CyanoBacteria (Blue – Green Algae)
The cyanobacteria are the largest and most diverse group of photosynthetic bacteria which are the only known oxygen producing prokaryotes.
They comprise of about 2500 species . section 19 of Bergey’s Manual of determinative Bacteriology describes 34 genera.
Diagnostic Features:
The cell constituting the thallus are prokaryotic
The flagella is completely absent (even reproductive cells are non-flagellated ) and movement in the members is accomoplished by gliding actions.
The phycobilin pigments are unique to this class are blue C-phycocyanin and red C-phycoerythrin.
The other pigments are chlorophyll-a, ß-carotene and unique xanthophyll namely myxoxanthin and mycoxanthophyll.
Their photosynthetic system resembles that of the eukaryotes because they have chlorophyll-a and photosystem II, and carry out oxygenic photosysnthesis
There are no membrane bound chromatophores
The unique food storage compounds are the myxophycean starch and a proteinaceous material cyanophycin
They have specialized cells known as heterocysts
Sexual reproduction is completely absent.
Distribution and Habitat
Cosmopolitan in distribution, with most of the species occurring throughout the world.
They are widely spread in aquatic habitat, some terrestrial species are also known. The aquatic forms mostly occur in fresh water, a few however are marine. Members are both planktonic and benthic.
The blue-green algae are the only organisms that grow in adverse and harsh environments. The grow in snow and also constitute the principal vegetative of hot springs (Colteronema, Synechococcus). In different hotsprings they can survive in water temperature upto 85oC.
They are good example of the adaptability of life to extreme environment Because of their gelatinous sheaths they can withstand long periods of desiccation.
A fairly large number of members like Scytonema, Cyclindrospermum are able to fix atmospheric nitrogen and are common in paddy fields of tropical and subtropical countires.
Many blue-green algae grow in association with other organisms showing endophytic, endozoic and symbiotic associations. Gleocapsa and Nostoc are phycobionts of lichens. Nostoc and Anabaena occur within the plant bodies of certain Bryophytes (Anthoceros, Sphagnum), Ferns (Azolla), Cycads and non-leguminous angiosperms (Gunnera)
The factors that contribute to their wide distribution are i) ability to withstand prolonged drying and extremes of temperature, ii) capacity of some to fix atmospheric nitrogen, iii) liable metabolism, iv) ability of many of them to enter into symbiotic relationship with other plants v) modes of reproduction.
Organization of Thallus:
Just as do they vary in the choice of their habitats, so do they in the range of vegetative structure.
1) Unicellular forms:
The thallus is a unicell which is usually spherical or oval Ex. Chroococcus, Synechococcus, Gelopcapsa.
2) Colonial forms:
In most blue-greens, the cells after division remain attached to their walls or are held in a common gelatinous matrix to form a loose organization of cells which is termed a colony. Such colony may be definite (Merismopedia ) or irregular and indefinite (Aphanothece). The colonies may be either filamentous or non-filamentous.
Non-filamentous colonies:
They are of various forms. They may be cubical, spherical, irregular depending on the planes and direction of division of cells. They cells may divide in two planes at right angles to each other or in three planes.
Filamentous:
This is the result of repeated cell divisions in a single plan and in a single direction forming a chain or a thread. It is known as trichome. The trichome with enclosing gelatinous sheath is called a filament.
The filament in some genera has a single trichome Ex- Oscillatoria , Lyngbya. In others, the filament contains several trichomes Ex- Microcoleus, Hydrocoleus.
The filament may be brached or unbranched in unbranched filamentous form there is differentiation into a base and an apex and growth is diffuse. Ex-Oscillatoria.
Branched filaments may be either false or true. In true branching, the cells constituting the filament divide into two planes. Ex- Stigonema , Hapalosiphon.
In false branching trichome is displaced to one side and passes into a branch of the mucilaginous sheath while the other trichome of the filament continue in the original sheath. Ex- Tolypothrix and Scytonema.
Most highly evolved thallus which is heterotrichous with true branching are found in Stigonema, Mastigocladus.
Cell Structure
The cyanophycean cell is characterized by a primitive prokaryotic cell. There are no organized nuclei, chromatophores, pyrenoids, mitochondria or true vacuole.
Under light microscope the cell is seen to consist of a cell envelope enclosing the tiny protoplast.
Cell envelope:
Typically the cell envelope consists of two parts namely, the sheath and the inner investment or cell wall.
Sheath:
It is a layer of extracellular mucilage external to the cell wall. It consists of 3 layers of non-cellulosic microfibrils arranged in a reticulate manner and embedded in a gelatinous maxtrix of homogenous appearance. It may become lamellated and pigmented.
The sheath is useful in many ways i) it served to hold the cells of the colonies together, ii) colour variation, iii) withstanding long period of desiccation by water absorbing and water retaining capacity, iv) gliding movement, v) in perennation.
Cell Wall or Inner Investment:
Internal to the mucilage sheath is a wall layer termed inner investment. It is differentialed into 4 layer LI –LIV.
Layer LI – innermost layer of cell wall, lies next to the plasmalemma. It is visible as a clear electron –transparent space.
Layer LII – it is thin, electron dense layer with lies next to Layer LI, it contains mucopeptide and muramic acid (peptidoglycan). It also contains glucosamine, diaminopimelic acied. Similar to mucopolymer layer of Gram negative bacteria. The cell wall owes its shape and mechanical strength to this layer.
Layer LIII – It is external to LII and again a clear transparent space.
Layer LIV – it is outermost layer. Contains both lipopolysaccharides and proteins.
