Environmental Engineering Reference
In-Depth Information
and transport from deeper horizons. Specific relationships between anammox
bacteria, denitrifiyers and nitrifiers will be discussed in the following sections.
With organic matter as the ultimate source of NH
4
+
for anammox, stoichio-
metric considerations may put certain constraints on the role of anammox in
the N cycle. In environments such as the intermediate depths in Golfo Dulce
where anammox scavenges all the ammonium released during mineralization,
which is ultimately coupled to denitrification, the contribution of the process to
N
2
production should be tied to the mineralization ratio of N to C. For exam-
ple, during the complete mineralization of Redfieldian organic matter through
denitrification and anammox 16 of 55.2 mol N
2
, or 29%, is produced through
anammox [11]:
94.4
H
+
→
94.4
NO
3
−
+
(
CH
2
O
)
106
(
NH
3
)
16
H
3
PO
4
+
(4)
16
NH
4
+
+
16
NO
2
−
+
106
CO
2
+
39.2
N
2
+
145.2
H
2
O
+
H
3
PO
4
16
NH
4
+
+
16
NO
2
−
→
32
H
2
O
(5)
The anammox contributions in Golfo Dulce, excluding the bottom depths,
were 58 and 32% at the two stations, which would indicate a higher N:C
ratio during mineralization than in the equations above [11]. Sediment trap
studies indicate a preferential mineralization of organic N during water-column
denitrification [43, 77], which may help explain these deviations.
16
N
2
+
5.3 Interaction with Denitrifiers
The biogeochemical relationship between anammox bacteria and denitrifiers
appears quite complex. As discussed above, anammox depends on ammonifi-
cation, which in environments such as Golfo Dulce may to a large extent be
carried out by denitrifiers that oxidize N-containing organics. Further interac-
tions with denitrifiers include the potential dependence on denitrification as a
source of NO
2
−
, as well as competition for NO
2
−
.
Nitrite has been shown as the electron acceptor utilized by anammox bacteria
for ammonium oxidation from wastewater systems [76], and patterns of isotope
pairing during marine anammox suggest that this is also true for anammox
bacteria in natural environments [12]; (see also above). Nitrate reduction is
the main source of NO
2
−
in Golfo Dulce and the Black Sea [12, 35], where
NO
2
−
accumulates in the anoxic waters, similar to observations in oceanic
oxygen minimum zones (e.g. [9]). Nitrite may also accumulate transiently to
high levels during anoxic incubations of NO
3
−
-amended sediment [70], while
in other cases NO
2
−
concentrations remain low [73].
It is not clear which factors regulate the accumulation of NO
2
−
during
NO
3
−
reduction in waters and sediments. Denitrifiers are often thought to
