Denitrification

Denitrification is a microbially facilitated process of nitrate reduction that may ultimately produce molecular nitrogen (N₂) through a series of intermediate gaseous nitrogen oxide products.

This respiratory process reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter. The preferred nitrogen electron acceptors in order of most to least thermodynamically favorable include nitrate (NO₃−), nitrite (NO₂−), nitric oxide (NO), nitrous oxide (N₂O) and dinitrogen (N₂). In terms of the general nitrogen cycle, denitrification completes the cycle by returning N₂ to the atmosphere.

The process is performed primarily by heterotrophic bacteria (such as Paracoccus denitrificans and various pseudomonads), although autotrophic denitrifiers have also been identified (e.g., Thiobacillus denitrificans). Denitrifiers are represented in all main phylogenetic groups. Generally several species of bacteria are involved in the complete reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway have been identified in the reduction process.

Direct reduction from nitrate to ammonium, a process known as dissimilatory nitrate reduction to ammonium or DNRA, is also possible for organisms that have the nrf-gene. This is less common than denitrification in most ecosystems as a means of nitrate reduction. Other genes known in microorganisms which denitrify include nir (nitrite reductase) and nos (nitrous oxide reductase) among others; organisms identified as having these genes include Alcaligenes faecalis, Alcaligenes xylosoxidans, many in the Pseudomonas genus, Bradyrhizobium japonicum, and Blastobacter denitrificans.