Environmental Engineering Reference
In-Depth Information
ization of Redfieldian organic matter through denitrification and anammox, is
produced through anammox (eqs. 4, 5) [11, 14]. The preferential mineralization
of nitrogen rich proteins through denitrification, releases more ammonium than
predicted by standard Redfield stoichiometry (eqs. 4) [11, 14]. If proteins are
the only substrate for denitrification, the relative contribution of anammox to
the total N
2
production increases to 48% [14]. In sediments and anoxic basins
like the Black Sea, ammonium diffuses upwards into the suboxic zone from
the underlying sulfidic zone [14, 35] where large amounts of ammonium are
released upon mineralization of organic nitrogen through fermentation/sulfate
reduction. In such environments where ammonium is supplied from outside the
suboxic zone, the relative importance of anammox can exceed 48% [11, 14,
19, 30, 70, 80].
Prognostic diagenetic modeling indicates that, globally, most benthic den-
itrification takes place in slope and hemipelagic sediments [37]. The role of
anammox in such locations awaits quantification. However, the general trend
towards increased relative importance of anammox with increasing water depth
and decreasing organic loading (Fig. 2) further substantiates that this process
may indeed be a major nitrogen sink in the ocean.
The extremely low concentration of ammonium and typical abundance of
nitrite could indicate that anammox bacteria also play an important role in
the nitrogen removal from oxygen minimum zone waters (e.g. [11, 14, 80]).
Nutrient measurements indicate that 30-50% of the total nitrogen loss in the
ocean occurs in these oxygen minimum zones [9, 25]. The pelagic nitrogen re-
moval takes place within only
0.1% of the ocean volume, and hence moderate
variations in the extent of oxygen minimum zones may have a large impact on
the global nitrogen cycle [9, 25]. This nitrogen loss has been fully attributed
to nitrate reduction to N
2
by heterotrophic bacteria (denitrification) [9, 18, 25]
because until recently there was no other process known that could transform
fixed inorganic nitrogen into N
2
. In fact, to the best of our knowledge there
is so far no published evidence from
15
N-labeling experiments that nitrate is
directly converted to N
2
by heterotrophic denitrifiers in the OMZ waters of the
ocean. Recent results show that anammox bacteria are responsible for massive
loss of fixed nitrogen as gaseous N
2
from the Benguela OMZ water [34]. The
possibility that anammox is also a dominant process for nitrogen removal in
other OMZ waters of the ocean should now be explored.
∼
References
[1]
Aller R.C. Bioturbation and manganese cycling in hemipelagic sediments, Philos Trans
R Soc of Lond A Math Phys Sci 1990; 331:51-58.
[2]
Aller R.C., Hall P.O.J., Rude P.D. and Aller J.Y. Biogeochemical heterogeneity and
suboxic diagenesis in hemipelagic sediments of the Panama Basin, Deep-Sea Res Pt I
1998; 45:133-65.
