The Nitrogen Cycle All life requires nitrogen-compounds

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Faktor Mikroorganisme Peran Rhizobium dalam
Pertumbuhan
Sri Wilarso Budi R Laboratorium
Silvikultur Fakultas Kehutanan IPB E-mail
wilarso62_at_yahoo.com
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The Nitrogen Cycle

• All life requires nitrogen-compounds, e.g.,
  proteins and nucleic acids.
• Air, which is 79 nitrogen gas (N2), is the major
  reservoir of nitrogen.
• But most organisms cannot use nitrogen in this
  form.
• Plants must secure their nitrogen in "fixed"
  form, i.e., incorporated in compounds such as
• nitrate ions (NO3-)
• ammonia (NH3)
• urea (NH2)2CO
• Animals secure their nitrogen (and all other)
  compounds from plants (or animals that have fed
  on plants).

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The Nitrogen Cycle
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• Four processes participate in the cycling of
  nitrogen through the biosphere
• nitrogen fixation
• decay
• nitrification
• denitrification
• Microorganisms play major roles in all four of
  these.

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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
• atmospheric fixation by lightning
• biological fixation by certain microbes alone
  or in a symbiotic relationship with plants
• industrial fixation

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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.
• N2 O2
  2NO
• 2NO O2
  2NO2
• 2NO2 H2O
  HNO3
• Ca or K HNO3
  CaNO3 or KNO3 (absorbed by roots plant)
• Atmospheric nitrogen fixation probably
  contributes some 5 8 of the total nitrogen
  fixed.

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Industrial Fixation

• Under great pressure, at a temperature of 600C,
  and with the use of a catalyst, atmospheric
  nitrogen and hydrogen (usually derived from
  natural gas or petroleum) can be combined to form
  ammonia (NH3). Ammonia can be used directly as
  fertilizer, but most of its is further processed
  to urea and ammonium nitrate (NH4NO3).

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Biological Fixation

• The ability to fix nitrogen is found only in
  certain bacteria.
• Some live in a symbiotic relationship with plants
  of the legume family (e.g., soybeans, alfalfa).
• Some establish symbiotic relationships with
  plants other than legumes (e.g., alders).
• Some nitrogen-fixing bacteria live free in the
  soil.
• Nitrogen-fixing cyanobacteria are essential to
  maintaining the fertility of semi-aquatic
  environments like rice paddies.
• 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.

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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.

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• 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 HNNH. In two further
  cycles of this process (each requiring electrons
  donated by ferredoxin) HNNH 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.

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The nitrogen-fixing organisms
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Symbiotic nitrogen fixation

• 1. Legume symbioses
• The most familiar examples of nitrogen-fixing
  symbioses are the root nodules of legumes
  (Paraserianthes falcataria, Accacia spp, peas,
  beans, clover, etc.).

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• Part of a crushed root nodule of a pea plant,
  showing four root cells containing colonies of
  Rhizobium. The nuclei (n) of two root cells are
  shown cw indicates the cell wall that separates
  two plant cells. Although it cannot be seen
  clearly in this image, the bacteria occur in
  clusters which are enclosed in membranes,
  separating them from the cytoplasm of the plant
  cells.
• In nodules where nitrogen-fixation is occurring,
  the plant tissues contain the oxygen-scavenging
  molecule, leghaemoglobin (serving the same
  function as the oxygen-carrying haemoglobin in
  blood). The function of this molecule in nodules
  is to reduce the amount of free oxygen, and
  thereby to protect the nitrogen-fixing enzyme
  nitrogenase, which is irreversibly inactivated by
  oxygen

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2. Associations with Frankia

• 2. Associations with Frankia
• Frankia is a genus of the bacterial group termed
  actinomycetes - filamentous bacteria that are
  noted for their production of air-borne spores.
  Included in this group are the common
  soil-dwelling Streptomyces species which produce
  many of the antibiotics used in medicine (see
  Streptomyces). Frankia species are slow-growing
  in culture, and require specialised media,
  suggesting that they are specialised symbionts.
  They form nitrogen-fixing root nodules (sometimes
  called actinorhizae) with several woody plants of
  different families, such as alder (Alnus
  species), sea buckthorn (Hippophae rhamnoides,
  which is common in sand-dune environments) and
  Casuarina (a Mediterranean tree genus). Figure A
  (below) shows a young alder tree (Alnus
  glutinosa) growing in a plant pot, and Figure B
  shows part of the root system of this tree,
  bearing the orange-yellow coloured nodules
  (arrowheads) containing Frankia.

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3. Cyanobacterial associations

• The photosynthetic cyanobacteria often live as
  free-living organisms in pioneer habitats such as
  desert soils (see cyanobacteria) or as symbionts
  with lichens in other pioneer habitats. 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. A symbiotic association of
  cyanobacteria with cycads is shown below. The
  first image shows a pot-grown plant. The second
  image shows a close-up of the soil surface in
  this pot. Short, club-shaped, branching roots
  have grown into the aerial environment. These
  aerial roots contain a nitrogen-fixing
  cyanobacterial symbiont.

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