Protoplast
The protoplast shows internal differentiation. It is divisible into two regions i) a centrally located clear area forming the core called the centroplasm and ii) the peripheral denser region surrounding it called chromoplasm.
Plasma membrane:
Internal to cell wall, the protoplast at its periphery is surrounded by plasma or cell membrane. It shows small undulations and consists of two electron-opaque layers separated by a translucent layer.
Lamallae or thylakoids:
In the peripheral region of the protoplast, within the plasma membrane, are elongated, flattened sac or disc-like structures called thylakoids or lamellae. They are closely appressed and organized in parallel stacks. However they are not organized into grana. The thylakoids act as the sites of both photosynthesis and respiration.
Adjacent thylakoids are characteristically separated one from other by a space of 50nm which is occupied by contagious rows of discoidal phycobilisomes. Phycobilisomes are unique light harvesting complexes found in cyanobacteria and red algae. They are made of stacks of pigment proteins (phycobiliproteins) linked together by colour protein called linkers. The phycobiliproteins make up about 85% of the phycobilisome complex, and occur in three major forms i.e., phycocyanin (PC), Phycoerythrin (PE) and allophycocyanin (APC).
Pigments:
The chief pigments are chlorophyll a (not chlorophyll –b), ß-carotene and phycobiliproteins. The best known phycobiliproteins or phycobilins are blue C-phycocyanin and red C-phycoerythrin. The Xanthophylls like Zeaxanthin, myxoxanthophyll, myxoxanthin and oscilloxanthin are also present.
The phycocyanin is the characteristic pigment of the blue greens. It imparts blue colour to the thallus and chlorophyll-a imparts green. These two pigments thus are responsible for the characteristic blue-green colouration.
Chromatic adaptation or Gaidakov phenomenon:
In many species of cyanobacteria, the pigmentation seems to be determined by the quality of light in which they are grown. For example, some species of Oscillatoria can assume a green colour in red light, a reddish hue in green light, and a bluish-green in yellow light.
Such a capacity to change colour complementary to that of the light is known as chromatic adaptation or Gaidakov phenomenon.
This adaptation is of definite advantage since, it enables to maximally absorb the light for photosynthesis.
Nucleus:
In the central protoplasm the circular fibrils of DNA are present. It is not separated from the surrounding cytoplasm by any membrane. Nucleoli is absent. Histones are absent. RNA is diffusedly distributed in the nucleoplasm.
Granules:
Besides thyalkoids and DNA various kinds of granules are found in the protoplast.
Ribosomes: 70s ribosomes are dispersed throughout the cell but are abundant in the central region around the nucleoplasm.
Cyanophycean granules: they are regarded as reserve food. They are large dense bodies composed of copolymers of aspartic acid and arginine in 1:1 ration. Theya re also known as structural granules.
Polyglucan granules: they are polymers of glucose similar to animal glycogen and are also known as Alpha granules. These are less dense, longer than ribosomes and are commonly associated with thylakoids.
Polyhedral bodies: they are polyhedral in shape similar to carboxysomes found in bacteria. Thy are usually associated with the DNA. They contain carbon dioxide fixing enzyme Ribulose 1,5-bisphosphate carboxylase.
Phosphate bodies: (Metachromatic or Volutin granules) are spherical and appear similar to the lipid bodies of eukaryotic cells. They contain stored phosphate.
Gas Vacuoles or Pseudo Vacuoles
They are hollow cylindrical vesicles with conical ends. The membranous rigid wall of the gas vacuoles is proteinaceous in nature. It is impermeable to water but freely permeable to gas. Thus, the gas vacuoles are filled with metabolic gases.
The gas vesicles provide a buoyancy regulating mechanism enabling the planktonic prokaryotes to poise at particular depths. At low light intensity, the gas vesicles are formed at a rapid rate making the thallus buoyant. Such cells rise to the surface where under high light intensity more sugar is produced leading to higher osmotic pressure. On increase in osmotic pressure, the gas vacuoles collapse with the conical end caps splitting away. Such cells lose their buoyancy and sink down to the bottom.
By determining buoyancy, gas vacuoles indirectly serve as a screen against intense light.
Heterocyst
Heterocysts are specialized cell which differ from ordinary vegetative cells in having a thickened wall, homogenous and pale yellowish contents and one or two pores. Occurrence of heterocysts is a feature unique to cyanophyceae.
The heterocyst usually occur singly. In some genera they occur in pairs (Anabaenopsis) and rarely in chains. When they occur singly they are either terminal (Gloeotrichia) or intercalary in position (Nostoc).
Heterocysts are formed from vegetative cells by cell enlargement, dissolution of storage granules, the deposition of a multilayered envelope outside cell wall, gradual loss of photosynthetic pigmenst and re-orientation of the photosynthetic lamellae into a complex reticulate pattern.
Heterocyst structure can be divided into envelope, photosynthetic lamellae and other cytoplasmic inclusions. Wall is distinct into 3 layers, the outermost fibrous, the middle homogenous and inner most laminated layers.
The contents of heterocysts are generally dense and yellowish in colour due to disappearance of photosynthetic pigments leaving only the carotenes. There are no phycobiliprotein pigments and photosystem II in hetercysts. They are unable to evolve oxygen and cannot fix carbondioxide. They have higher respiratory rate than vegetative cells.
At the junction between the heterocyst and the vegetative cell there is a deep constriction. The constriction region has a pore for communication with adjacent cells. The constriction region has a pore for communication with the adjacent cells.
Plasmodesmata connect the plasma membranes of the heterocyst and the adjacent cells.
