Role of nitrogen in the biosphere
The growth of all organisms depends on the availability of mineral nutrients, and
none is more important than nitrogen, which is required in large amounts as an
essential component of proteins, nucleic acids and other cellular constituents.
There is an abundant supply of nitrogen in the earth's atmosphere - nearly 79%
in the form of N2 gas. However, N2 is unavailable for use by most organisms
because there is a triple bond between the two nitrogen atoms, making the
molecule almost inert. In order for nitrogen to be used for growth it must be
"fixed" (combined) in the form of ammonium (NH4) or nitrate (NO3) ions. The
weathering of rocks releases these ions so slowly that it has a negligible effect on
the availability of fixed nitrogen. So, nitrogen is often the limiting factor for
growth and biomass production in all environments where there is suitable
climate and availability of water to support life.
Microorganisms have a central role in almost all aspects of nitrogen availability
and thus for life support on earth:
a. some bacteria can convert N2 into ammonia by the process termed
nitrogen fixation; these bacteria are either free-living or form symbiotic
associations with plants or other organisms (e.g. termites, protozoa)
b. other bacteria bring about transformations of ammonia to nitrate, and of
nitrate to N2 or other nitrogen gases
c. many bacteria and fungi degrade organic matter, releasing fixed nitrogen
for reuse by other organisms.
All these processes contribute to the nitrogen cycle.
We shall deal first with the process of nitrogen fixation and the nitrogen-fixing
organisms, then consider the microbial processes involved in the cycling of
nitrogen in the biosphere.
The nitrogen-fixing organisms
All the nitrogen-fixing organisms are prokaryotes (bacteria). Some of them live
independently of other organisms - the so-called free-living nitrogen-fixing
bacteria. Others live in intimate symbiotic associations with plants or with
other organisms (e.g. protozoa). Examples are shown below.
Examples of Nitrogen fixing bacteria
Symbiotic with
plants
Free Living
Aerobic
Azotobacter
Klebsiella
some cyanobacteria
Anaerobic
Clostridium
Purple Sulphur bacteria
Legumes
Rhizobium
other
plants
Azospirillum
Frankia
NITROGEN CYCLE
Nitrogen fixation
The nitrogen molecule (N2) is quite inert. To break it apart so that its atoms can
combine with other atoms requires the input of substantial amounts of energy.
Three processes are responsible for most of the nitrogen fixation in the
biosphere:
1. biological fixation by certain microbes — alone or in a symbiotic
relationship with some plants and animals
2. industrial fixation
3. atmospheric fixation by lightning
Mechanism of biological nitrogen fixation
Picture of root nodule
Nitrogen-fixing Rhizobium lives in the root nodules of leguminous plants
Biological nitrogen fixation can be represented by the following equation, in
which two moles of ammonia are produced from one mole of nitrogen gas, at the
expense of 16 moles of ATP and a supply of electrons and protons (hydrogen
ions):
N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi
A point of special interest is that the nitrogenase enzyme complex is highly
sensitive to oxygen. It is inactivated when exposed to oxygen, because this
reacts with the iron component of the proteins.
Biological nitrogen fixation requires a complex set of enzymes and a huge
expenditure of ATP.
Although the first stable product of the process is ammonia, this is quickly
incorporated into protein and other organic nitrogen compounds.
In the symbiotic nitrogen-fixing organisms such as Rhizobium, the root nodules
of leguminous plants contain oxygen-scavenging molecules such as
Leghaemoglobin,(L b) which shows as a pink colour when the active nitrogenfixing nodules of legume roots are cut open. Leghaemoglobin regulate the supply
of oxygen to the nodule tissues in the same way as haemoglobin regulates the
supply of oxygen to mammalian tissues.
They also form symbiotic associations with other organisms such as the water
fern Azolla, and cycads. The association with Azolla, where cyanobacteria
(Anabaena azollae) are harboured in the leaves, has sometimes been shown to be
important for nitrogen inputs in rice paddies, especially if the fern is allowed to
grow and then ploughed into the soil to release nitrogen before the rice crop is
sown.
Industrial FixationHaber –Bosch process
Haber-Bosch was the first industrial chemical process to use high pressure for a
chemical reaction. It directly combines nitrogen from the air with hydrogen
under extremely high pressures and moderately high temperatures.
N2 (g) + 3 H2 (g) ---------------------2 NH3 (g)
Atmospheric Fixation
The enormous energy of lightning breaks nitrogen molecules and enables their
atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve
in rain, forming nitrates, that are carried to the earth.
Atmospheric nitrogen fixation probably contributes some 5– 8% of the total
nitrogen fixed.
Decay
The proteins made by plants enter and pass through food webs just as
carbohydrates do. At each trophic level, their metabolism produces organic
nitrogen compounds that return to the environment, chiefly in excretions. The
final beneficiaries of these materials are microorganisms of decay. They break
down the molecules in excretions and dead organisms into ammonia.
Nitrification
Ammonia can be taken up directly by plants — usually through their roots.
However, most of the ammonia produced by decay is converted into nitrates.
This is accomplished in two steps:
-Bacteria of the genus Nitrosomonas oxidize NH3 to nitrites (NO2−).
-Bacteria of the genus Nitrobacter , Nitrococcus oxidize the nitrites to nitrates
(NO3−).
These two groups of autotrophic bacteria are called nitrifying bacteria. Through
their activities (which supply them with all their energy needs), nitrogen is made
available to the roots of plants. Nitrification occurs in well aerated soil and
water. Many legumes, in addition to fixing atmospheric nitrogen, also perform
nitrification — converting some of their organic nitrogen to nitrites and nitrates.
These reach the soil when they shed their leaves.
Excess of nitrate ions can be washed to water bodies causing Eutrophication.
Denitrification
The above processes remove nitrogen from the atmosphere and pass it through
ecosystems.
Denitrification reduces nitrates to nitrogen gas, thus replenishing the
atmosphere.
Denitrifying bacteria live in soil and water where oxygen supply is low and use
nitrate ions as a source of oxygen for aerobic respiration examplePseudomaonas denitrificans.
Once again, bacteria are the agents. They live deep in soil and in aquatic
sediments where conditions are anaerobic. They use nitrates as an alternative to
oxygen for the final electron acceptor in their respiration.
Thus they close the nitrogen cycle.