Denitrification is the utilization of nitrate () as a terminal electron acceptor.  It is a widespread process that is used by many members of the Pseudomonadota. Many facultative anaerobes use denitrification because nitrate, like oxygen, has a high reduction potential.  Many denitrifying bacteria can also use ferric iron () and some organic electron acceptors.  Denitrification involves the stepwise reduction of nitrate to nitrite (), nitric oxide (NO), nitrous oxide (), and dinitrogen () by the enzymes nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase, respectively.  Protons are transported across the membrane by the initial NADH reductase, quinones, and nitrous oxide reductase to produce the electrochemical gradient critical for respiration.  Some organisms (e.g. E. coli) only produce nitrate reductase and therefore can accomplish only the first reduction leading to the accumulation of nitrite.  Others (e.g. Paracoccus denitrificans or Pseudomonas stutzeri) reduce nitrate completely.  Complete denitrification is an environmentally significant process because some intermediates of denitrification (nitric oxide and nitrous oxide) are important greenhouse gases that react with sunlight and ozone to produce nitric acid, a component of acid rain.  Denitrification is also important in biological wastewater treatment where it is used to reduce the amount of nitrogen released into the environment thereby reducing eutrophication. Denitrification can be determined via a nitrate reductase test.