In mature heterocyst the pore channel is plugged with a polar nodule which is located between the plasma membrane and the inner envelope of the heterocyst.
The intercalary heterocyst has two pores, one at each pole whereas the terminal heterocyst has a single pore toward the trichome.
Functions:
1. Sites of nitrogen fixation.
2. Point of breakage of trichome.
3. They stimulate the formation of Akinetes.
4. They stimulate growth and cell division
5. Storage of food reserves.
Reproduction:
Vegetative and asexual modes of reproduction are commonly present in cyanobacteria. True sexuality or occurrence of sex cells and organs is absent.
Vegetative reproduction:
Vegetatively the cyanophytes multiply by i) cell division or fission ii) fragmentation and iii) by formation of hormogones or hormogonia
Cell Division or Fission
In unicellular blue-greens cell division leads to multiplication of the species. The division of the nuclear material (DNA) precedes the cell division. This is followed by the division of cytoplasm and separation of the resultant two daughter protoplasts.
Fragmentation:
During favourable conditions, propogation by fragmentation is common in both non-filamentous colony and filamentous forms.
Reaching a certain size, the non-filamentous colony splits into small parts. Each part which is called a fragment by repeated cell division grown into a new colony.
In filamentous form, the trichome may break into fragments by mechanical means such as by the bite of animals or stress caused by water currents or death of certain cells.
Hormogonia formation:
It is a specialized process of vegetative propogation characteristic of the cyanophyceae. The filamentous genera regularly multiply by breaking of their trichomes within the sheath into short fragments of one to many cells known as the hormogones or hormogonia.
Hormogonia are generally motile. The hormogones are formed in two ways:
1) By intercalary heterocyst:
Many heterocystous genera of blue-green have intercalary heterocyst separated a few cells apart from each other in the trichome. The trichome breaks at the sites of intercalary heterocysts because of weaker adhesion between a heterocyst and a vegetative cell.
2) By formation of separation discs:
In some genera one or few cells in the trichome undergoes lysis due to secretion of muric acid. As a consequence the protoplast of the cell degenerates to form a viscous substance. These mucilage filled dead cells are called necridia.
The walls of adjacent cells bulge and become convex. The dead cell (necridium) which is wedged between the bulging walls of the two adjacent cells appear biconcave. These biconcave, dead cells are called the intercalary biconcave separation discs. The latter provide weak links at which the trichome breaks and forms hormogonia.
The hormogonia finally settle down and grow into new filaments by cell division.
Asexual Reproduction:
Many cyanobacteria reproduce asexually by the formation of non-motile asexual spores. The spores are produced in ordinary vegetative cell. No meiosis or mitosis is involved in their formation.
Akinetes:
These are specialized thick-walled resistant cells formed in some heterocystous blue-green algae. They are larger than the vegetative cells. Mostly, they are developed singly next to a heterocyst at the base of a trichome.
During the formation of the akinete, the vegetative cell increase in size, accumulates food reserves, secrete a thick, multilayered spore wall surrounding the parent cell wall. It is often yellow or brown. Finally the modified cells round off and separate each constituting a reproductive unit. They are in reality the resting cells.
With the onset of conditions favourable for growth the akinete germinate and forms new filament.
Endospores:
These are small spores formed endogenously with the vegetative cell. At the time of endospore formation the vegetative cells increase in size and protoplast divides. A large number os small, uninucleate daughter protoplasts are formed.
Each daughter protoplast secretes a wall around it and become a endospore. The liberated endospore germinates immediately without a resting period.
Exospores:
In some generea, the cell wall ruptures at the distal end of the vegetative cell. The spores are successively pinched off at the exposed end of the extruded protoplast. They are called exospores. Ex. Chamaesiphon.
Nannocytes:
In some non-filamentous blue-greens the cell contents divide repeatedly without any cell enlargement. Numerous daughter cell are produced in each parent cell. These are called nannocytes. They differ from endospores in having naked protoplasts and extremely small size. The nannocytes germinate in situ to give rise to the colonies. Ex. Microcystis.
Oscillatoria
Division: Thallophyta
Sub Div: Algae
Class : Cyanophyceae
Order : Harmogonales
Family : Oscillatoriaceae
Genus: Oscillatoria
Distribution and Habitat:
The Genus Oscillatoria includes 100 species, which are wide in distribution. Oscillatoria is common, freshwater, filamentous dark blue-green algae.
They occur in varied habitat. Usually found in temporary rain water pools, dampsoil, road side ditches. Bottom of shallow temporary puddles and ditches, drain or sewers are usually covered with large patches of Oscillatoria. Few species are marine (O. sancta), halophillic (O. tambi), planktonic bloom forming (O. rubescenes, O. personata).
Vegetative Structure:
The thallus consists of free-living trichomes which often form a compact, tangled floating mass. Each trichome is long, thread-like unbranched structures.
Mostly it appears naked because the sheaths around it is very delicate and poorly developed. The trichomes are septate, the septa are faintly visible and often marked by rows of granules on either side.
Except the end cell, all are alike and discoid. Terminal cell is curved or convex at the free end. In a few species the apical cell may be swollen into a cap-like structure or capped capitates. In some apical cell may be thickened into a calyptra.
Cell structure:
All the cells in the trichome are similar in structure. The cells are broader than their length.
The cell wall being stable there is hardly any conspicuous mucilage sheath external to it. The cell protoplast shows distinct outer colured chromoplasm which surrounds the central colourless centroplasm.
