4. ļ¼ Cyanobacteria are aquatic and photosynthetic, that is,
they live in the water, and can manufacture their own
food.
ļ¼ Because they are bacteria, they are quite small and
usually unicellular, though they often grow in
colonies large enough to see.
ļ¼ They have the distinction of being the oldest known
fossils, more than 3.5 billion years old.
ļ¼ It may surprise you then to know that the
cyanobacteria are still around; they are one of the
largest and most important groups of bacteria on
earth.
5. ļ¼ Many Proterozoic oil deposits are attributed to the activity of cyanobacteria.
ļ¼ They are also important providers of nitrogen fertilizer in the cultivation of rice
and beans.
ļ¼ The cyanobacteria have also been tremendously important in shaping the course
of evolution and ecological change throughout earth's history.
ļ¼ The oxygen atmosphere that we depend on was generated by numerous
cyanobacteria during the Archaean and Proterozoic Eras.
ļ¼ Before that time, the atmosphere had a very different chemistry, unsuitable for
life as we know it today.
6. ļ¼ The other great contribution of the cyanobacteria is the origin of plants.
ļ¼ The chloroplast with which plants make food for themselves is actually a
cyanobacterium living within the plant's cells.
ļ¼ Sometime in the late Proterozoic, or in the early Cambrian, cyanobacteria
began to take up residence within certain eukaryote cells, making food for the
eukaryote host in return for a home.
ļ¼ This event is known as endosymbiosis, and is also the origin of the
eukaryotic mitochondrion.
ļ¼ Because they are photosynthetic and aquatic, cyanobacteria are often called
"blue-green algae"
8. A typical cell of blue-green algae is
composed of the following components:
1. Outer cellular covering.
2. Cytoplasm.
3. Nucleic material.
9. 1. Outer Cellular Covering:
The outer covering of cell
includes:
(a) Mucilaginous layer
(b) Cell wall and
(c) Innermost plasma membrane.
10. (a) Mucilaginous layer:
o Mucilaginous sheath is the outermost layer covering
the cell wall.
o In some cases the mucilaginous layer is very
conspicuous and forms mucilaginous sheath but in
others it may be inconspicuous.
o It protects the cell from the injurious factors of the
environment.
(b) Cell wall:
o Just below the mucilaginous layer is present cell wall.
o Electron microscopy has revealed that the cell wall is
relatively complex structure.
o The cell wall is 2 or 3-layered and the inner layer lies
in between outer wall layer and plasma membrane.
o The cell wall is formed of polysaccharides and
mucopeptides.
11.
12. (c) Plasma membrane:
o The plasma membrane is selectively permeable
living membrane enclosing the cytoplasm and is
lipoproteinic in nature.
2. Cytoplasm:
o Below the plasma membrane is seen the
groundplasm which contains structures of
different shapes and functions.
o In the peripheral region of cytoplasm are located
lamellae which contain pigmemts .
o Fine structure study has made it clear that the
pigmented lamellae are not organised into
plastid.
o Lamellae or membranes are derived from plasma
membrane.
13. ļ¼ The pigments in lamellae include
o chlorophylls,
o Xanthophylls,
o c-phycoerythrin and
o c-phycocyanin,
o the last two are characteristically
found in blue-green algae only.
ļ¼ In addition to lamellae, several
membrane bound vesicles may also
be seen in the cytoplasm and they
may sometimes be stacked in layers.
ļ¼ Besides, ribosomes may be found
scattered in the groundplasm.
14. 3. Nuclear Material:
o The nucleoplasm or DNA containing
region is centrally located in the cell and
shows a fibrillar structure.
o Nucleoplasm is Feulgen - positive but is
not organized into an electron
micrograph of cell, nucleus,
o i.e., there is no nuclear boundary and no
nucleolus.
o During division the nucleoplasmic
material dispersed throughout the cell
divides into two and no spindle
apparatus participates in this process.
15.
16.
17.
18. Nostoc Structure
ā¢Nostoc are filamentous and unbranched.
ā¢Numerous filaments are found in a gelatinous mass as a colony. The colonies
may be as big as an egg.
ā¢The filament consists of a chain of cells, which appear like a bead. They are
called trichomes
ā¢Cells are oval, spherical or cylindrical
ā¢Some of the cells in the filament are differentiated, they are
called heterocyst. They are sites for nitrogen fixation. Nitrogenase enzyme
fixes nitrogen
ā¢Each filament is covered in a mucilaginous sheath, which is a protective layer.
It absorbs and retains water.
ā¢Colonies are of different shapes, sizes and colours. They are mostly greenish
or bluish-green in colour and also have red-brown or yellow-green colour
19. ā¢Each cell has a thick cell wall made up of peptidoglycan
ā¢The cytoplasm of a cell is differentiated into outer coloured
due to peripherally arranged chromoplast and inner clear
cytoplasm
ā¢Cells have various pigments. Cells contain chlorophyll (green
pigment). Phycocyanin (blue) and phycoerythrin (red) are also
present
ā¢Inner cytoplasm contains incipient nucleus or a nuclear body,
DNA is without histones
20. Nostoc Reproduction
ļ± Nostoc reproduce vegetatively or asexually by spore formation.
ļ± The vegetative reproduction is by fragmentation.
ļ± Small colonies can grow attached to a large colony and later form
separated colonies.
ļ± Hormogonia are short and free filaments.
ļ± They are formed when a filament breaks.
ļ± It retains the gelatinous sheath. New trichomes are developed inside
the colony.
21. ļ±Asexual reproduction is by the formation of resting spores known
as akinetes.
ļ±Some of the cells become thick-walled due to accumulation of
food. They can withstand unfavourable conditions for many
years.
ļ±Under favourable conditions, they germinate to form a new
filament.
ļ±Nostoc also reproduce by heterocysts.
ļ±Heterocysts separate from the filament. They divide and
germinate into a new filament.
22. The life cycle of Nostoc is given as follows:
1. Initially, fresh thalli present having thin colourless envelopes.
2. After 8 weeks, macroscopic spherical and subspherical forms of the thalli are
observed.
o When observed under the microscope, trichomes with heterocytes were found.
3. Next, the thalli released hormogonia along with heterocytes.
4. Hormogonia developed envelope and thereby formed colonies- increased and
formed large thalli.
5. These growing thalli formed buds.
o Aseriate phase starts-->coiling of trichome--> compact mass of cells.
6. Detachment of trichomes at a mature stage and formation of akinetes( enclosed
in a sheath).
23.
24. Ecological Importance
ā¢Nostoc are important for their nitrogen-fixing ability.
o They are used in paddy fields and are also used to increase the nutrient value of soil
ā¢They are rich in proteins and vitamin C and are used as a delicacy in various Asian
countries, e.g. N. flagelliforme, N. commune, etc.
ā¢N. muscorum has shown to accumulate polyhydroxy butyrate, which is a precursor of
plastic. It may have useful application in the industry
ā¢Cyanobacteria can convert CO2 to biofuels. Nostoc have shown to produce hydrogen
ā¢They can be used for bioremediation of wastewater and degrade environmental pollutants
ā¢Various species, e.g. N. muscorum, N. commune, N. insulare, etc. extracts have shown
antibacterial or antiviral activity and may be used in future to prepare drugs