Introduction to Cyanophyceae:
It is a primitive group of
algae, consists of 150 genera and about 2,500 species. In India, the division
is represented by 98 genera and about 833 species. Members of the class
Myxophyceae (Cyanophyceae) are commonly known as Blue Green Algae(BGA).
The name blue green algae is given because of the presence of a dominant
pigment c-phycocyanin, the blue green pigment.
In addition, other pigments
like chlorophyll a (green), c-phycoerythrin (red), β-carotene and different
xanthophylls are also present. The members of this class are the simplest
living autotrophic prokaryotes
Important Characteristics of Cyanophyceae:
The important characteristics
of the division are as follows:
1. The individual cells are prokaryotic in nature. The
nucleus is incipient type and they lack membrane bound organelles.
2. Both vegetative and reproductive cells are non-flagellate.
3. Cell wall is made up of microfibrils and is differentiated
into four (4) layers. The cell wall composed of mucopeptide, along with
carbohydrates, amino acids and fatty acids.
4. Locomotion is generally absent, but when occurs, it is of
gliding or jerky type.
5. The principal pigments are chlorophylls a (green),
c-phycocyanin (blue) and c-phyco- erythrin (red). In addition, other pigments
like β-carotene and different xanthophylls like myxoxanthin and myxoxanthophyll
are also present.
6. Membrane bound chromatophore are absent. Pigments are
found embedded in thylakoids.
7. The reserve foods are cyanophycean starch and cyanophycean
granules (protein).
8.
Many filamentous members possess specialized cells of disputed function
(supposed to be the centre of N2 fixation) known as heterocysts.
9. Reproduction takes place by vegetative and asexual
methods. Vegetative reproduction takes place by cell division, fragmentation
etc. Asexual reproduction takes place by endospores, exospores, akinetes,
nannospores etc.
10. Sexual reproduction is completely absent. Genetic
recombination is reported.
Occurrence of Cyanophyceae:
Members of Cyanophyceae are
available in different habitats. Most of the species are fresh water (e.g.,
Oscillatoria, Rivularia), a few are marine (e.g., Trichodesmium, Darmocarpa),
and some species of Oscillatoria and Nostoc are grown on terrestrial habitat.
Species of some members like Anabaena grow as endophytes in thallus of
Anthoceros (Bryophyta) and in leaves of Azolla (Pteridophyta) and Nostoc in the
root of Cycas (Gymnosperm).
Species of Nostoc, Scytonema,
Gloeocapsa, and Chroococcus grow symbiotically with different fungi and form
lichen. Some members like Nostoc, Anabaena etc. can fix atmospheric nitrogen
and increase soil fertility.
Thallus Organisation in Cyanophyceae:
Plants of this group show much
variation in their thallus organisation.
The thallus may be of
unicellular or colonial forms:
1. Unicellular Form:
In unicellular form, the cells may be oval or spherical. Common members are
Gloeocapsa (Fig. 3.23A), Chroococcus and Synechococcus.
2. Colonial Form: In most of the members the cells
after division remain attached by their cell wall or remain together in a
common gelatinous matrix, called a colony.
The colonies may be of two types:
a. Non- filamentous, and
b.
Filamentous.
a. NonFilamentous Type: The cells of this type divide
either alternately or in three planes, thereby they form spherical
(Gomphosphaera, Coelosphaerum), cubical (Eucapsis alpine, Fig. 3.23C), squarish
(Merismopedia) or irregular (Microcystis, Fig. 3.23B) colony.
b. Filamentous Type:
By the repeated cell division in one plane, single row of cells are formed,
known as trichome. e.g., Oscillatoria (Fig. 3.23D), Spirulina, Arthosporia etc.
The trichome when covered by mucilaginous sheath is called a filament. The
filament may contain single trichome (Oscillatoria, Lyngbya) or several
trichomes (Hydrocoleus, Microcoleus, Fig. 3.23E).
The trichomes may be unbranched (Oscillatoria, Lyngbya), branched
(Mastigocladus limilosus, Fig. 3.23J) and falsely branched (Scytonema, Fig.
3.23K and Tolypothrix).
The cyanophycean cells are prokaryotic in nature, which rarely
exceeds 10p in diameter. Each cell consists of outer covering of cell envelop
which surrounds the membrane covered protoplast (Fig. 3.24C).
1. Envelop:
a. Mucilagenous Sheath: Presence of mucilaginous
sheath is common in all cyanophycean members. It consists of three layers of
microfibrils arranged reticulately within an amorphous matrix.
b. Cell Wall: The cell wall consists of four (4) (Fig.
3.24A) layers (under E.M.) named as LI, LII, LIII and LIV by Carr and Whitton
(1973). Each layer is about 10µ in thickness. The LI is the layer situated near
cell membrane and LIV is the outermost.
Cell
wall is composed of mucopeptide together with carbohydrates, amino acids and
fatty acids like Gram-positive bacteria. The LI and LIII layers are electron
transparent, but the LII and LIV layers are electron opaque (impervious).
