nitrogen cycle is a cycle where nitrogen is used and replenish
over and over again.
The processes of the nitrogen cycle
Nitrogen is present in the environment in a wide variety of
chemical forms including organic nitrogen, ammonium (NH4+), nitrite
(NO2-), nitrate (NO3-), nitrous oxide (N2O), nitric oxide (NO) or
inorganic nitrogen gas (N2). Organic nitrogen may be in the form of
a living organism, humus or in the intermediate products of organic
matter decomposition. The processes of the nitrogen cycle transform
nitrogen from one form to another. Many of those processes are
carried out by microbes, either in their effort to harvest energy
or to accumulate nitrogen in a form needed for their growth. The
diagram above shows how these processes fit together to form the
nitrogen cycle.
[edit]Nitrogen fixation
Main article: Nitrogen fixation
Atmospheric nitrogen must be processed, or "fixed" (see page on
nitrogen fixation), to be used by plants. Some fixation occurs in
lightning strikes, but most fixation is done by free-living or
symbiotic bacteria. These bacteria have the nitrogenase enzyme that
combines gaseous nitrogen with hydrogen to produce ammonia, which
is then further converted by the bacteria to make their own organic
compounds. Most biological nitrogen fixation occurs by the activity
of Mo-nitrogenase, found in a wide variety of bacteria and some
Archaea. Mo-nitrogenase is a complex two component enzyme that has
multiple metal-containing prosthetic groups.[7] Some nitrogen
fixing bacteria, such as Rhizobium, live in the root nodules of
legumes (such as peas or beans). Here they form a mutualistic
relationship with the plant, producing ammonia in exchange for
carbohydrates. Nutrient-poor soils can be planted with legumes to
enrich them with nitrogen. A few other plants can form such
symbioses. Today, about 30% of the total fixed nitrogen is
manufactured in ammonia chemical plants.[8]
[edit]Conversion of N2
The conversion of nitrogen (N2) from the atmosphere into a form
readily available to plants and hence to animals is an important
step in the nitrogen cycle, which distributes the supply of this
essential nutrient. There are four ways to convert N2 (atmospheric
nitrogen gas) into more chemically reactive forms:[5]
Biological fixation: some symbiotic bacteria (most often
associated with leguminous plants) and some free-living bacteria
are able to fix nitrogen as organic nitrogen. An example of
mutualistic nitrogen fixing bacteria are the Rhizobium bacteria,
which live in legume root nodules. These species are diazotrophs.
An example of the free-living bacteria is Azotobacter.
Industrial N-fixation: Under great pressure, at a temperature of
600 C, and with the use of an iron catalyst, hydrogen (usually
derived from natural gas or petroleum) and atmospheric nitrogen can
be combined to form ammonia (NH3) in the Haber-Bosch process which
is used to make fertilizer and explosives.
Combustion of fossil fuels: automobile engines and thermal power
plants, which release various nitrogen oxides (NOx).
Other processes: In addition, the formation of NO from N2 and O2
due to photons and especially lightning, can fix nitrogen.
[edit]Assimilation
Main article: Assimilation (biology)
Plants take nitrogen from the soil, by absorption through their
roots in the form of either nitrate ions or ammonium ions. All
nitrogen obtained by animals can be traced back to the eating of
plants at some stage of the food chain.
Plants can absorb nitrate or ammonium ions from the soil via
their root hairs. If nitrate is absorbed, it is first reduced to
nitrite ions and then ammonium ions for incorporation into amino
acids, nucleic acids, and chlorophyll.[5] In plants that have a
mutualistic relationship with rhizobia, some nitrogen is
assimilated in the form of ammonium ions directly from the nodules.
Animals, fungi, and other heterotrophic organisms obtain nitrogen
by ingestion of amino acids, nucleotides and other small organic
molecules.
[edit]Ammonification
When a plant or animal dies, or an animal expels waste, the
initial form of nitrogen is organic. Bacteria, or fungi in some
cases, convert the organic nitrogen within the remains back into
ammonium (NH4+), a process called ammonification or mineralization.
Enzymes Involved:
GS: Gln Synthetase (Cytosolic & PLastid)
GOGAT: Glu 2-oxoglutarate aminotransferase (Ferredoxin &
NADH dependent)
GDH: Glu Dehydrogenase:
Minor Role in ammonium assimilation.
Important in amino acid catabolism.
[edit]Nitrification
Main article: Nitrification
The conversion of ammonium to nitrate is performed primarily by
soil-living bacteria and other nitrifying bacteria. In the primary
stage of nitrification, the oxidation of ammonium (NH4+) is
performed by bacteria such as the Nitrosomonas species, which
converts ammonia to nitrites (NO2-). Other bacterial species, such
as the Nitrobacter, are responsible for the oxidation of the
nitrites into nitrates (NO3-).[5] It is important for the nitrites
to be converted to nitrates because accumulated nitrites are toxic
to plant life.
Due to their very high solubility, and because soils are largely
unable to retain anions, nitrates can enter groundwater. Elevated
nitrate in groundwater is a concern for drinking water use because
nitrate can interfere with blood-oxygen levels in infants and cause
methemoglobinemia or blue-baby syndrome.[9] [10] Where groundwater
recharges stream flow, nitrate-enriched groundwater can contribute
to eutrophication, a process that leads to high algal, especially
blue-green algal populations. While not directly toxic to fish
life, like ammonia, nitrate can have indirect effects on fish if it
contributes to this eutrophication. Nitrogen has contributed to
severe eutrophication problems in some water bodies. Since 2006,
the application of nitrogen fertilizer has been increasingly
controlled in Britain and the United States. This is occurring
along the same lines as control of phosphorus fertilizer,
restriction of which is normally considered essential to the
recovery of eutrophied waterbodies.