The chromoplasm is invested bya two layered plasma membrane. The pigments located in flattened sac-like lamellae or thylakoids embedded in chromoplasm. The food reserves are in form of cyanophycean starch and cyanophycin granules. The planktonic species have pseudo-vacuoles.
The centroplasm has DNA fibrils, lacks nucleolus and nuclear membrane. There are no heterocyst is Oscillatoria.
The growth is intercalary.
Movement:
The characteristic feature of Oscillatoria is slow, rhythmic but active movement of its trichomes. Oscillatoria shows both autonomous and induced movements. Athe movements are of following three types:
1. Gliding or creeping movements:
These rhythmic movement take place in the direction of the long axis of the trichome. They are called axial movement. The trichome glides forward and backwards.
2. Oscillatory Movements:
Oscillatoria also exhibit slow waing movements. These are jerky, pendulum like oscillations of the front end. The genera got its name because of these movements (Oscillare- to swing).
3. Bending movements:
At the end of each oscillation there is generally a rapid bending of the apex.
Reproduction:
Oscillatoria reproduces vegetatively. The only know method is by the formation of hormogones.
The hormogonia are short sections of tiny cells separated from the trichomes. The hormogonia are formed by dead cell(necridia) which form biconcave separation discs.
These biconcave dead cells or separation discs provide weak links in the trichome and thus marks the points of disjunction of the trichome into hormogones.
The hormogonia by repeated cell division in all the cells grows into a full-length trichome.
Occasionally, propagation by fragmentation also takes place.
Nostoc
Division: Thallophyta
Sub Div: Algae
Class : Cyanophyceae
Order : Harmogonales
Family : Nostocacae
Genus: Nostoc
Distribution and Habitat:
The Genus Nostoc has 30 species. It is filamentous form of both terrestrial and aquatic habitats. The aquatic habitat include both fresh and sea water. Terrestrial species (N.commune) grow on dampsoil and forms leather sheets. Some species of Nostoc grow in paddy fields. Notoc ocuurs is symbiotic association with fungi to form lichens. They also occur in symbiotic association with Anthoceros (Bryophytes), Cycas (N.punctiformae), Gunnera manicata (Angiosperms).
Vegetative Structure:
Bead-like chains form the trichome. Each trichome is usually enclosed by its own mucilaginous sheath and is called a filament. The filament doesnot occur in single but grows in large colonies. The numerous filaments in the colony are held together by a mucilaginous envelope formed by the fusion of the individual trichome sheaths. The mucilage lump is bounded externally by a firm, tough pellicle like bonding membrane to form a definite colony. Each colony appear like bluish or yellowish mass of jelly.
Structure of trichome:
Each trichome is composed of numerous rounded or oval cells. The cells are joined loosely from end to end resembling a string of beads. At places, the trichome bear colourless, empty looking cells called the heterocysts, y are slightly large and have thicker walls than the vegetative cells.. spherical or balled shaped. They are intercalary and have two pores. The filament shows akinetes
Cell structure:
The cell consists of the cell wall surrounding the protoplast. The protoplast is differentiated into coloured chromoplasm and inner colourless centroplasm.
The chromoplasm has pigments (Chlorphyll a, Xanthophyll , Phycobilins) located in lamellae. The protoplast also contains the colourless granules of cyanophycean starch and cyanophycin granules of proteineous nature.
Reproduction:
Nostoc reproduces vegetatively and asexually.
Vegetative Reproduction:
Occurs by following methods-
Colony fragmentation:
Nostoc colony breaks up into flat expanses as result of storm or other disturbances. Each of these grow up to the size of the parent colony
Hormogonia;
Trichome ruptures at plaes where a heterocyst and the vegetative cell adjoin. This junction is the weakest link in the chain. In this way short segments of living cells called the hormogonia are formed.
The hormogonia move away from the enclosing gelatin matrix by gliding and establishes new colonies by division. Sometimes the hormogones develop into fresh trichome without being liberated from the colonial sheath and increase the number of trichomes in the adult colony.
Asexual Reproduction
Akinetes:
Under unfavourable conditions, any cell or some cells of the trichome become enlarge and each secretes a thick, highly resistant wall around it. Reserve food material accumulates in such cells and lose 90% of their photosynthetic and respiratory capabilities.
Such specially modified vegetative cells are called the akinetes or resting spores. It is not unusual to find all the cells between the two heterocysts and occasionally entire trichoe converted into akinetes.
In favourable conditions akinetes germinates and give rise to a new filament.
Endospore:
The internal division of the protoplast results in an irregular mass of spores called endospores. The endospores on liberation give rise to new filaments. Ex. N. commune, N. microscopicum.
Anabaena
Division: Thallophyta
Sub Div: Algae
Class : Cyanophyceae
Order : Harmogonales
Family : Nostocaceae
Genus: Anabaena
Distribution and Habitat:
Anabaena is a blue-green algae found floating in water along with Nostoc in permanent and semi-permanent pools. Most species are aquatic with planktonic nature. Many species constitute important components of water blooms along with Microcystis aeruginia. Few species grows in paddy fields. Many species show symbiotic association with variety of plants like Azolla, Cycas.
Vegetative Structure:
It forms unbranched filaments which occur singly or in flocoose colonies.No firm colony is formed in Anabaena as in Nostoc. The trichomes are straight and occur singly with a sheath. Sheath surrounding trichomes are hyaline and of water nature.
The cells are usually barrel shaped and spherical. The protoplast of vegetative cells are either homogenous or granulose filled with numerous pseudovacuoles.
The heterocysts are intercalary and are larger than vegetative cells.
Reproduction:
Vegetative reproduction by fragmentation and Hormogonia formation.
