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Dr. Mohammad Asadul Haque
Associate Professor
Department of Soil Science, PSTU
Nitrogen fixation in nonlegume plants
Nitrogen Fixation by Free-living Blue Green Algae
Characteristics of BGA
 Blue green algae are unicellular, colonial or filamentous
 Filamentous BGA consist of many cells arranged one after another like a
chain.
 Some of these filaments form special types of cells known as heterocyst and
the organisms are called heterocystous BGA. Heterocysts are associated with
N2 fixation.
 Heterocysts are large, thick walled, apparently empty cells and usually
yellow in color.
 A mature heterocyst is surrounded by a multilayered envelope and connected
by cytoplasmic bridges to neighboring photosynthetic vegetative cells. These
bridges regulate the flow of molecules between two types of cells and
therefore a series of physiological and biochemical changes lead to nitrogen
fixation and assimilation.
 Heterocysts may terminal, intercalary and lateral
Fig. Heterocystous BGA
 Heterocysts derive some reductant such as glucose 6 phosphate, pyruvate
from photosynthetic vegetative cells to reduce atmospheric nitrogen to fixed
nitrogen.
 The vegetative cells depend on heterocysts for nitrogen nutrition in the form
of glutamine, glutamate and other amino acids.
Dr. Mohammad Asadul Haque
Associate Professor
Department of Soil Science, PSTU
Functions of heterocysts
1. Site of nitrogen fixation. BGA can fix N2 with the help of nitrogenase
enzyme which is located in heterocysts of heterocystous BGA.
2. Heterocysts protect the nitrogenase enzyme from damage by oxygen.
Nitrogenase enzyme is very sensitive to oxygen. It is quickly inactivated by
oxygen, but heterocysts possess the reducing environment and there no
photosynthesis takes place and does not evolve oxygen.
3. Heterocysts maintain the protoplasmic connection with vegetative cells with
the help of cytoplasmic bridge situated at one or both ends.
4. They regulate spore formation.
5. Heterocysts along function as reproductive cell.
List of cyanobacteria
A. Heterocystous N2 fixing BGA
Anabaena, Anabaenopsis, Aulosira, Chlorogloea, Cylindrospermum, Nostoc,
Calothrix, Scytonema, Tolypothrix, Fischerella, Hapalosiphon, Mastigocladus,
Stigonema and Westiellopsis
B. Non-heterocystous N2 fixing BGA
Plectonema, Trichodesmium, Lyngbya and Oscillatoria