Environmental Engineering Reference
In-Depth Information
dominate NO
3
−
reduction and thereby NO
2
−
production. They only form a
subset, however, of a large variety of organisms capable of nitrate reduction to
nitrite [84], including at least some anammox bacteria [26]. An identification
of the dominating NO
2
−
-producing organisms in natural environments would
contribute to an understanding of NO
2
−
dynamics and their significance for
anammox.
Under the assumption that nitrite consumption by anammox can be described
by Michaelis-Menten kinetics, the apparent half-saturation concentration, K
m
,
for NO
2
−
during anammox in natural environments has been constrained to
<
3 µM [12, 73]. For the one sedimentary setting where kinetic studies were
performed in some detail, by examining the time course of NO
2
−
depletion, the
apparent K
m
for NO
2
−
during denitrification was similarly low and there was no
change in the relative importance of anammox and denitrification during NO
2
−
depletion [12]. With maximum NO
2
−
concentrations in natural environments
of only a few µmol per liter, tighter constraints on the K
m
values are however
needed for a determination of how competition for NO
2
−
may affect the balance
between anammox and denitrification.
5.4 Oxygen Sensitivity and the Interaction with Aerobic
Ammonium Oxidizers
Little is known about the oxygen tolerance of anammox bacteria in the ma-
rine environment. Experimental work with enrichment cultures showed that the
anammox metabolism is inhibited by oxygen concentrations as low as 1 µM
[67]. The inhibition is fully reversible and anammox enrichment cultures that
have been exposed to oxygen, resume activity immediately after the reestab-
lishment of anaerobic conditions [67]. As such anammox bacteria might be
able to thrive in oxygen deficient environments that regularly experience in-
cursions of oxygenated water, such as the oxygen minimum zones of coastal
upwelling areas. At higher oxygen concentrations anammox bacteria could be
dormant, and become active again under anaerobic conditions. Alternatively,
marine snow aggregates are abundant in many oxygen deficient environments,
and could provide the anammox bacteria with anaerobic micro-environments
at low ambient oxygen concentrations (
<
25 µM) [45].
Intriguingly, in the so called Completely Autotrophic Nitrogen removal Over
Nitrite (CANON) process in oxygen limited bioreactors, aerobic and anaerobic
ammonia oxidizing bacteria cooperate to remove ammonium at ambient oxygen
concentrations as high as 10 µM [42, 63, 71]. In these bioreactors, anaerobic
and aerobic ammonia oxidizing bacteria form small (micrometer to millimeter
size) aggregates, with the aerobic ammonia oxidizers restricted to the outer
shell (
<
100 µM) while the anammox bacteria occur in the central anoxic part
