5.2.5: Denitrification

Nitrate can be reduced for assimilatory or dissimilatory purposes. For assimilatory nitrate reduction, nitrate is reduced to ammonia, which can then be used to build biological molecules that contain nitrogen (N_org) (Fig. \(5.3\)). For dissimilatory reduction, there are two pathways: denitrification and dissimilatory nitrate reduction to ammonia (DNRA; see section 5.2.6).

Denitrification involves the reduction of nitrate via a chain of microbial reactions ultimately to dinitrogen \(\left(\text{N}_{2}\right)\), as illustrated by the following reduction half reactions: \[\begin{align} & \text{NO}_{3}^{-} + 2 \ \text{H}^{+} + 2 \ e^{-} \longleftrightarrow \text{NO}_{2}^{-} + \text{H}_{2} \text{O} \\ & \text{NO}_{2}^{-} + 2 \ \text{H}^{+} + e^{-} \longleftrightarrow \text{NO} + \text{H}_{2} \text{O} \\ & 2 \ \text{NO} + 2 \ \text{H}^{+} + 2 \ e^{-} \longleftrightarrow \text{N}_{2} \text{O} + \text{H}_{2} \text{O} \\ & \text{N}_{2} \text{O} + 2 \ \text{H}^{+} + 2 \ e^{-} \longleftrightarrow \text{N}_{2} + \text{H}_{2} \text{O} \end{align}\]

Denitrifying microorganisms can reduce nitrate rapidly to dinitrogen under favorable conditions. However, the chain of reactions may also stop at one of the intermediate nitrogen species such as nitrous oxide \(\left(\text{N}_{2} \text{O}\right)\) or nitric oxide \((\text{NO})\). Electrons necessary to drive the reduction reactions can come from a wide range of organic compounds as well as dihydrogen \(\left(\text{H}_{2}\right)\), ferrous iron \((\text{Fe(II)})\), reduced sulfur compounds (e.g., sulfide, elemental sulfur, and thiosulfate), iron sulfide solid-phases (e.g., \(\text{FeS}\)), arsenite \((\text{As(III)})\), and reduced manganese \((\text{Mn(II)})\) (Pang and Wang, 2021). Denitrification is thought to primarily occur in anoxic environments. However, some bacteria can use the reaction in oxic environments (Ji et al., 2015).

Denitrification can be carried out by numerous groups of Bacteria, including organotrophs, lithotrophs, phototrophs (Eldor, 2015) as well as Archaea and Eukaryotes. Some eukaryotic microorganisms are able to store nitrate intracellularly and even use it for dissimilatory nitrate reduction where oxygen is not available (Kamp et al., 2015)

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