Environmental Engineering Reference
In-Depth Information
Mulder and co-workers [38] were the first to provide direct evidence for the
anaerobic oxidation of ammonium in a waste water bioreactor. They argued
that ammonium was oxidized with nitrate to N
2
according to equation (2) and
named this biological process 'Anammox.' Van de Graaf et al. [76] showed
that in these bioreactors ammonium is oxidized to N
2
with nitrite instead of
nitrate as the electron acceptor. Recently, Strous et al. (1999) [65] discovered
organisms capable of anaerobic ammonium oxidation ('anammox') in waste
waters treatment plants. These so-called 'anammox' bacteria belonging to the
Order
Planctomycetales
directly oxidize ammonium to N
2
with nitrite as the
electron acceptor:
NH
4
+
+
NO
2
−
→
2
H
2
O
(3)
Anammox bacteria are chemoautotrophs [55, 59, 65] and 0.07 mol CO
2
is
fixed per mol ammonium oxidized [66].
Although the anammox process is generally seen as a promising process
for waste water treatment, it was believed to be insignificant in the natural
environment due to the extremely slow generation times (more than 2 weeks)
of the anammox organisms [82]. However, recent studies of a variety of marine
settings using biogeochemical and microbiological techniques [11, 12, 19, 35,
51, 53, 70, 73] indicate that the anaerobic oxidation of ammonium by nitrate
and/or nitrite may contribute significantly to the loss of reactive nitrogen in the
ocean.
N
2
+
2. TRACING ANAMMOX IN THE ENVIRONMENT
2.1 Labeling Experiments
Incubation experiments with addition of
15
N-labeled nitrogen species have
been a useful tool for elucidating anaerobic ammonium transformations and
quantifying the rates in both sediments and anoxic water columns. Consump-
tion of
15
NH
4
+
and concomitant production of
15
N-labeled N
2
provided the
first experimental evidence for anaerobic ammonium oxidation in marine sed-
iments [70]. This transformation was only observed in the presence of nitrate,
which was reduced via nitrite. Through the separate analysis of
14
N
15
N and
15
N
15
N production, experiments with labeling of nitrate, ammonium, or both,
demonstrated that N
2
formed through a 1-to-1 pairing of N from both sources,
as characteristic of anammox. The sediment process was further tied to anam-
mox through the demonstration of a typical microbial temperature response,
and by showing that nitrite could be used en lieu of nitrate [12, 73].
Labeling experiments also provided evidence of anammox in anoxic waters.
Anaerobic oxidation of
15
NH
4
+
to N
2
was observed in both the anoxic basin
of Golfo Dulce, Costa Rica, and in the suboxic zone of the Black Sea [11,
35]. The results from Golfo Dulce exhibited three features characteristic of
