Environmental Engineering Reference
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[42]. The ammonium is partly oxidized under oxygen limitation to nitrite by
aerobic ammonium oxidizers, such as Nitrosomonas and Nitrosospira (eq. 6).
NH 4 + +
NO 2 +
2 H +
1.5 O 2
H 2 O
+
(6)
The produced nitrite is subsequently used as an electron acceptor by the
anammox bacteria to oxidize ammonium to dinitrogen gas (eq. 3). The stoi-
chiometry of the CANON process can be represented by eq. 7.
2 NH 4 + +
2 H +
1.5 O 2
N 2 +
3 H 2 O
+
(7)
There is so-far no direct evidence for the CANON process in the marine
environment. However, the CANON process combined with enzyme catalyzed
hydrolysis of organic nitrogen would provide an alternative explanation for
the fact that the relative contribution of anammox to the total N 2 production
is far larger [11, 70] in certain environments than the 29% expected from the
stoichiometry (eqs. 4 and 5) of the complete mineralization of Redfieldian
organic matter through denitrification and anammox (see also chapter 5.2).
6. ANAMMOX AND THE CHEMICAL EVOLUTION
OF EARTH'S SURFACE
The present chemical composition of the oceans and atmosphere is the result
of evolving biological processes and geological activity interacting throughout
Earth's history. During some stages, nitrogen limitation of biological produc-
tion is thought to have played an important role in the configuration of the
biogeochemical cycles and as a driving force in biological evolution [3, 20].
Anammox introduces the possibility of NH 4 + removal under anoxic conditions
to the N cycle. This shunt seems of particular relevance on early Earth, where
the atmosphere and oceans are thought to have been anoxic (e.g. [3]). Anam-
mox depends on NO 2 , and before the rise of O 2 ,NO 2 and NO 3 may mainly
have been supplied at a low flux from atmospheric processes [33]. Biological N
fixation as a source of NH 4 + is believed to have evolved early in Earth's history
and in the diversification of life [6, 20, 21]. Thus, a niche for anammox bacteria
may have existed since the Archean, but due to the low flux of NO 2 /NO 3 the
process may not have had a large impact on the N cycle [30]. As O 2 accumu-
lated, nitrification became possible and likely supplied substantial amounts of
NO 2 /NO 3 for denitrification and anammox, leading to a configuration of the
N cycle similar to that of modern euxinic basins. In conjunction with possible
trace-metal limitation of N 2 fixation in the sulfidic oceans, this stimulation of
the oxidative N cycle may have led to substantial limitation of primary pro-
duction [3, 20]. The unique ladderane lipids of the anammoxosome membrane
potentially provide a specific biomarker of anammox activity in the geological
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