Anammox

Anaerobic lithotrophs that oxidize ammonium using nitrite as electron acceptor (anammox) are noted later in the section on anaerobic bacteria. 

Although they have not been obtained in pure culture, chemolithotrophic anaerobic bacteria (anammox) that oxidize ammonia using nitrite as electron donor and CO2 as a source of carbon have been described. In addition, they can oxidize propionate to CO2 (Giiven et al. 2005) by a pathway that has not yet been resolved. 

An important development is the appreciation of the role of anammox bacteria. These carry out the anaerobic reaction between NH4 and NO2 in which nitrite is the electron donor with the production of N2 (Schmid et al. 2005). Several groups of bacteria have been implicated, and all of them belong to the phylum Planctomyces, although quite distinct organisms may be involved. They... 

Schmid MC et al. (2005) Biomarkers for in situ detection of anaerobic ammonium-oxidizing (Anammox) bacteria. Appl Environ Microbiol 71 1677-1684. 

As we mentioned in Chapter 1, we are unsure of the time of the origin of NO in the atmosphere and hence of the beginnings of oxidative processes in membranes associated with nitrogen oxides. One example is provided by Planctomycetes, the anammox bacteria, which carries out the reaction... 

Kuypers, M. et al. (2003). Anaerobic ammonia oxidation by anammox bacteria in the Black Sea. Nature, 422, 608-611... 

Roper, A.J. (1999). Reconstructing early events in eukaryotic evolution. Am. Nat., 154, S146—S163 van Niftrick, L.A., Fuerst, J.A., Sinninghe Damste, J.S., Kuenen, J.G., Jetten, M.G.M. and Srauss, M. (2004). The anammoxone An introcytoplasmic compartment in anammox bacteria. FEBS Microbiol. Lett., 233, 7-13... 

Most recently, a highly unusual membrane composition was reported from anaerobic ammonium-oxidising (anammox) bacteria. In these bacteria, nitrite is reduced, nitrogen gas generated, and carbon dioxide is converted into organic carbon, as the consequence of ammonia reduction. This central energygenerating process can be described as ... 

The anammox catabolism, an exceptionally slow process generating toxic intermediates (hydroxylamine and hydrazin), takes place in an intracytoplasmic compartment called the anammoxosome. A surrounding impermeable membrane protects the cytoplasm from the toxic molecules produced inside this organelle-like structure. Such a tight barrier against diffusion seems to be realised by four-membered aliphatic cyclobutane rings that have been found... 

Recent research suggests that denitrification may involve the reduction of nitrate to nitrite followed by the reaction of nitrite with ammonium in which the nitrogen in nitrite oxidizes the nitrogen in ammonium thereby generating a molecule of NjCg). This is termed the anammox reaction. 

Some of the missing nitrogen is also likely due to the anammox reaction, in which ammonia is oxidized to N2 by anaerobic microbes. 

Recent research has identified some other microbial routes for denitrification that are not heterotrophic. One, called the anammox reaction, involves the oxidation of ammonium to N2 using either nitrite or nitrate as the electron donor. The second has bacteria using Mn " to reduce nitrate to N2. As noted earlier, N2 is generated by the oxidation of ammonium using Mn02 as the electron acceptor. [Denitrification may also be supported by Fe " (aq) oxidation.] These reactions are summarized in Table 12.2. The overall consequence of these reactions is that ammonium does not accumulate in the pore waters where Mn respiration and denitrification are occurring. 

The biogeochemical cycling of nitrogen is very much controlled by redox reactions. This perspective is presented in Figure 24.3 for the redox reactions that take place in the water column and sediments. The major pathways of reduction are nitrogen fixation, assimilatory nitrogen reduction, and denitrification. The major oxidation processes are nitrification and anaerobic ammonium oxidation (anammox). Each of these is described next in further detail. 

Before the recent discovery of anammox, most of the loss of fixed nitrogen from the ocean was assumed to be occurring via denitrification. We now know that some, if not most, of this loss is truly due to anammox. Anammox occurs in coastal sediments, anoxic marginal seas, and in OMZs. The bacteria responsible for anammox are similar to the ones found in wastewater bioreactors (Planctomycetales). 

The global ocean balance between N2 fixation and the loss of fixed nitrogen through anammox and denitrification. Source From Arrigo, K. R. (2005). Nature 437, 349-351. 

Anammox, which involves the anaerobic NH4+ oxidation with NC>3, may be another mechanism whereby N is lost form estuarine/coastal systems as N2. 

Rysgaard, S., Glud, R.N., Risgaard-Petersen, N., and Dalsgaard, T. (2004) Denitrification and anammox activity in Arctic marine sediments. Lirnnol. Oceanogr. 49, 1493-1502. 

Mascitti, V. and Corey, E.J. (2006) Enantio-selective synthesis of pentacyclo-anammoxic acid. Journal of the American Chemical Society, 128, 3118-3119. 

The single stereogenic center of y-silylsubstituted cyclopentenone 12 allowed for an excellent stereocontrol in the [2 + 2]-photocycloaddition to the strained cyclobutene 13, which in turn had been obtained by a [2 + 2]-photocycloaddition-ring contraction sequence (Scheme 6.6) [26]. Alkene 13 dictates the exo-approach of the cyclopentenone, but as a meso-compound is of course not capable of controlling the absolute configuration. Further elaboration of product 14 led to (+)-pentacyclo-anammoxic acid (15). 

The decrease in the sum of nitrate, nitrite, and ammonium that is observed in the 0 = 15.90-16.00kgm-3 density layer is usually explained by denitrification [26,46], consumption by chemosynthesis [23], or/and anammox, the reaction between nitrite and ammonia [47,48]. From the comparison of the vertical gradients (Fig. 3) the role of nitrate becomes comparable with that of oxygen only in the lower part of the redox layer. Nitrate can be consumed for denitrification and reduction by thiosulfate, elemental sulfur, and sulfide [49,50]. The role of nitrate as a potential oxidizer of reduced manganese and iron is actively being discussed now [51], but the presence of these reactions has not been proved or widely accepted. If they exist, these reactions... 

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