[edit]Denitrification
Main article: Denitrification
Denitrification is the reduction of nitrates back into the
largely inert nitrogen gas (N2), completing the nitrogen cycle.
This process is performed by bacterial species such as Pseudomonas
and Clostridium in anaerobic conditions.[5] They use the nitrate as
an electron acceptor in the place of oxygen during respiration.
These facultatively anaerobic bacteria can also live in aerobic
conditions.
[edit]Anaerobic ammonium oxidation
Main article: Anammox
In this biological process, nitrite and ammonium are converted
directly into elemental nitrogen (N2) gas. This process makes up a
major proportion of elemental nitrogen conversion in the
oceans.
[edit]Marine nitrogen cycle
A schematic representing the Marine Nitrogen Cycle
The nitrogen cycle is an important process in the ocean as well.
While the overall cycle is similar, there are different players and
modes of transfer for nitrogen in the ocean. Nitrogen enters the
water through precipitation, runoff, or as N2 from the atmosphere.
Nitrogen cannot be utilized by phytoplankton as N2 so it must
undergo nitrogen fixation which is performed predominately by
cyanobacteria.[11] Without supplies of fixed nitrogen entering the
marine cycle the fixed nitrogen would be used up in about 2000
years.[12] Phytoplankton need nitrogen in biologically available
forms for the initial synthesis of organic matter. Ammonia and urea
are released into the water by excretion from plankton. Nitrogen
sources are removed from the euphotic zone by the downward movement
of the organic matter. This can occur from sinking of
phytoplankton, vertical mixing, or sinking of waste of vertical
migrators. The sinking results in ammonia being introduced at lower
depths below the euphotic zone. Bacteria are able to convert
ammonia to nitrite and nitrate but they are inhibited by light so
this must occur below the euphotic zone.[13] Ammonification or
Mineralization is performed by bacteria to convert the ammonia to
ammonium. Nitrification can then occur to convert the ammonium to
nitrite and nitrate.[14] Nitrate can be returned to the euphotic
zone by vertical mixing and upwelling where it can be taken up by
phytoplankton to continue the cycle. N2 can be returned to the
atmosphere through denitrification.
NH4+ is thought to be the preferred source of fixed nitrogen for
phytoplankton because its assimilation does not involve a redox
reaction and therefore requires little energy. However NO3 is more
abundant so most phytoplankton have adapted to have the enzymes
necessary to undertake this reduction (nitrate reductase). There
are a few notable and well-known exceptions that include
Prochlorococcus and some Synechococcus.[12] These species can only
take up nitrogen as NH4+.
The nutrients in the ocean are not uniformly distributed. Areas
of upwelling provide supplies of nitrogen from below the euphotic
zone. Coastal zones provide nitrogen from runoff and upwelling
occurs readily along the coast. However, the rate at which nitrogen
can be taken up by phytoplankton is decreased in oligotrophic
waters all year-round and temperate water in the summer resulting
in lower primary production.[15] The distribution of the different
forms of nitrogen varies throughout the oceans as well.
Nitrate is depleted in near-surface water except in upwelling
regions. Coastal upwelling regions usually have high nitrate and
chlorophyll levels as a result of the increased production.
However, there are regions of high surface nitrate but low
chlorophyll that are referred to as HNLC (high nitrogen, low
chlorophyll) regions. As of now the best explanation for HNLC
regions relates to iron limitation in the ocean. In recent years
iron has become an important player when discussing ocean dynamics
and nutrient cycles. The input of iron varies by region and is
delivered to the ocean by dust (from dust storms) and is leached
out of rocks. Iron is under consideration as the true limiting
element in the ocean.
NH4+ and NO2 show a maximum concentration at 50-80 m (lower end
of the euphotic zone) with decreasing concentration below that
depth. This distribution can be accounted for by the fact that NO2
and NH4+ are intermediate species. They are both rapidly produced
and consumed through the water column.[12] The amount of NH4+ in
the ocean is about 3 orders of magnitude less than nitrate.[12]
Between NH4+, NO2, and NO3, NO2 has the fastest turnover rate. It
can be produced during NO3 assimilation, nitrification, and
denitrification; however, it is immediately consumed again.
[edit]New vs. regenerated nitrogen
Nitrogen entering the euphotic zone is referred to as new
nitrogen because it is newly arrived from outside the productive
layer. [11] The new nitrogen can come from below the euphotic zone
or from outside sources. Outside sources are considered to be
upwelling from deep water or by nitrogen fixation. If the organic
matter is eaten, respired, delivered to the water as ammonia, and
re-incorporated into organic matter by phytoplankton it is
considered recycled/regenerated production.
New production is an important component of the marine
environment. One reason is that only continual input of new
nitrogen can determine the total capacity of the ocean to produce a
sustainable fish harvest.[16] Harvesting fish from regenerated
nitrogen areas will lead to a decrease in nitrogen and therefore a
decrease in primary production. This will have a negative effect on
the system. However, if fish are harvested from areas of new
nitrogen the nitrogen will be replenished.