Denitrifiers

Denitrifiers are microorganisms that perform denitrification, a step in the nitrogen cycle in which nitrate (NO3−) is reduced to gaseous nitrogen compounds such as nitrite (NO2−), nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2). In most denitrifiers, nitrate serves as a terminal electron acceptor when oxygen is limited.

Denitrification proceeds through a sequential set of enzyme-catalyzed reductions: nitrate reductases (nar or nap) reduce NO3− to NO2−; nitrite reductases (nir) reduce NO2− to NO; nitric oxide reductases (nor) convert NO to N2O; and nitrous oxide reductases (nos) reduce N2O to N2. The balance of end products depends on environmental conditions and the genetic makeup of the organism; incomplete denitrification can release N2O, a potent greenhouse gas.

Denitrification is carried out by diverse bacteria and some archaea. Commonly cited laboratory denitrifiers include Paracoccus denitrificans and Pseudomonas stutzeri; Thiobacillus denitrificans is an example of a chemolithoautotrophic denitrifier. Denitrification occurs in many habitats, including soils, freshwater and marine sediments, wetlands, and groundwater.

Ecologically, denitrification helps remove excess nitrate from soils and aquatic systems, contributing to nitrogen retention and reducing pollution. In sediments and wetlands it can be a major pathway for nitrate removal. However, incomplete denitrification can release nitrous oxide, a greenhouse gas, and reliance on nitrate as the electron acceptor links denitrification to fluctuations in oxygen and carbon availability. Denitrification is also important in wastewater treatment, where controlled denitrification reduces nitrate levels before discharge.

Genetics and regulation. The enzymes are encoded by nar and nap nitrate reductases, nir nitrite reductase, nor nitric oxide reductase, and nos nitrous oxide reductase. Their expression is regulated by oxygen levels, nitrate availability, carbon source, and redox state.