Environmental Engineering Reference
In-Depth Information
of the mineralization of organic N with Mn oxides as terminal electron acceptor.
Further indications of anaerobic ammonium oxidation by Mn oxides include the
observation of an ammonium deficiency in anoxic Mn-oxide rich sediment from
the Panama Basin [2] and of transient accumulation of nitrate in anoxic sed-
iment incubations, with the maximum nitrate concentrations correlating with
the Mn content of the sediment [31]. Both these findings were suggested to
result from an anaerobic Mn oxide-dependent nitrification-like process in con-
junction with denitrification. Alternatively, Luther and co-workers [36], based
on differences in porewater chemistry and nutrient fluxes between Mn-rich and
Mn-poor sediments, as well as abiotic experiments with Mn oxides, suggested
that Mn oxides oxidize ammonium directly to N
2
. Since this reaction requires
fresh oxide surfaces, it was suggested to be most important in the presence of
O
2
, where surfaces would be continuously regenerated through Mn
2
+
oxida-
tion. Manganese oxide-based ammonium oxidation has also been suggested to
be active in the Black Sea water column [40].
In order to directly demonstrate and quantify ammonium oxidation cou-
pled to Mn reduction in sediments, Thamdrup and Dalsgaard [69] amended a
metabolically active anoxic manganese-rich sediment with
15
N-labeled ammo-
nium, and monitored the production of
15
N labeled N
2
. No such production was
detectable, however, and it was concluded that the process was insignificant for
nitrogen cycling in this sediment. Subsequent studies at several other locations
with manganese concentrations more typical of coastal sediments also failed to
detect any anaerobic ammonium oxidation in the absence of nitrate or nitrite
[19, 70].
As discussed previously (See chapter 3.2), anaerobic ammonium oxidation
in the water column of Golfo Dulce was attributed to the anammox process
with no indications of a coupling to Mn oxides [11], despite relatively high
concentrations of particulate Mn and soluble Mn
2
+
[68]. Thus, there is as yet
no direct evidence for the process in either sediments or anoxic water columns.
It remains to be determined to which extent the anammox process may explain
the phenomena that have been attributed to ammonium oxidation by Mn oxides.
5. FACTORS CONTROLLING ANAMMOX IN THE
MARINE ENVIRONMENT
5.1 Temperature
With recent reports of anammox activity in melted sea ice and high-Arctic
sediments [53, 54], the process has been found at temperatures of -1.8 - 16
◦
C
in natural environments, the upper limit being reached in Golfo Dulce [68]. In
sediment from Skagerrak and Greenland with a stable
in situ
temperature of
∼
6
◦
C and
-1
◦
C, respectively, anammox exhibited a psychrotolerant temperature
response in short-term experiments with highest rates at
∼
15
◦
C and no activity
∼
