5.2.4: Anaerobic ammonia oxidation (anammox)

Anammox provides an anaerobic pathway for ammonia oxidation. In the reaction, nitrite produced during nitrification (reaction \((5.2.3.1)\)) is reduced with ammonia as the electron donor, as shown in the following example reaction: \[\text{NH}_{4}^{+} + \text{NO}_{2}^{-} \longleftrightarrow \text{N}_{2} + 2 \ \text{H}_{2} \text{O}\]

The reaction proceeds through two intermediate compounds, nitric oxide \((\text{NO})\) and hydrazine \(\left(\text{N}_{2} \text{H}_{4}\right)\) (Kartal and Keltjens, 2016; van Niftrik and Jetten, 2012) (Fig. \(5.3\)). Interestingly, hydrazine is used as a rocket fuel.

So far, anammox has only been demonstrated in members of the Brocadiales, a bacterial order within phylum Planctomycetes (Jetten et al., 2010; Strous et al., 1999). Their metabolism is chemolithotrophic and has been documented in a wide range of environments, including marine sediments, aquifers, and freshwater aquatic habitats. In marine environments, it has been shown to be capable of consuming more than 50% of the fixed nitrogen in dead biomass (Dalsgaard et al., 2003; Kuypers et al., 2005; Thamdrup and Dalsgaard, 2002). As a second example, Kumar et al. (2017) found that anammox was response for 83% of the nitrogen loss from low-oxygen zones of a pristine limestone aquifer in Germany.

Anammox involves both oxidized and reduced forms of nitrogen and may be particularly common near transitions between oxic and anoxic conditions (Kuenen, 2020; Zhu et al., 2013). The nitrite can be generated by nitrification and denitrification and the ammonium from ammonification, nitrogen fixation, and dissimilatory nitrate reduction to ammonia (DNRA).

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