Asexual reproduction by Akinetes.
It is very hard to distinguish Anabaena from Nostoc. Nostoc canbe identify by a firm gelatinous envelope in which trichome are always much contorted, whereas Anabaeana does not have contorted trichome. Anbaena has a watery gelatinous sheath and never form colonies of definite form.
The cyanobacteria are the largest and most diverse group of photosynthetic bacteria which are the only known oxygen producing prokaryotes.
They comprise of about 2500 species . section 19 of Bergey’s Manual of determinative Bacteriology describes 34 genera.
Diagnostic Features:
The cell constituting the thallus are prokaryotic
The flagella is completely absent (even reproductive cells are non-flagellated ) and movement in the members is accomoplished by gliding actions.
The phycobilin pigments are unique to this class are blue C-phycocyanin and red C-phycoerythrin.
The other pigments are chlorophyll-a, ß-carotene and unique xanthophyll namely myxoxanthin and mycoxanthophyll.
Their photosynthetic system resembles that of the eukaryotes because they have chlorophyll-a and photosystem II, and carry out oxygenic photosysnthesis
There are no membrane bound chromatophores
The unique food storage compounds are the myxophycean starch and a proteinaceous material cyanophycin
They have specialized cells known as heterocysts
Sexual reproduction is completely absent.
Distribution and Habitat
Cosmopolitan in distribution, with most of the species occurring throughout the world.
They are widely spread in aquatic habitat, some terrestrial species are also known. The aquatic forms mostly occur in fresh water, a few however are marine. Members are both planktonic and benthic.
The blue-green algae are the only organisms that grow in adverse and harsh environments. The grow in snow and also constitute the principal vegetative of hot springs (Colteronema, Synechococcus). In different hotsprings they can survive in water temperature upto 85oC.
They are good example of the adaptability of life to extreme environment Because of their gelatinous sheaths they can withstand long periods of desiccation.
A fairly large number of members like Scytonema, Cyclindrospermum are able to fix atmospheric nitrogen and are common in paddy fields of tropical and subtropical countires.
Many blue-green algae grow in association with other organisms showing endophytic, endozoic and symbiotic associations. Gleocapsa and Nostoc are phycobionts of lichens. Nostoc and Anabaena occur within the plant bodies of certain Bryophytes (Anthoceros, Sphagnum), Ferns (Azolla), Cycads and non-leguminous angiosperms (Gunnera)
The factors that contribute to their wide distribution are i) ability to withstand prolonged drying and extremes of temperature, ii) capacity of some to fix atmospheric nitrogen, iii) liable metabolism, iv) ability of many of them to enter into symbiotic relationship with other plants v) modes of reproduction.
Organization of Thallus:
Just as do they vary in the choice of their habitats, so do they in the range of vegetative structure.
1) Unicellular forms:
The thallus is a unicell which is usually spherical or oval Ex. Chroococcus, Synechococcus, Gelopcapsa.
2) Colonial forms:
In most blue-greens, the cells after division remain attached to their walls or are held in a common gelatinous matrix to form a loose organization of cells which is termed a colony. Such colony may be definite (Merismopedia ) or irregular and indefinite (Aphanothece). The colonies may be either filamentous or non-filamentous.
Non-filamentous colonies:
They are of various forms. They may be cubical, spherical, irregular depending on the planes and direction of division of cells. They cells may divide in two planes at right angles to each other or in three planes.
Filamentous:
This is the result of repeated cell divisions in a single plan and in a single direction forming a chain or a thread. It is known as trichome. The trichome with enclosing gelatinous sheath is called a filament.
The filament in some genera has a single trichome Ex- Oscillatoria , Lyngbya. In others, the filament contains several trichomes Ex- Microcoleus, Hydrocoleus.
The filament may be brached or unbranched in unbranched filamentous form there is differentiation into a base and an apex and growth is diffuse. Ex-Oscillatoria.
Branched filaments may be either false or true. In true branching, the cells constituting the filament divide into two planes. Ex- Stigonema , Hapalosiphon.
In false branching trichome is displaced to one side and passes into a branch of the mucilaginous sheath while the other trichome of the filament continue in the original sheath. Ex- Tolypothrix and Scytonema.
Most highly evolved thallus which is heterotrichous with true branching are found in Stigonema, Mastigocladus.
Cell Structure
The cyanophycean cell is characterized by a primitive prokaryotic cell. There are no organized nuclei, chromatophores, pyrenoids, mitochondria or true vacuole.
Under light microscope the cell is seen to consist of a cell envelope enclosing the tiny protoplast.
Cell envelope:
Typically the cell envelope consists of two parts namely, the sheath and the inner investment or cell wall.
Sheath:
It is a layer of extracellular mucilage external to the cell wall. It consists of 3 layers of non-cellulosic microfibrils arranged in a reticulate manner and embedded in a gelatinous maxtrix of homogenous appearance. It may become lamellated and pigmented.
The sheath is useful in many ways i) it served to hold the cells of the colonies together, ii) colour variation, iii) withstanding long period of desiccation by water absorbing and water retaining capacity, iv) gliding movement, v) in perennation.
Cell Wall or Inner Investment:
Internal to the mucilage sheath is a wall layer termed inner investment. It is differentialed into 4 layer LI –LIV.
Layer LI – innermost layer of cell wall, lies next to the plasmalemma. It is visible as a clear electron –transparent space.
Layer LII – it is thin, electron dense layer with lies next to Layer LI, it contains mucopeptide and muramic acid (peptidoglycan). It also contains glucosamine, diaminopimelic acied. Similar to mucopolymer layer of Gram negative bacteria. The cell wall owes its shape and mechanical strength to this layer.
