Anammox

Anammox, an abbreviation for "anaerobic ammonium oxidation", is a globally important microbial process of the nitrogen cycle[1] that takes place in many natural environments.

In this biological process, which is a redox comproportionation reaction, nitrite and ammonium ions are converted directly into a diatomic molecule of nitrogen and water.

[18] In 1932, it was reported that dinitrogen gas was generated via an unknown mechanism during fermentation in the sediments of Lake Mendota, Wisconsin, USA.

[21] In 1977, Engelbert Broda predicted the existence of two chemolithoautotrophic microorganisms capable of oxidizing ammonium to dinitrogen gas on the basis of thermodynamic calculations.

[22][23] It was thought that anaerobic oxidation of ammonium would not be feasible, assuming that the predecessors had tried and failed to establish a biological basis for those reactions.

Based on a previous study, Strous et al.[28] calculated the stoichiometry of anammox process by mass balancing, which is widely accepted by other groups.

[30] In 2002 however, anammox was found to play an important part in the biological nitrogen cycle, accounting for 24–67% of the total N2 production in the continental shelf sediments that were studied.

According to 15N labeling experiments carried out in 1997, ammonium is biologically oxidized by hydroxylamine, most likely derived from nitrite, as the probable electron acceptor.

[37] Recently, using detailed molecular analyses and combining complementary methods, Kartal and coworkers published strong evidence supporting the latter mechanism.

Environmental genomics analysis of the species Candidatus Kuenenia stuttgartiensis, through a slightly different and complementary metabolism mechanism, suggested NO to be the intermediate instead of hydroxylamine (Figure 4).

In the absence of dissolved oxygen, nitrite, or nitrate, microbes living in the anode compartment are able to oxidize ammonium to dinitrogen gas (N2) just as in the classical anammox process.

The sequence identities of the anammox 16S rRNA genes range from 87 to 99%, and phylogenetic analysis places them all within the phylum Planctomycetota,[61] which form the PVC superphylum together with Verrucomicrobia and Chlamydiae.

Their phylogenetic position together with a broad range of specific physiological, cellular, and molecular traits give anammox bacteria their own order Brocadiales.

[64][65] For the enrichment of the anammox organisms a granular biomass or biofilm system seems to be especially suited in which the necessary sludge age of more than 20 days can be ensured.

[68] Conventional nitrogen removal from ammonium-rich wastewater is accomplished in two separate steps: nitrification, which is mediated by aerobic ammonia- and nitrite-oxidizing bacteria and denitrification carried out by denitrifiers, which reduce nitrate to N2 with the input of suitable electron donors.

However, in partial nitritation/anammox systems, oxygen demand is greatly reduced because only half of the ammonium needs to be oxidized to nitrite instead of full conversion to nitrate.

[69] Additionally, anammox bacteria easily form stable self-aggregated biofilm (granules) allowing reliable operation of compact systems characterized by high biomass concentration and conversion rate up to 5–10 kg N m−3.

[70] Overall, it has been shown that efficient application of the anammox process in wastewater treatment results in a cost reduction of up to 60%[71][72] as well as lower CO2 emissions.

A bioreactor containing the anammox bacterium Kuenenia stuttgartiensis
C17-C20 ladderane lipids from anammox bacteria containing either three linearly fused cyclobutane rings and one cyclohexane or five cyclobutane rings. Fatty acids are esterified with methanol or the glycerol backbone, and the ladderane alcohols are ether-linked with glycerol, all in different combinations. [ 7 ]
Figure 2. The biological nitrogen cycle, with dissimilatory nitrate reduction to ammonium
Figure 3. Possible biochemical pathway and cellular localization of the enzyme systems involved in anammox reaction.
Figure 4. Hypothetical metabolic pathways and reversed electron transport in the anammoxosome. (a) Anammox catabolism that uses nitrite as the electron acceptor for the creation of a proton motive force over the anammoxosomal membrane. (b) Proton motive force- driven reversed electron transport combines central catabolism with nitrate reductase (NAR) to generate ferredoxin for carbon dioxide reduction in the acetyl-CoA pathway. HAO, hydrazine oxidoreductase ; HD, hydrazine dehydrogenase; HH, hydrazine hydrolase; NIR, nitrite oxidoreductase; Q, quinine. Light blue diamonds, cytochromes; blue arrows, reductions; pink arrows, oxidations.