Aerobic denitrification or co-respiration the simultaneous use of both oxygen (O2) and nitrate (NO3-) as oxidizing agents, performed by various genera of microorganisms. This process differs from anaerobic denitrification not only in its insensitivity to the presence of oxygen, but also in that it has a higher potential to create the harmful byproduct nitrous oxide.
Nitrogen, acting as an oxidant, is therefore reduced in a succession of four reactions performed by the enzymes nitrate, nitrite, nitric-oxide, and nitrous oxide reductases. The pathway ultimately yields reduced molecular nitrogen (N2), as well as, when the reaction does not reach completion, the intermediate species nitrous oxide (N2O). A simple denitrification reaction proceeds as:
NO3− → NO2− → NO + N2O → N2 (g)
The respiration reaction which utilizes oxygen as the oxidant is:
C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O
Classically, it was thought that denitrification would not occur in the presence of oxygen since there seems to be no energetic advantage to using nitrate as an oxidant when oxygen is available. Experiments have since proven that denitrifiers are often only facultative anaerobes and that aerobic denitrification does indeed occur in a broad range of microbial organisms with varying levels of productivity, usually lower productivity than results from purely aerobic respiration. The advantages of being able to perform denitrification in the presence of oxygen are uncertain, though it is possible that the ability to adapt to changes in oxygen levels plays a role.
Denitrifying bacteria are used in waste water treatment plants to remove carcinogenic nitrate ions from drinking water.
Aerobic denitrification tends to produce more nitrous oxide (N2O) than anaerobic denitrification. The nitrous oxide will in turn yield a highly reactive free oxygen radical, which reacts with and depletes ozone. Nitrous oxide also contributes to global warming.