Layer LIII – It is external to LII and again a clear transparent space.
Layer LIV – it is outermost layer. Contains both lipopolysaccharides and proteins.
Protoplast
The protoplast shows internal differentiation. It is divisible into two regions i) a centrally located clear area forming the core called the centroplasm and ii) the peripheral denser region surrounding it called chromoplasm.
Plasma membrane:
Internal to cell wall, the protoplast at its periphery is surrounded by plasma or cell membrane. It shows small undulations and consists of two electron-opaque layers separated by a translucent layer.
Lamallae or thylakoids:
In the peripheral region of the protoplast, within the plasma membrane, are elongated, flattened sac or disc-like structures called thylakoids or lamellae. They are closely appressed and organized in parallel stacks. However they are not organized into grana. The thylakoids act as the sites of both photosynthesis and respiration.
Adjacent thylakoids are characteristically separated one from other by a space of 50nm which is occupied by contagious rows of discoidal phycobilisomes. Phycobilisomes are unique light harvesting complexes found in cyanobacteria and red algae. They are made of stacks of pigment proteins (phycobiliproteins) linked together by colour protein called linkers. The phycobiliproteins make up about 85% of the phycobilisome complex, and occur in three major forms i.e., phycocyanin (PC), Phycoerythrin (PE) and allophycocyanin (APC).
Pigments:
The chief pigments are chlorophyll a (not chlorophyll –b), ß-carotene and phycobiliproteins. The best known phycobiliproteins or phycobilins are blue C-phycocyanin and red C-phycoerythrin. The Xanthophylls like Zeaxanthin, myxoxanthophyll, myxoxanthin and oscilloxanthin are also present.
The phycocyanin is the characteristic pigment of the blue greens. It imparts blue colour to the thallus and chlorophyll-a imparts green. These two pigments thus are responsible for the characteristic blue-green colouration.
Chromatic adaptation or Gaidakov phenomenon:
In many species of cyanobacteria, the pigmentation seems to be determined by the quality of light in which they are grown. For example, some species of Oscillatoria can assume a green colour in red light, a reddish hue in green light, and a bluish-green in yellow light.
Such a capacity to change colour complementary to that of the light is known as chromatic adaptation or Gaidakov phenomenon.
This adaptation is of definite advantage since, it enables to maximally absorb the light for photosynthesis.
Nucleus:
In the central protoplasm the circular fibrils of DNA are present. It is not separated from the surrounding cytoplasm by any membrane. Nucleoli is absent. Histones are absent. RNA is diffusedly distributed in the nucleoplasm.
Granules:
Besides thyalkoids and DNA various kinds of granules are found in the protoplast.
Ribosomes: 70s ribosomes are dispersed throughout the cell but are abundant in the central region around the nucleoplasm.
Cyanophycean granules: they are regarded as reserve food. They are large dense bodies composed of copolymers of aspartic acid and arginine in 1:1 ration. Theya re also known as structural granules.
Polyglucan granules: they are polymers of glucose similar to animal glycogen and are also known as Alpha granules. These are less dense, longer than ribosomes and are commonly associated with thylakoids.
Polyhedral bodies: they are polyhedral in shape similar to carboxysomes found in bacteria. Thy are usually associated with the DNA. They contain carbon dioxide fixing enzyme Ribulose 1,5-bisphosphate carboxylase.
Phosphate bodies: (Metachromatic or Volutin granules) are spherical and appear similar to the lipid bodies of eukaryotic cells. They contain stored phosphate.
Gas Vacuoles or Pseudo Vacuoles
They are hollow cylindrical vesicles with conical ends. The membranous rigid wall of the gas vacuoles is proteinaceous in nature. It is impermeable to water but freely permeable to gas. Thus, the gas vacuoles are filled with metabolic gases.
The gas vesicles provide a buoyancy regulating mechanism enabling the planktonic prokaryotes to poise at particular depths. At low light intensity, the gas vesicles are formed at a rapid rate making the thallus buoyant. Such cells rise to the surface where under high light intensity more sugar is produced leading to higher osmotic pressure. On increase in osmotic pressure, the gas vacuoles collapse with the conical end caps splitting away. Such cells lose their buoyancy and sink down to the bottom.
By determining buoyancy, gas vacuoles indirectly serve as a screen against intense light.
Heterocyst
Heterocysts are specialized cell which differ from ordinary vegetative cells in having a thickened wall, homogenous and pale yellowish contents and one or two pores. Occurrence of heterocysts is a feature unique to cyanophyceae.
The heterocyst usually occur singly. In some genera they occur in pairs (Anabaenopsis) and rarely in chains. When they occur singly they are either terminal (Gloeotrichia) or intercalary in position (Nostoc).
Heterocysts are formed from vegetative cells by cell enlargement, dissolution of storage granules, the deposition of a multilayered envelope outside cell wall, gradual loss of photosynthetic pigmenst and re-orientation of the photosynthetic lamellae into a complex reticulate pattern.
Heterocyst structure can be divided into envelope, photosynthetic lamellae and other cytoplasmic inclusions. Wall is distinct into 3 layers, the outermost fibrous, the middle homogenous and inner most laminated layers.
The contents of heterocysts are generally dense and yellowish in colour due to disappearance of photosynthetic pigments leaving only the carotenes. There are no phycobiliprotein pigments and photosystem II in hetercysts. They are unable to evolve oxygen and cannot fix carbondioxide. They have higher respiratory rate than vegetative cells.
