Nitrogen anammox

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... 

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 ... 

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. 

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. 

Biomolecules containing [3]- and [5]-ladderanes have been discovered within intracellular hpid membranes of anammox bacteria" that recycle atmospheric nitrogen. The ladderane lipids are found in the membrane surrounding a unique cytoplasmic organelle, the anammoxosome, within which the anaerobic oxidation of ammonia into molecular nitrogen occurs. The oxidation, known as the anammox process, follows the reaction (equation 24) ... 

Another anaerobic process that removes bio-available nitrogen from the water is the anaerobic oxidation of ammonium (Anammox), in which ammonium and nitrite are combined to form N2, thus... 

From a geochemical perspective, denitrification and anammox have the same implication, i.e., they both lead to a loss of fixed nitrogen from the ocean, albeit with a somewhat different stoichiometry. This loss requires a counterbalancing source elsewhere if fixed nitrogen in the ocean is to be maintained at roughly constant levels through time. 

In order to quantify the loss of fixed nitrogen via denitrification and anammox we definitely need more N2 concentration measurements and more formation rate measurements in suboxic zones such as found in the Arabian Sea and the eastern tropical Pacific Ocean. 

Like canonical denitrification anammox favorable conditions are those where oxygen has been depleted, and, like canonical denitrification, anammox has been found both in marine sediments and within the water column. Nitrogen stable isotope incubations are commonly used to distinguish between anammox and canonical denitrification. Typically, additions of + N03, and N03 ... 

Potential processes that may be called upon to explain the large discrepancy between excess N2 and NOs deficits within the SNM are (1) sedimentary denitrification, (2) anammox, (3) metal (Fe, Mn)-catalyzed denitrification, and (4) non-Redfieldian organic matter mineralization (Codispoti et al., 2001 Devol et al., 2006a,b). The first of these possibilities requires a decoupling between nitrogen and phosphorus cycles (e.g. possible greater burial of phosphorus in continental margin sediments). The less likely additional requirement is that the maximal inputs from the sediments should occur at the same depth as the water column peak of N02. In other words, the exact coincidence of the extrema in NOs , N02, ... 

Figure 21.1 Microbial nitrogen cycling processes in sedimentary environments on a coral reef (A) nitrogen fixation (B) ammonification (C) nitrification (D) dissimilatory nitrate reduction and denitrification (E) assimilatory nitrite/nitrate reduction (F) ammonium immobilization and assimilation. Adapted from D Elia and Wiebe (1990). Anammox (the anaerobic oxidation of NH4" with NO2 yielding N2 ) is not represented, as it has not yet been shown to occur on coral reefs, but may be found to be important in reef sediments.

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