Mirror site of http://gause.biology.ualberta.ca/bio108.hp/lect30.html, Univ. of Alberta, Biology 108
Nitrogen cycle shown in Fig. 49.12 [1, p. 1144]
80% of the earth's atmosphere is N2, but no eukaryotes can assimilate this vital nutrient. Only certain prokaryotes can fix N2- that is, reduce it to ammonia (NH3) that can be used in the synthesis of nitrogenous organic compounds such as amino acids [1, p. 1143]
This reduction is carried out by a complex enzyme called nitrogenase via a multistep process with a very high energy requirement. The process that can be written as: [1, p. 726]
Nitrogen-fixation is carried out by a diverse group of prokaryotes that may be free-living (non-symbiotic) or in a symbiotic association with plants [1, p. 1143].
Among the free-living microorganisms are: [2, Table 16.12, p. 618]
(A) aerobes that are:
(B) anaerobes that are:
Symbiotic associations can occur between
(A) leguminous plants (e.g. beans, peas, clover etc.) and a bacterium
of the genus Rhizobium or Bradyrhizobium.
(B) non-leguminous plants and actinomycetes (filamentous bacteria) of the genus Frankia.
[2, Table 16.12, p. 618]
Another feature of nitrogenase is its sensitivity to O2.
Nitrogenase is rapidly and irreversibly inactivated by O2. [2, p. 618]
In aerobic bacteria, this enzyme is protected from O2 by various mechanisms: [2, p. 618]
The best studied system is the legume-Rhizobium symbiosis in which the roots have swellings, called nodules composed of plant cells that contain the N2-fixing bacteria in a form called bacteroids. [1, p. 727] There is a marked specificity between the species of legume and strains of Rhizobium. A single Rhizobium strain is generally able to infect certain species of legumes and not others. [2, p. 682] Neither the bacterium nor the plant can fix N2 alone, it is only after the formation of the nodule that N2-fixation occurs. [2, p. 682]
Stages of nodule formation: [2, p. 683]
The symbiotic relationship between the legume and the N2-fixing bacteria is mutualistic, with both partners benefiting. [1, p. 727] The bacteria supply the plant with fixed nitrogen (NH4+), and the plant provides the bacteria with carbohydrates (an energy source) and other organic compounds.
The exquisite coevolution of these partners is evident in their cooperative synthesis of a protein called leghemoglobin, with the plant and the bacterium each synthesizing part of the molecule. [1, p. 727]
Leghemoglobin is an iron-containing protein that, like hemoglobin in red blood cells, reversibly binds O2. It provides O2 to the bacteroids for their intense respiration required to produce ATP for N2 fixation. In addition, leghemoglobin keeps the concentration of free O2 low in the nodules to protect the nitrogenase from O2 inhibition. [1, p. 728]
Most of the ammonium produced by the symbiotic N2-fixation is used by the plant for amino acid synthesis. [1, p. 728] When conditions are favorable, the nodules fix so much nitrogen that they secrete excess ammonium to the soil, which increases the fertility of the soil. [1, p. 728].
Returning to the nitrogen cycle Fig. 49.12 [1, p. 1144], we see that ammonium can also be produced by a process known as ammonification, which occurs during the decomposition of nitrogen-containing organic compounds (proteins, amino acids and nucleic acids). Ammonification is carried out many bacteria and eukaryotes under aerobic and anaerobic conditions.
Ammonium can be used as an energy source by certain aerobic bacteria that oxidize it to nitrite [1, p. 1144]. (e.g. Nitrosomonas spp.) In turn, the nitrite serves as an energy source for another group of aerobic bacteria that oxidize it to nitrate. (e.g. Nitrobacter spp.) Nitrite seldom accumulates in soil because both groups of bacteria usually live together in the soil.
The oxidation of ammonium to nitrate is known as nitrification.
It forms nitrate which can be readily assimilated by plants. However, because nitrate
is rapidly leached from soils receiving high rainfall, nitrification is not beneficial
in agricultural practice. [2, p. 658]
Under anaerobic conditions (e.g. waterlogged soil), nitrate can serve as a terminal electron acceptor for some facultative anaerobes. Nitrate is reduced to N2, and this process is called denitrification. [1, p. 1144] It is considered to be a detrimental process because N2 is not available to plants and animals.
The activities of the prokaryotic N2-fixing microorganisms are required to reduce N2 to ammonium for so the nitrogen cycle can continue.
Nitrogenase: the enzyme responsible for fixing atmospheric Nitrogen gas.
Nodule: a swollen outgrowth of a plant root induced by and containing Nitrogen fixing bacteria.
Bacteroid: a bacterial cell living symbiotically in a root nodule. It is somewhat larger and more irregular in shape than the free living bacterium.
Leghemoglobin: a protein found in root nodules of legumes which reversibly bind Oxygen, thus controlling its concentration.
Nitrification: the conversion of ammonium to nitrate and nitrite by bacteria.
Denitrification:the conversion of nitrate to nitrogen gas.
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