The Microbial World:
The Nitrogen cycle and Nitrogen fixation
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.
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:
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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)
other bacteria bring about transformations of ammonia to nitrate, and of
nitrate to N2 or other nitrogen gases
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.
Nitrogen is an incredibly versatile element, existing in both inorganic and
organic forms as well as many different oxidation states. The movement of
nitrogen between the atmosphere, biosphere, and geosphere in different forms
is described by the nitrogen cycle (Figure 1),
Five main processes cycle nitrogen through the biosphere, atmosphere, and
geosphere: nitrogen fixation, nitrogen uptake (organismal growth), nitrogen
mineralization (decay), nitrification, and denitrification
Nitrogen fixation
The conversion of N2 into ammonia, and thence into proteins, is achieved by
microorganisms in the process called nitrogen fixation
Mechanism of biological nitrogen fixation
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
This reaction is performed exclusively by prokaryotes (the bacteria and related
organisms), using an enzyme complex termed nitrogenase. This enzyme consists of
two proteins - an iron protein and a molybdenum-iron protein, as shown below
The reactions occur while N2 is bound to the nitrogenase enzyme complex. The Fe
protein is first reduced by electrons donated by ferredoxin. Then the reduced Fe protein
binds ATP and reduces the molybdenum-iron protein, which donates electrons to N2,
producing HN=NH. In two further cycles of this process (each requiring electrons
donated by ferredoxin) HN=NH is reduced to H2N-NH2, and this in turn is reduced to
2NH3.
Depending on the type of microorganism, the reduced ferredoxin which supplies
electrons for this process is generated by photosynthesis, respiration or fermentation.
Nitrogen Uptake
NH4+
Organic N
The ammonia produced by nitrogen fixing bacteria is usually quickly incorporated into
protein and other organic nitrogen compounds, either by a host plant, the bacteria
itself, or another soil organism. When organisms nearer the top of the food chain (like
us!) eat, we are using nitrogen that has been fixed initially by nitrogen fixing bacteria.
Nitrogen Mineralization
Organic N
NH4+
After nitrogen is incorporated into organic matter, it is often converted back into
inorganic nitrogen by a process called nitrogen mineralization, otherwise known as
decay. When organisms die, decomposers (such as bacteria and fungi) consume the
organic matter and lead to the process of decomposition. During this process, a
significant amount of the nitrogen contained within the dead organism is converted to
ammonium. Once in the form of ammonium, nitrogen is available for use by plants or
for further transformation into nitrate (NO3-) through the process called nitrification.
Nitrification
NH4+
NO3-
Some of the ammonium produced by decomposition is converted to nitrate via a
process called nitrification. The bacteria that carry out this reaction gain energy from
it. Nitrification requires the presence of oxygen, so nitrification can happen only in
oxygen-rich environments like circulating or flowing waters and the very surface layers
of soils and sediments.
The process of nitrification has some important consequences. Ammonium ions are
positively charged and therefore stick (are sorbed) to negatively charged clay particles
and soil organic matter. The positive charge prevents ammonium nitrogen from being
washed out of the soil (or leached) by rainfall. In contrast, the negatively charged
nitrate ion is not held by soil particles and so can be washed down the soil profile,
leading to decreased soil fertility and nitrate enrichment of downstream surface and
groundwaters.
Denitrification
NO3-
N2+ N2O
Through denitrification, oxidized forms of nitrogen such as nitrate and nitrite (NO2-) are
converted to dinitrogen (N2) and, to a lesser extent, nitrous oxide gas. Denitrification is
an anaerobic process that is carried out by denitrifying bacteria, which convert nitrate
to dinitrogen in the following sequence:
NO3NO2NO
N2O
N2.
Nitric oxide and nitrous oxide are both environmentally important gases. Nitric oxide
(NO) contributes to smog, and nitrous oxide (N2O) is an important greenhouse gas,
thereby contributing to global climate change
Once converted to dinitrogen, nitrogen is unlikely to be reconverted to a biologically
available form because it is a gas and is rapidly lost to the atmosphere. Denitrification is
the only nitrogen transformation that removes nitrogen from ecosystems (essentially
irreversibly), and it roughly balances the amount of nitrogen fixed by the nitrogen fixers
described above