Denitrification

Respiration is a chemical process in which energy is released when electrons are passed from a donor molecule to an acceptor molecule. In addition to energy being released, the respiratory process results in the donor molecule being converted to an oxidized molecule, meaning it has lost electrons, and the acceptor molecule being converted to a reduced molecule, meaning it has gained electrons. Typically, the acceptor molecule is oxygen, but in anaerobic environments, which lack oxygen, bacteria may reduce molecules other than oxygen that have high reduction potentials or ability to accept electrons in a process known as anaerobic respiration. Denitrification occurs when bacteria reduce nitrate or nitrite by this process. In a sequence of four reductions, nitrate is converted to dinitrogen gas, the molecular form in which nitrogen escapes from soils and aquatic systems. The four-step sequence is: 1.) Nitrate is reduced to nitrite. 2.) Nitrite is reduced to nitric oxide. 3.) Nitric oxide (NO) is reduced to nitrous oxide. 4.) Nitrous oxide is reduced to dinitrogen. Depending on its physiological capabilities, a single organism may carry out all of these reductions, or it may carry out only a few.

In addition to dinitrogen, small amounts of nitrous oxide leave aquatic and soil systems. This happens because not all bacteria that produce nitrous oxide can subsequently reduce it to dinitrogen. Therefore, some nitrous oxide can leak out of cells into the atmosphere, if it is not first reduced to dinitrogen by other organisms. Nitric oxide is also a gas, but organisms that have the ability to reduce nitrite to nitric oxide always have the ability to reduce nitric oxide to nitrousoxide. For this reason, nitric oxide is not an important product of denitrification.

In aquatic and soil systems fixed nitrogen primarily exists as a component of three inorganic molecules; nitrate, nitrite, and ammonium (NH₄⁺), and in the proteins and other types of organic molecules that comprise living and dead organisms. Although only nitrogen from the molecules nitrate and nitrite is converted to a gaseous form and removed from these systems, nitrogen from proteins and ammonium can also be removed if it is first oxidized to nitrate or nitrite. This conversion begins in a process termed ammonification, when nitrogen is released from the biomass of dead organisms, which produces ammonium. Ammonium can then be converted to nitrate in an aerobic respiratory reaction called nitrification, in which the ammonium serves as an electron donor, and oxygen an electron acceptor.