Denitrification is a microbially facilitated process of nitrate reduction
that may ultimately produce molecular nitrogen through a series of
intermediate gaseous nitrogen oxide products. This respiratory process
reduces oxidized forms of nitrogen in response to the oxidation of an electron
donor such as organic matter. The preferred nitrogen electron acceptors in
order of most to least thermodynamically favorable include nitrate, nitrite,
nitric oxide, nitrous oxide finally resulting in the production of
dinitrogen completing the nitrogen cycle. Denitrifying microbes require a
very low oxygen concentration of less than 10%, as well as organic C for
energy. Since denitrification can lower leaching of NO3 to groundwater, it can
be strategically used to treat sewage or animal residues of high nitrogen
content. Denitrification allows for the production of N2O, which is a greenhouse
gas that can have a considerable influence on global warming.
The process is performed primarily by heterotrophic bacteria, although
autotrophic denitrifiers have also been identified. Denitrifiers are represented
in all main phylogenetic groups. Generally several species of bacteria
are involved in the complete reduction of nitrate to molecular nitrogen, and
more than one enzymatic pathway has been identified in the reduction process.
Direct reduction from nitrate to ammonium, a process known as
dissimilatory nitrate reduction to ammonium or DNRA, is also possible for
organisms that have the nrf-gene. This is less common than denitrification in
most ecosystems as a means of nitrate reduction. Other genes known in
microorganisms which denitrify include nir and nos among others; organisms
identified as having these genes include Alcaligenes faecalis, Alcaligenes
xylosoxidans, many in the Pseudomonas genus, Bradyrhizobium japonicum, and
Blastobacter denitrificans. Nutrient limitation
All organisms require certain nutrients in their surroundings for survival.
Depending upon the ecosystem, nitrogen is most likely the limiting nutrient,
although phosphorus is the other primary limiting nutrient and these two elements
interact chemically. Some organisms appear to be able to denitrify and
remove phosphorus. The triple bond of N2 makes this a very stable compound; most
organisms depend upon others to break this down to make it available for
biochemical reactions. See Nitrification. Symbiotic relationships
between Rhizobium species and legumes are well-documented.
Conditions required Denitrification takes place under
special conditions in both terrestrial and marine ecosystems. In general, it
occurs where oxygen, a more energetically favourable electron
acceptor, is depleted, and bacteria respire nitrate as a substitute terminal
electron acceptor. Due to the high concentration of oxygen in our
atmosphere denitrification only takes place in anoxic environments where
oxygen consumption exceeds the oxygen supply and where sufficient quantities
of nitrate are present. These environments may include certain soils
and groundwater, wetlands, oil reservoirs, poorly ventilated corners of
the ocean, and in seafloor sediments. Denitrification generally proceeds
through some combination of the following intermediate forms:
NO 3− → NO
2− → NO + N 2O → N
The complete denitrification process can be expressed as a redox reaction:
2 NO3− + 10 e− + 12 H+ → N2 + 6 H2O This reaction shows a fractionation in
isotope composition. Lighter isotopes of nitrogen are preferred in the reaction,
leaving the heavier nitrogen isotopes in the residual matter. The process can
cause delta-values of up to −40, where delta is a representation of the
difference in isotopic composition. This can be used to identify denitrification
processes in nature. Use in wastewater treatment
Denitrification is commonly used to remove nitrogen from sewage and
municipal wastewater. It is also an instrumental process in constructed
wetlands and riparian zones for the prevention of groundwater pollution with
nitrate resulting from excessive agricultural or residential fertilizer
usage. Wood chip bioreactors have been studied since the 2000s and are
effective in removing nitrate from agricultural run off and even manure.
Reduction under anoxic conditions can also occur through process called
anaerobic ammonium oxidation: NH4+ + NO2− → N2 + 2 H2O
In some wastewater treatment plants, small amounts of methanol, ethanol,
acetate, glycerin, or proprietary products are added to the wastewater to
provide a carbon source for the denitrification bacteria.
Denitrification processes are also used in the treatment of industrial
wastewater. Influence on global climate change
Increasing carbon dioxide levels within the atmosphere will influence global
nutrient cycling, yet it is difficult to predict what those interactions might
be. Chemical interactions between soils and the atmosphere will be influenced by
changes in atmospheric composition. There are indications that increased
fertilization of soils with nitrogen causes a decrease in carbon
sequestration. It has been shown that streams and
rivers receiving nitrogen inputs from urban and agricultural land uses are a
significant source of nitrous oxide to the atmosphere.
See also Aerobic denitrification
Anaerobic respiration Bioremediation
Climate change Hypoxia
Nitrogen fixation Simultaneous
nitrification-denitrification References
Literature Atlas, R.M., Barthas, R. Microbial
Ecology: Fundamentals and Applications. 3rd Ed. Benjamin-Cummings Publishing.
ISBN 0-8053-0653-6 Zumft, W.G.: Cell biology and molecular
basis of denitrification. In: Microbiol. Mol. Biol. Rev. Bd. 61, Nr. 4, S.
533-616. PMID 9409151 PDF