Anammox Physiology and Metabolism

All currently known bacteria capable of anaerobic ammonium oxidization belong to a deep-branching lineage of the order Planctomycetales with high genus level diversity (Freitag and Prosser 2003; Schmid et al. 2003). The evolutionary distance among the anammox genera is large (<85% 16S rRNA gene nucleotide identity), though they share the same basic anammox metabolism and cell structure. There are currently four Candidatus genera whose grouping is largely based on 16S rRNA sequences: the "freshwater" Kuenenia (K. stuttgartiensis; Schmid et al. 2000) and Brocadia (B. anammoxidans (5) and B. fulgida (22) ), and the "marine" anammox Scalindua (S. sorokinii, S. brodae, and S. wagneri; Schmid et al. 2003). The fourth Candidatus genus has one member, Anammoxoglobus propionicus (Kartal et al. 2007b), which exhibits an alternative metabolism. Anammox bacteria are characterized by a membrane-bound organelle called the anammoxosome that comprises more than 30% of the cell volume. This intracytoplasmic compartment is surrounded by unique lipids, called ladderanes (Sinninghe Damste et al. 2002) that are unique to the anammox bacteria. Ether and ester linkages tie the lipids to a glycerol backbone in the membrane which has historically only been found in members of the domain Archaea and may reflect an early divergence of anammox in the bacterial lineage (Brochier and Philippe 2002). Due to a very dense arrangement of carbon atoms, the ladderane lipids serve as a diffusion barrier (Sinninghe Damste et al. 2002). This may serve to protect the bacteria from the toxic anammox reaction intermediates hydroxylamine and hydrazine (Jetten et al. 2003). Due to their unique characteristics, ladderane lipids have also been used as a biomarker for the presence of anammox bacteria (Kuypers et al. 2003).

Evidence from the genome of Candidatus K. stuttgartiensis (Strous et al. 2006) indicates that the anammox reaction proceeds via the following steps:

The anammox hydroxylamine oxidoreductase (HAO) enzyme is responsible for the oxidation of hydrazine to N2 gas and is located exclusively within the anammoxo-some (Lindsay et al. 2001), a possible target for future molecular studies. The highly reactive hydrazine intermediate is stored inside the anammoxosome (Sinninghe Damste et al. 2002), which is especially important considering the slow enzymatic turnover, resulting in a doubling time of 9 days in optimal conditions for the "freshwater" anammox (Strous et al. 1999a, b). Anammox are reversibly inhibited by O2, and reaction rates are the same after as before aeration (Jetten et al. 1999).

Anammox bacteria have been found to be metabolically flexible, exhibiting alternative metabolic pathways. For instance, anammox can subsequently reduce nitrate to nitrite to ammonium, followed by the conversion of ammonium and nitrite to N2 through the anammox pathway, allowing anammox bacteria to overcome ammonium limitation. Anammox bacteria are also a potential source of N2O production by nitric oxide detoxification (Kartal et al. 2007a). Currently the other known processes that produce N2O are nitrification and denitrification (Fig. 10.1. As such, classical denitrification measures that depend exclusively on N2O measures may overstate the role of denitrification in the system. Another alternative pathway is carried out by Candidatus Anammoxoglobus propionicus, which has been shown to co-oxidize propionate and ammonium, and out-compete denitrifiers and other anammox bacteria in the process (Kartal et al. 2007b). This supports the niche differentiation of anammox in which different "ecotypes" dominate specific habitats, and may be the reason why two different anammox species are not commonly found in the same sample. Lastly, iron and manganese oxides have also been found to be respired with formate as an electron donor (Strous et al. 2006), further expanding the metabolic diversity of the anammox bacteria.

Organic N

Organic N

Aerobic

Anaerobic

Fig. 10.1 Anaerobic ammonium oxidation pathway of nitrogen removal in context of the current nitrogen cycle

Aerobic

Anaerobic

Conical DerVrtffö03^

Fig. 10.1 Anaerobic ammonium oxidation pathway of nitrogen removal in context of the current nitrogen cycle

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