The Denitrifying Prokaryotes: Defining the Denitrifiers

Abstract

One of the hallmarks of bacterial physiology is the ability to use a wide variety of substrates as oxidants or terminal oxidants for respiration. Because of its high redox potential and prevalence, oxygen is generally the preferred reductant for respiratory bacteria. However, bacteria can utilize many other compounds as terminal respiratory oxidant. One of these compounds is nitrate. Nitrate respiration occurs via two dissimilar pathways that utilize the same initial substrate but produce different end products. One of these pathways, termed ammonification, is carried out by bacteria such as Escherichia coli, and is marked by reduction of nitrate to nitrite and then to ammonia. The second pathway of nitrate respiration is denitrification, which is the reduction of nitrate to gaseous nitrogen oxides, principally nitrogen gas (Fig. 1). The initial step in denitrification is the reduction of nitrate to nitrite, as occurs in ammonification. In the next step, the defining reaction, nitrite is reduced to nitric oxide, a gaseous nitrogen oxide. This conversion of a fixed, non-gaseous form of nitrogen to gaseous forms has led this respiratory process to be termed “denitrification” because biologically preferred forms of nitrogen are lost. Once nitric oxide is produced, it is further reduced to nitrous oxide and then to nitrogen gas. The production of nitrogen gas connects denitrification to the nitrogen cycle via nitrogen fixation. The ammonia produced by nitrogen fixation can be converted by nitrifying bacteria to nitrite and nitrate, the substrates of denitrification. This series of reductions and oxidation reactions constitute the nitrogen cycle (cont in article)