Geoscience Reference
In-Depth Information
state is equivalent to that of cellular nitrogen (
3) and thus requires the least energy
for assimilation.
12,13
Ammonia concentrations above 1-2
−
M tend to inhibit assim-
ilation of other nitrogen species.
10
On the other hand, if nitrate is to be assimilated
for the synthesis of cellular materials, it should be reduced to ammonia with the aid
of several enzymes including nitrate reductase (enzyme catalyzed reduction) within
the cell. This reduction process is called “assimilatory nitrate reduction” and requires
energy.
7,14
Nitrogen uptake can be an important process. For example, in Basin d'Arcachon
in southern France, due to the high nitrogen uptake rates of the seagrass
Zostera
noltii,
nitrogen uptake is quantitatively more important than denitrification as a
nitrogen sink.
15
In shallow water systems, biological organisms larger than phytoplankton
turn over slowly, and their metabolism is lower. Nevertheless, these organisms
store large amounts of nitrogen, because a substantial amount of nitrogen is tied
up in their biology. Thus, nitrogen concentrations in the shallow systems tend to
be lower.
16
Nutrient assimilation by macrophytes can be significantly different from that by
phytoplankton because macrophytes have the ability to grow for long periods on
stored nutrients. Rooted seagrasses can assimilate nutrients from sediment and
possibly serve as nutrient pumps
10
(
see Chapter 5 for details).
µ
4.1.1.2
Nitrification
Nitrification is the microbiological oxidation of ammonium to nitrite and then to
nitrate under aerobic conditions, to satisfy the energy requirements of autotrophic
microorganisms. Much of the energy released by this oxidation is used to reduce
the carbon present in CO
2
to the oxidation state of cellular carbon, during the
formation of organic matter.
As indicated previously, the first step in nitrification is oxidation of ammonium
to nitrite, which is accomplished by
Nitrosomonas
bacteria.
+
+
−
NH
+
1
1
2
O
2
H
+
NO
+
H O
(4.2)
4
2
2
2
The second step is oxidation of nitrite to nitrate by
Nitrobacter
. This is a faster
process.
−
1
2
−
NO
+
O
NO
3
(4.3)
2
2
The overall nitrification reaction is therefore
+
−
+
NH
+
2
O
NO
+
H O
+
2
H
(4.4)
4
2
3
2
The nitrification process can influence marine primary production by competing with
heterotrophs for the limited supply of dissolved oxygen and by decreasing the amount
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