2. INTRODUCTION:
• They are also known as blue green algae are one of the
important ubiquitous oxygen evolving photosynthetic
prokaryotes and ultimate source of nitrogen for paddy fields
• They use photosynthetic pigments, such
as carotenoids, phycobilins, and various forms of chlorophyll,
which absorb energy from light
• Also, they are variable in morphology, ranging
from unicellular and filamentous to colonial forms.
• Filamentous forms exhibit functional cell differentiation such
as heterocysts (for nitrogen fixation), akinetes (resting stage
cells), and hormogonia (reproductive, motile filaments).
3. CHARACTERISTICS:
• Prokaryotic cells
This characteristic is shared by all Cyanobacteria. They are prokaryotic, meaning
they lack a membrane-bound nucleus, which is a fundamental feature of a
prokaryote.
• Photoautotrophs
Cyanobacteria are the earliest cells capable of harnessing solar energy to drive their
own food synthesis.
• Cell wall
Cyanobacteria have a cell wall comprised mainly of peptidoglycan on its inner wall
and lipopolysaccharides on the outer wall. On the outermost surface of the outer wall
is a layer of fibrous material.
• Blue-green color
Cyanobacteria are characterized by the presence of pigments that account for their
color. In fact, they got their name ‘Cyanobacteria’ from their color, cyan (which is
greenish blue).
4. • Pigments
Phycobilisomes are pigments responsible for the blue-green pigmentation of most cyanobacteria. Other
pigments are chlorophyll. These photosynthetic pigments enable them to synthesize their own sugar
through photosynthesis.
Some cyanobacterial species (e.g. Prochlorothrix, Prochlorococcus, Prochloron) lack phycobilisomes but
they have chlorophyll b.
• Motility
Some of them are nonmotile whereas others can move by gliding motility. Motile filaments of
cyanobacterial cells are called hormogonia. Individual cells may break away from this filament to start a
new colony elsewhere.
• Gas vesicle (gas vacuoles)
In order to float, they form a gas vesicle (a vesicle bounded by a protein sheath and not by a lipid
membrane). Many planktonic cyanobacteria use them for floating toward the surface to collect light or
for sinking toward the sediments to avoid the harsh effects of UV radiation.
• Carboxysomes
They have microcompartments, such as the carboxysome. A carboxysome is a compartmentalized cage-
like structure surrounded by a protein shell. Cyanobacteria use it for concentrating CO2 and therefore
increase the efficiency of RuBisCo (the CO2-fixing enzyme).
5. Structure:
• Cell Sheath
The cyanobacterial cell is surrounded by a hygroscopic gelatinous sheath, which is made up of three
layers of microfibrils. The microfibrils are composed of pectic acids and mucopolysaccharides.
Sheaths are reticulately arranged within a matrix to give a homogeneous appearance.
The function of the cell sheath is to absorb and retain water.
• Cell Wall
A multi-layered wall is present below the gelatinous sheath. The cell wall is made up of four layers
which are designated numerically as LI, LII, LIII, and LIV.
The layers LI and LIII are electron transparent, while LII and LIV are electron-dense
• Plasma Membrane
Below the cell wall, a thin cytoplasmic bilayer membrane called the plasma membrane is present.
The plasma membrane consists of two electron-opaque layers separated by a translucent layer. It has
a thickness of 7 nm, is selectively permeable, and maintains the physiological integrity of the cell.
• Cytoplasm
The cytoplasm is differentiated into two regions, the outer colored peripheral region which is
called the chromoplasm, and the central colourless region called the centroplasm or central
body.
6. Reproduction:
Cyanobacteria reproduce asexually and the commonest mode of reproduction in them
is transverse binary fission. In addition, there are certain specialized structures such
as akinetes, hormogonia, hormocysts and spores, which are partly involved in the
process of reproduction
1. Vegetative reproduction: It continues within the colony until such time as the
moisture disappears. Then many of the cells form spores. These spores form the
blackish mass seen in dry weather. These spores are capable of surviving drought and
ultraviolet radiation in the summer. These spores grown in spring and form new
colonies.
2. Hormogones formation: It is the most common method of asexual reproduction
in Nostoc. Hormogones are short length filaments. They develop into larger
lengths by vegetative reproduction. Heterocysts are the point at which the filament
breaks into hormogones. Hormogones may also formed by the breaking of
filament or decay of filament at some other points. Hormogones may retain
gelatinous envelope. But in some cases, hormogone come out of the gelatinous
envelope and develop into new colony. In some cases, hormogone do not come
out of the colony and grow to form new trichome within the parent colony. Thus a
large number of trichomes are developed in the same colony.
7. 4. Akinetes or Anthrospores formation: Akinetes are formed during unfavorable
conditions. Akinetes are thick walled spores with a large amount of reserved food
material. Their wall is two to three layers thick. They have granular protoplasm. Akinetes
have reserved food material in the form of cyanophycin starch grains. Akinetcs are capable
of forming new filaments. The Akinetes can survive dry conditions. These ultimately start
new colonies elsewhere where rain comes.
5.Spores: Non-filamentous cyanobacteria generally produces pores such as endospores,
exospores and nano cysts which contribute by germinating and giving rise to new vegetative
cells when the unfavourable condition is over. Endospores are produced endogenously like
those in bacteria; exospores are the result to exogenous budding of cells, and the nanocysts
are produced endogenously like endospores. The difference between an endospore and a
nanocyst is that in endospore formation the parent cell concomitantly enlarges in size,
whereas in nanocyst formation there is no such enlargement of the cell
6.Heterocyst: Some cyanobacteria produce hormocysts, which are multicellular structures
having a thick and massives heath. They may be intercalary or terminal in position and may
germinate from either end or both the ends to give rise to the new filaments.
8. Economic Importance of Cyanobacteria:
1. Cyanobacteria are one of the early colonizers of bare and barren
areas and generate such conditions that favour the growth of other
organisms even in the most hostile environment.
2. They are good food source for several aquatic animals.
Moreover, the cyanobacteria are now-the-days exploited as food
for animals including humans. Spirulina, a filamentous
cyanobacterium, is now incorporated in food supplement as well as
animal feed through ‘single cell protein’ manufacture because of its
high protein content (upto 70%).
3. Nostoc, Anabaena are often used for reclamation of ‘user’ soils.
They produce acidic chemicals for counteracting alkalinity of the
soil and they supply nitrogen compounds which are generally
deficient in these soils.
4. Species of Anabaena and Aulosira do not allow mosquito larvae
to grow nearby. Such cyanobacteria can be inoculated in village
ponds to prevent the growth of mosquitoes.
5. Extracts of Lyngbia are used to manufacture antibiotic-like
compounds.
