Environmental Engineering Reference
In-Depth Information
1
/
2
N
2
Nitrogen fixation
(nitrogenase)
3 e
-
+ 6 ATP
NH
4
+
NO
2
-
NO
3
-
Nitrite reductase
Nitrate reductase
2 e
-
6 e
-
Glutamine
synthetase
Glutamic acid
Glutamine
ATP
Glutamine oxglutaric
amino transferase (GOGAT)
Cellular organic N
products
Cellular C
production
Glutamic acid
α
- Oxoglutaric acid
FIGURE 13.2
Nitrogen assimilation. This figure illustrates that nitrogen must be assimilated
in the form of ammonium, and energy requirements for assimilation are N
2
NO
3
NO
2
NH
4
.
costly reducing equivalents, in the form of NADH, NADPH, or ferridoxin.
Nitrate reductase requires molybdenum to function properly, so limitations
of molybdenum may lead to an inability to utilize nitrate. In oxic envi-
ronments ammonium has higher potential energy than nitrate, so energy is
required to convert nitrate to ammonium before assimilation. For this rea-
son, many aquatic bacteria and primary producers prefer ammonium, and
affinity for ammonium is relatively high.
Many bacteria, including some cyanobacteria (Young, 1992), have the
capacity to assimilate N
2
. This capacity is known as
nitrogen fixation
. The
transformation does not occur spontaneously because it requires an ex-
tremely high activation energy. The conversion of N
2
to ammonium is ac-
complished with the enzyme nitrogenase, which requires molybdenum as
an essential component. The process is one of the most energetically ex-
pensive metabolic reactions, requiring at least six ATP molecules and three
electrons (reducing equivalents) for each ammonium produced. Another
important property of nitrogenase is that it is inactivated by O
2
. Organ-
isms either have to inhabit anoxic habitats to fix nitrogen or they have to
protect the enzyme from exposure (Bothe, 1982).
Some groups of cyanobacteria have formed specialized cells called het-
erocysts to protect nitrogenase from O
2
(Figs. 13.1 and 13.3). These cells
are clearly different in appearance under the microscope and have a vari-
ety of adaptations that allow for nitrogenase activity in the heterocysts and
photosynthetic O
2
evolution in adjacent cells. The adaptations include high
respiratory rates in heterocysts to consume O
2
, thick gel or mucilage
around the heterocysts to retard inward diffusion of O
2
, and loss of O
2
evolution in photosynthesis with retention of cyclic photophosphorylation
(i.e., generation of ATP by photosystem I) in heterocysts (Haselkorn and
Buikema, 1992). Other groups of cyanobacteria have no heterocysts but
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