At the junction between the heterocyst and the vegetative cell there is a deep constriction. The constriction region has a pore for communication with adjacent cells. The constriction region has a pore for communication with the adjacent cells.
Plasmodesmata connect the plasma membranes of the heterocyst and the adjacent cells.
In mature heterocyst the pore channel is plugged with a polar nodule which is located between the plasma membrane and the inner envelope of the heterocyst.
The intercalary heterocyst has two pores, one at each pole whereas the terminal heterocyst has a single pore toward the trichome.
Functions:
1. Sites of nitrogen fixation.
2. Point of breakage of trichome.
3. They stimulate the formation of Akinetes.
4. They stimulate growth and cell division
5. Storage of food reserves.
Reproduction:
Vegetative and asexual modes of reproduction are commonly present in cyanobacteria. True sexuality or occurrence of sex cells and organs is absent.
Vegetative reproduction:
Vegetatively the cyanophytes multiply by i) cell division or fission ii) fragmentation and iii) by formation of hormogones or hormogonia
Cell Division or Fission
In unicellular blue-greens cell division leads to multiplication of the species. The division of the nuclear material (DNA) precedes the cell division. This is followed by the division of cytoplasm and separation of the resultant two daughter protoplasts.
Fragmentation:
During favourable conditions, propogation by fragmentation is common in both non-filamentous colony and filamentous forms.
Reaching a certain size, the non-filamentous colony splits into small parts. Each part which is called a fragment by repeated cell division grown into a new colony.
In filamentous form, the trichome may break into fragments by mechanical means such as by the bite of animals or stress caused by water currents or death of certain cells.
Hormogonia formation:
It is a specialized process of vegetative propogation characteristic of the cyanophyceae. The filamentous genera regularly multiply by breaking of their trichomes within the sheath into short fragments of one to many cells known as the hormogones or hormogonia.
Hormogonia are generally motile. The hormogones are formed in two ways:
1) By intercalary heterocyst:
Many heterocystous genera of blue-green have intercalary heterocyst separated a few cells apart from each other in the trichome. The trichome breaks at the sites of intercalary heterocysts because of weaker adhesion between a heterocyst and a vegetative cell.
2) By formation of separation discs:
In some genera one or few cells in the trichome undergoes lysis due to secretion of muric acid. As a consequence the protoplast of the cell degenerates to form a viscous substance. These mucilage filled dead cells are called necridia.
The walls of adjacent cells bulge and become convex. The dead cell (necridium) which is wedged between the bulging walls of the two adjacent cells appear biconcave. These biconcave, dead cells are called the intercalary biconcave separation discs. The latter provide weak links at which the trichome breaks and forms hormogonia.
The hormogonia finally settle down and grow into new filaments by cell division.
Asexual Reproduction:
Many cyanobacteria reproduce asexually by the formation of non-motile asexual spores. The spores are produced in ordinary vegetative cell. No meiosis or mitosis is involved in their formation.
Akinetes:
These are specialized thick-walled resistant cells formed in some heterocystous blue-green algae. They are larger than the vegetative cells. Mostly, they are developed singly next to a heterocyst at the base of a trichome.
During the formation of the akinete, the vegetative cell increase in size, accumulates food reserves, secrete a thick, multilayered spore wall surrounding the parent cell wall. It is often yellow or brown. Finally the modified cells round off and separate each constituting a reproductive unit. They are in reality the resting cells.
With the onset of conditions favourable for growth the akinete germinate and forms new filament.
Endospores:
These are small spores formed endogenously with the vegetative cell. At the time of endospore formation the vegetative cells increase in size and protoplast divides. A large number os small, uninucleate daughter protoplasts are formed.
Each daughter protoplast secretes a wall around it and become a endospore. The liberated endospore germinates immediately without a resting period.
Exospores:
In some generea, the cell wall ruptures at the distal end of the vegetative cell. The spores are successively pinched off at the exposed end of the extruded protoplast. They are called exospores. Ex. Chamaesiphon.
Nannocytes:
In some non-filamentous blue-greens the cell contents divide repeatedly without any cell enlargement. Numerous daughter cell are produced in each parent cell. These are called nannocytes. They differ from endospores in having naked protoplasts and extremely small size. The nannocytes germinate in situ to give rise to the colonies. Ex. Microcystis.
Oscillatoria
Division: Thallophyta
Sub Div: Algae
Class : Cyanophyceae
Order : Harmogonales
Family : Oscillatoriaceae
Genus: Oscillatoria
Distribution and Habitat:
The Genus Oscillatoria includes 100 species, which are wide in distribution. Oscillatoria is common, freshwater, filamentous dark blue-green algae.
They occur in varied habitat. Usually found in temporary rain water pools, dampsoil, road side ditches. Bottom of shallow temporary puddles and ditches, drain or sewers are usually covered with large patches of Oscillatoria. Few species are marine (O. sancta), halophillic (O. tambi), planktonic bloom forming (O. rubescenes, O. personata).
Vegetative Structure:
The thallus consists of free-living trichomes which often form a compact, tangled floating mass. Each trichome is long, thread-like unbranched structures.
Mostly it appears naked because the sheaths around it is very delicate and poorly developed. The trichomes are septate, the septa are faintly visible and often marked by rows of granules on either side.
Except the end cell, all are alike and discoid. Terminal cell is curved or convex at the free end. In a few species the apical cell may be swollen into a cap-like structure or capped capitates. In some apical cell may be thickened into a calyptra.
Cell structure:
All the cells in the trichome are similar in structure. The cells are broader than their length.