2. Cytoplasmic Membrane:
The cytoplasmic membrane is
also known as plasmalemma present just inner to the cell wall. It consists of
two electron opaque layers separated by a translucent layer
3. Protoplast:
Studies with Electron Microscope by Wilden and Mercer,
Pankrats and Bowen and many others show that the protoplast consists of
thylakoids, cytoplasmic inclusion and nucleoplasm.
a. Thylakoids:
These are the complex lamellar system, which functions like the protoplasts of
eukaryotes. Thylakoids are not bounded by membrane instead they appear as
elongated and flattened sacs composed of two unit membranes.
b. Cytoplasmic Inclusions:
The cytoplasmic inclusions present in the cyanophycean cell are ribosomes,
cyanophycean granules, polyhedral bodies, polyphosphate bodies, polyglucoside
bodies, α-granules, β-granules and gas vacuoles
c. Nucleoplasm: The
nucleoplasm is usually centrally located and contains numerous fine randomly
oriented fibres of DNA.
Heterocysts
These are specialized cells of
the filament distinguished from others by their thick wall, polar nodule(s) and
homogenous contents.
Heterocysts are commonly found
in the members of Stigonematales and Nostocales (except Oscillatoriaceae). They
grow in the filament either terminally (Gloeotrichia) or intercalary (Nostoc)
or both terminal and intercalary (Anabaena desikacharyiensis).
Structure: Pale yellow, thick-walled specialized
cells, larger than vegetative cells are called heterocysts. The wall of the
heterocyst is differentiated into three regions, an outer fibrous, middle homogenous
and inner lamellar layer.
It has pore(s) at the pole(s)
where it remains attached with the vegetative cell. The pores are plugged with
polar nodule(s) or polar granules. The wall of the heterocyst is thicker
towards the polar regions.
The inner content is dense and
appears to be homogeneous. The thylakoides i.e., the photosynthetic lamellae
are present, but the ribosomes are less in number. Other granular inclusions
are appears to be absent. Due to absence of phycobillins and chlorophyll a,
photosynthesis does not take place.
Function of Heterocyst:
1.
Some considered it as the store-house of food materials.
2. Some others considered that they help in hormogonia
formation.
3. According to Brand (1903), it is of spore-like structure.
4. Fritsch (1951) told that during vegetative period it
secretes certain substances which promote growth and cell division.
6. Singh and Kumar (1970) pointed out that in addition to N2 fixation;
it also helps in growth and development of the
filament.
Gaidukov phenomenon or
complementary chromatic adaptation:
The efficiency to change the
pigment composition, to absorb maximum light for photosynthesis, with the
variation of the incident light is called complementary chromatic adaptation.
Many members of Cyanophyceae
have the capacity to change their colour in relation to the wave length of incident
light. Due to variation of the wavelength of incident light they can change
their pigment composition. It may appear blue green in yellow light, green in
red light and reddish in green light. Gaidukov (1903) first invented the
phenomenon and according to his name it is also known as “Gaidukov phenomenon”.
.
Importance of Cyanophyceae
1. They are one of the early colonizers of bare and barren areas. They
provide suitable conditions for the growth of other organisms even in the most
hostile environment.
2. Blue green algae function as food to several aquatic animals. Spirulina
is regularly collected for human consumption in parts of Africa. Nostoc is
similarly used in China. In Rajasthan Anabaena and Spirulina are collected from
Sambar Lake and used as fodder and manure. Spirulina is very easily cultivated
in tanks and can be used as a palatable protein rich food supplement for humans
and animals.
3. Several cyanobacteria have the
ability of nitrogen fixation. The filamentous forms possess special large pale
cells or heterocyst’s for this. Some of the fixed nitrogen comes out as
excretion. After death of cyanobacteria the substratum becomes rich in nitrogen.
Such nitrogen fixing cyanobacteria are now regularly inoculated in the rice
fields. This saves consumption of nitrogen fertilizers.
4. Nitrogen fixing cyanobacteria are often used for reclaiming usar soils,
e.g., Nostoc, Anabaena. These cyanobacteria produce acidic chemicals for
counteracting alkalinity of the soil and nitrogenous compounds which are generally
deficient in these soils.
5. Antibiotic can be
manufactured from extract of Lyngbia.
6. Species of Anabaena and
Aulosira do not allow mosquito larvae to grow nearby. Such cyanobacteria can be
inoculated in village ponds and rice fields to prevent the growth of
mosquitoes.
7. Cyanobacteria can grow on the walls and roofs
of buildings during the rainy seasons causing discolouration, corrosion and
leakage.
8. They produce water blooms,
imparting bad odour and colour to water bodies.
9. Some cyanobacteria produce toxins harmful to most aquatic animals. They
may prove equally toxic to human beings drinking or bathing in such water. The
important toxins producing cyanobacteria are Microcytic aeruginosa (= Anacystis
cyanea), Anabaena flosaquae, Aphanizomenon flos-aquae.
Source: biology discussion/botanyvvc.blogspot.in/Sridhar Uddamari
