10.2.3: Interactions of anammox microorganisms with other nitrogen cycles

Anaerobic ammonia oxidation (anammox) is thought to be responsible for consuming a large portion of the fixed nitrogen in a wide range of environments, as discussed in Section 5.2.4. The reaction combines oxidation of ammonia with reduction of nitrite: \[\text{NH}_{4}^{-} + \text{NO}_{2}^{-} \longleftrightarrow \text{N}_{2} + 2 \ \text{H}_{2} \text{O}\]Therefore, anammox microorganisms can only drive the reaction forward where ammonium and nitrite are simultaneously available.

Multiple groups of nitrogen cyclers influence the availability of ammonium and nitrate, creating opportunities for interactions with microorganisms that catalyze anammox. Nitrifying microorganisms can supply nitrite for anammox, but nitrifiers also consume nitrite as well as ammonium (Fig. \(10.2\)). Similarly, denitrifying microorganisms can also supply and consume nitrite. Microbes that catalyze dissimilatory nitrate reduction to ammonia (DNRA) can supply ammonia, but they also produce and consume nitrite, like denitrifiers and nitrifiers. Lastly, numerous groups of microorganisms also consume and produce ammonium and other reactive nitrogen compounds in assimilation and ammonification reactions. Thus, several groups of nitrogen-cycling microorganisms can have both mutualistic and competitive interactions with microorganisms that catalyze anammox (Kuenen, 2020).

One of the main environmental factors that influences the outcome of these interactions is oxygen availability. Anammox often occurs at the interface of oxic and anoxic conditions (Kuenen, 2020). Near the interface, where oxygen concentration are low, nitrifiers may form a positive relationship with anammox microorganisms by oxidizing only part of the available ammonium to nitrite (Yan et al., 2012). Moreover, on the anoxic side of the interface, anaerobic nitrate reduction via denitrification and DNRA can occur, which collectively have the potential supply both nitrite and ammonia for anammox.

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