Prokaryotes fix nitrogen into a form that can be used by eukaryotes.
LEARNING OBJECTIVE [ edit ]
Explain the need for nitrogen fixation and how it is accomplished
KEY POINTS [ edit ]
Prokaryotes perform biological nitrogen fixation (BNF) to convert nitrogen gas from the
atmosphere into ammonia, which can be used by eukaryotes to form important biomolecules such
as amino acids and nucleic acids.
Although most nitrogen fixation is performed by prokaryotes,abiotic processes, such as industrial
processes and lightning, can also fix nitrogen.
Some bacteria form symbiotic relationships with legumes, which provide oxygen-free nodules on
their roots for nitrogen fixation to occur; this process allows ammonia to form naturally from
atomspheric nitrogen to act as fertilizer for soils.
TERMS [ edit ]
legume
a large family of herbs, shrubs, and trees that bear nodules on the roots that contain nitrogenfixing bacteria
nitrogen fixation
the conversion of atmospheric nitrogen into ammonia and organic derivatives, by natural means,
especially by microorganisms in the soil, into a form that can be assimilated by plants
abiotic
nonliving, inanimate, characterized by the absence of life; ofinorganic matter
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Cooperation between Bacteria and Eukaryotes: Nitrogen Fixation
Nitrogen is a very important element for
living things because it is part of
the nucleotides and amino acids that are
the building blocks of nucleic acids
and proteins, respectively. Nitrogen is
usually the most limiting element in
terrestrial ecosystems. Atmospheric
nitrogen, N2, provides the largest pool of
available nitrogen. However, eukaryotes
cannot use atmospheric, gaseous nitrogen
to synthesize macromolecules.
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Fortunately, nitrogen can be "fixed," meaning it is converted into ammonia (NH3) either
biologically or abiotically. Abiotic nitrogen fixation occurs as a result of lightning or by
industrial processes.
Bacterial Nitrogen Fixation
Biological nitrogen fixation (BNF) is exclusively carried out by prokaryotes: soil
bacteria, cyanobacteria, and Frankia spp.(filamentous bacteria interacting with actinorhizal
plants such as alder, bayberry, and sweet fern). After photosynthesis andcellular respiration,
BNF is the second most important biological process on Earth. The equation representing the
process is as follows, where Pi stands for inorganic phosphate: N2 +
16ATP + 8− + 8H+ 2NH3 + 16ADP + 16Pi + H2
The total fixed nitrogen through BNF is about 100 to 180 million metric tons per year.
Biological processes contribute 65 percent of the nitrogen used in agriculture.
Types of Bacteria
Cyanobacteria are the most important nitrogen fixers in aquatic environments. In soil,
members of the genus Clostridium are examples of free-living, nitrogen-fixing bacteria. Other
bacteria live symbiotically with legume plants, providing the most important source of BNF.
Symbionts may fix more nitrogen in soils than free-living organisms by a factor of 10. Soil
bacteria, collectively called rhizobia, are able to symbiotically interact with legumes to form
nodules: specialized structures where nitrogen fixation occurs . Nitrogenase, the enzyme that
fixes nitrogen, is inactivated by oxygen, so the nodule provides an oxygen-free area for
nitrogen fixation to take place. This process provides a natural and inexpensive plant
fertilizer as it converts (reduces) atmospheric nitrogen to ammonia, which is easily usable by
plants. The use of legumes is an excellent alternative to chemical fertilization and is of
special interest to sustainable agriculture, which seeks to minimize the use of chemicals and
conserve natural resources. Through symbiotic nitrogen fixation, the plant benefits from
using an endless source of nitrogen: the atmosphere. Bacteria benefit from
usingphotosynthates (carbohydrates produced during photosynthesis) from the plant and
having a protected niche. Additionally, the soil benefits from being naturally fertilized.
Therefore, the use of rhizobia as biofertilizers is a sustainable practice.
Location of Nitrogen Fixation
Soybean (Glycine max) is a legume that interacts symbiotically with the soil bacterium Bradyrhizobium
japonicum to form specialized structures on the roots called nodules where nitrogen fixation occurs.
Some legumes, like soybeans, are also key sources of agricultural protein. Some of the most
important legumes are soybean, peanuts, peas, chickpeas, and beans. Other legumes, such as
alfalfa, are used to feed cattle.