO3− + 2 H+ + 2 e−→ − + H2O  (Nitrate reductase)
 * − + 2 H+ + e− →  NO + H2O  (Nitrite reductase)
 * 2 NO + 2 H+ + 2 e− →   + H2O (Nitric oxide reductase)
 * + 2 H+ + 2 e− →  + H2O  (Nitrous oxide reductase)
 * 2 NO3− + 10 e− + 12 H+ → N2 + 6 H2O

Conditions of Denitrification
In nature, denitrification can take place in both terrestrial and marine ecosystems. Typically, denitrification occurs in anoxic environments, where the concentration of dissolved and freely available oxygen is depleted. In these areas, nitrate (NO3−) or nitrite (−) can be used as a substitute terminal electron acceptor instead of oxygen (O2), a more energetically favourable electron acceptor. Terminal electron acceptor is a compound that gets reduced in the reaction by receiving electrons. Examples of anoxic environments can include soils, groundwater , wetlands, oil reservoirs , poorly ventilated corners of the ocean and seafloor sediments.

Furthermore, denitrification can occur in oxic environments as well. High activity of denitrifers can be observed in the intertidal zones, where the tidal cycles cause fluctuations of oxygen concentration in sandy coastal sediments. For example, the bacterial species Paracoccus denitrificans engages in denitrification under both oxic and anoxic conditions simultaneously. Upon oxygen exposure, the bacteria is able to utilize nitrous oxide reductase, an enzyme that catalyzes the last step of denitrification.

Denitrification can lead to a condition called isotopic fractionation in the soil environment. The two stable isotopes of nitrogen, N14 and N15 are both found in the sediment profiles. The lighter isotope of nitrogen, N14, is preferred during denitrification, leaving the heavier nitrogen isotope, N15, in the residual matter. This selectivity leads to the enrichment of N14 in the biomass compared to N15. Moreover, the relative abundance of N14 can be analyzed to distinguish denitrification apart from other processes in nature.

se110301 (talk) 07:47, 20 November 2017 (UTC)