Transcript Nitrogen Fixation in the Ocean: The Role of Cyanobacteria

Nitrogen Fixation by
Cyanobacteria
KSU Students
Faculty of Science
Botany & Microbiology Dept.
Supervisor
Prof .Dr. Ibraheem IBN
Nitrogen Fixation
• The growth of all organisms depend on the
availability of Nitrogen (e.g. amino acids)
• Nitrogen in the form of Dinitrogen (N2) makes
up 80% of the air we breathe but is
essentially inert due to the triple bond (NN)
• 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 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
• biological fixation
• industrial fixation
Nitrogen fixing bacteria
In biological nitrogen fixation two moles of ammonia are
produced from one mole of nitrogen gas, using 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.
A point of special interest is that the nitrogenase
enzyme complex is highly sensitive to oxygen
Some nitrogen fixing organisms
• Free living aerobic bacteria
– Azotobacter
– Beijerinckia
– Klebsiella
– Cyanobacteria (lichens)
• Free living associative bacteria
– Azospirillum
• Free living anaerobic bacteria
• Symbionts
– Desulfovibrio
– Rhizobium (legumes)
– Purple sulphur bacteria
– Frankia (alden trees)
– Purple non-sulphur bacteria
– Green sulphur bacteria
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 Fixation cont.
• 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.
• Scientist estimate that biological fixation globally adds
approximately 140 million metric tons of nitrogen to
ecosystems every year.
Cyanobacteria
• Gram negative
• Contain photosytem I and II (fix CO2, produce O2)
– Note:
•Photosystem I provides the cells with ATP
• Photosystem II breaks water down to O2
Cyanobacteria
• Diversity: Most diverse photosynthetic bacteria
– Split into five subsections
•I: Unicellular rods or cocci
•II: Unicellular, aggregate with use of outer wall, from
reproductive baeocytes
•III: Vegetative unbranched trichomes
•IV: Filamentous unbranched trichomes with heterocycst
formation
•V: Filamentous branched trichomes with heterocycst
formation
Cyanobacteria
•Heterocysts
•Trichomes
– When the cell is deprived of
fixed inorganic nitrogen
(ammonia)…
•Thick cell wall formation
•Photosystem I for ATP
production
•Degredation of photosysten II
– Involved in O2 production
Nitrogen
– O2 inhibits nitrogenase
Fixation
Anabaena with heterocysts
In some
cyanobacteria nitogen
fixation occurs in
heterocycts. These
cells only have
Photosystem I
Anabaena sp. with symbiont bacteria
(possibly Zoogloea) around heterocysts
The other cells have both photosystem I and
photosystem II, which generates oxygen when
light energy is used to split water to supply H2 for
synthesis of organic compounds.
Nitrogen Fixation
• All nitrogen fixing bacteria use highly conserved
enzyme complex called Nitrogenase
• Nitrogenase is composed of of two subunits: an
iron-sulfur protein and a molybdenum-iron-sulfur
protein
• Aerobic organisms face special challenges to
nitrogen fixation because nitrogenase is
inactivated when oxygen reacts with the iron
component of the proteins
Nitrogenase
Nitrogenase (encoded by nifH gene)
2ATP
2ADP + 2Pi
Fe Protein
O2 Inhibition
FeMo
Protein
e-
N2
NH3
Heterocyst
Characteristic features of
Heterocyst:
1. The Heterocyst is the site for cyanobacterial nitrogen fixation which
is an enlarged cell, and may be present terminally or intercalary in
the filamentous cyanophycean algae.
2. In the process of cyanobacterial nitrogen fixation, hydrogen gas (H2)
is also evolved as a by product and 40% of it is recycled by the hup
gene (hydrogen uptake gene), whereas remaining 60% hydrogen
gas can be used by biotechnologists as a source of future clean fuel.
3. The Heterocyst is made up of three (3) different cell wall layers- the
outer fibrous and middle homogenous layers are made up of noncellulose polysaccharide. Whereas, the inner laminated layer is
made up of glycolipids.
Characteristic features of
Heterocyst: Cont.
4. On one hand, these special cell wall layers permit the
atmospheric N2 (g) to diffuse inside, whereas on the
other hand they stop the atmospheric O2 (g) to come
inside.
5. This is a damage-control mechanism for the enzyme
nitrogenase, as the nitrogenase is sensitive to O2 and
cold, and cannot function in the presence of O2 (g).