Environmental Engineering Reference
In-Depth Information
ficient Namibian shelf waters. Ammonium should have accumulated in these
waters if heterotrophic denitrification was solely responsible for the nitrogen
loss [48, 50]. The low ammonium concentrations (below detection limit) in the
suboxic zone could indicate that anammox bacteria play an important role in
the nitrogen removal from the Benguela OMZ waters [11, 14, 80].
The N-deficit in the OMZ clearly shows that large amounts of fixed inorganic
nitrogen are being removed from the Namibian suboxic waters.
In situ
15
N-
labeling experiments indicate that nitrate is not directly converted to N
2
by
heterotrophic denitrification in the suboxic zone. Instead, nutrient profiles,
anammox rates, abundances of anammox cells, and specific biomarker lipids
indicate that fixed nitrogen is lost through anammox coupled to a) reduction
of nitrate to nitrite by heterotrophic denitrifiyers or anammox bacteria and b)
aerobic ammonium oxidation. Data from the Chilean OMZ show a similar
dominance of anammox over heterotrophic denitrification (B. Thamdrup, T.
Dalsgaard, M.M. Jensen, O. Ulloa, unpublished results). Depth integrated rates
indicated that 1-5 mmol fixed inorganic nitrogen per square meter per day
was lost from the suboxic waters of the Benguela upwelling system due to
anammox. Assuming that the main area (
100,000 km
2
) of suboxic shelf water
∼
extends from 28
◦
to 18
◦
south [8], 1.4
1 Tg fixed nitrogen per year might be
lost through anammox from the Benguela system.
±
3.4 Arctic Sea Ice
Anammox activity was recently also reported for Arctic sea ice [53]. Anaer-
obic processes occur in the lower 0.5 m of sea ice, in brine systems, which
become anoxic as a result of oxygen dilution by melting of deoxygenated
ice crystals and oxygen consumption associated with decomposition of ice
algae and/or detritus incorporated during ice formation. The organic matter
degradation also leads to high concentrations of dissolved organic carbon and
ammonium in sea ice. Incubation experiments with addition of
15
N-labeled
nitrogen species to melted sea ice indicate that substantial amounts of fixed
inorganic nitrogen (100-300 nmol N L
−
1
sea ice d
−
1
) are lost from sea ice
as N
2
as a result of heterotrophic denitrification and anammox. Anammox
accounts for up to 19% of the total N
2
production in the investigated sea
ice.
4. ANAEROBIC OXIDATION OF AMMONIUM WITH
MANGANESE OXIDES
With standard reduction potentials for Mn oxide/Mn
2
+
couples only slightly
lower than that of the NO
3
−
/N
2
couple (e.g. [69]), Mn oxides are potential elec-
tron acceptors in anaerobic ammonium oxidation. In their seminal study of early
diagenesis, Froehlich and coworkers [24] assumed that N
2
was the end product