The cell wall being stable there is hardly any conspicuous mucilage sheath external to it. The cell protoplast shows distinct outer colured chromoplasm which surrounds the central colourless centroplasm.
The chromoplasm is invested bya two layered plasma membrane. The pigments located in flattened sac-like lamellae or thylakoids embedded in chromoplasm. The food reserves are in form of cyanophycean starch and cyanophycin granules. The planktonic species have pseudo-vacuoles.
The centroplasm has DNA fibrils, lacks nucleolus and nuclear membrane. There are no heterocyst is Oscillatoria.
The growth is intercalary.
Movement:
The characteristic feature of Oscillatoria is slow, rhythmic but active movement of its trichomes. Oscillatoria shows both autonomous and induced movements. Athe movements are of following three types:
1. Gliding or creeping movements:
These rhythmic movement take place in the direction of the long axis of the trichome. They are called axial movement. The trichome glides forward and backwards.
2. Oscillatory Movements:
Oscillatoria also exhibit slow waing movements. These are jerky, pendulum like oscillations of the front end. The genera got its name because of these movements (Oscillare- to swing).
3. Bending movements:
At the end of each oscillation there is generally a rapid bending of the apex.
Reproduction:
Oscillatoria reproduces vegetatively. The only know method is by the formation of hormogones.
The hormogonia are short sections of tiny cells separated from the trichomes. The hormogonia are formed by dead cell(necridia) which form biconcave separation discs.
These biconcave dead cells or separation discs provide weak links in the trichome and thus marks the points of disjunction of the trichome into hormogones.
The hormogonia by repeated cell division in all the cells grows into a full-length trichome.
Occasionally, propagation by fragmentation also takes place.
Nostoc
Division: Thallophyta
Sub Div: Algae
Class : Cyanophyceae
Order : Harmogonales
Family : Nostocacae
Genus: Nostoc
Distribution and Habitat:
The Genus Nostoc has 30 species. It is filamentous form of both terrestrial and aquatic habitats. The aquatic habitat include both fresh and sea water. Terrestrial species (N.commune) grow on dampsoil and forms leather sheets. Some species of Nostoc grow in paddy fields. Notoc ocuurs is symbiotic association with fungi to form lichens. They also occur in symbiotic association with Anthoceros (Bryophytes), Cycas (N.punctiformae), Gunnera manicata (Angiosperms).
Vegetative Structure:
Bead-like chains form the trichome. Each trichome is usually enclosed by its own mucilaginous sheath and is called a filament. The filament doesnot occur in single but grows in large colonies. The numerous filaments in the colony are held together by a mucilaginous envelope formed by the fusion of the individual trichome sheaths. The mucilage lump is bounded externally by a firm, tough pellicle like bonding membrane to form a definite colony. Each colony appear like bluish or yellowish mass of jelly.
Structure of trichome:
Each trichome is composed of numerous rounded or oval cells. The cells are joined loosely from end to end resembling a string of beads. At places, the trichome bear colourless, empty looking cells called the heterocysts, y are slightly large and have thicker walls than the vegetative cells.. spherical or balled shaped. They are intercalary and have two pores. The filament shows akinetes
Cell structure:
The cell consists of the cell wall surrounding the protoplast. The protoplast is differentiated into coloured chromoplasm and inner colourless centroplasm.
The chromoplasm has pigments (Chlorphyll a, Xanthophyll , Phycobilins) located in lamellae. The protoplast also contains the colourless granules of cyanophycean starch and cyanophycin granules of proteineous nature.
Reproduction:
Nostoc reproduces vegetatively and asexually.
Vegetative Reproduction:
Occurs by following methods-
Colony fragmentation:
Nostoc colony breaks up into flat expanses as result of storm or other disturbances. Each of these grow up to the size of the parent colony
Hormogonia;
Trichome ruptures at plaes where a heterocyst and the vegetative cell adjoin. This junction is the weakest link in the chain. In this way short segments of living cells called the hormogonia are formed.
The hormogonia move away from the enclosing gelatin matrix by gliding and establishes new colonies by division. Sometimes the hormogones develop into fresh trichome without being liberated from the colonial sheath and increase the number of trichomes in the adult colony.
Asexual Reproduction
Akinetes:
Under unfavourable conditions, any cell or some cells of the trichome become enlarge and each secretes a thick, highly resistant wall around it. Reserve food material accumulates in such cells and lose 90% of their photosynthetic and respiratory capabilities.
Such specially modified vegetative cells are called the akinetes or resting spores. It is not unusual to find all the cells between the two heterocysts and occasionally entire trichoe converted into akinetes.
In favourable conditions akinetes germinates and give rise to a new filament.
Endospore:
The internal division of the protoplast results in an irregular mass of spores called endospores. The endospores on liberation give rise to new filaments. Ex. N. commune, N. microscopicum.
Anabaena
Division: Thallophyta
Sub Div: Algae
Class : Cyanophyceae
Order : Harmogonales
Family : Nostocaceae
Genus: Anabaena
Distribution and Habitat:
Anabaena is a blue-green algae found floating in water along with Nostoc in permanent and semi-permanent pools. Most species are aquatic with planktonic nature. Many species constitute important components of water blooms along with Microcystis aeruginia. Few species grows in paddy fields. Many species show symbiotic association with variety of plants like Azolla, Cycas.
Vegetative Structure:
It forms unbranched filaments which occur singly or in flocoose colonies.No firm colony is formed in Anabaena as in Nostoc. The trichomes are straight and occur singly with a sheath. Sheath surrounding trichomes are hyaline and of water nature.
The cells are usually barrel shaped and spherical. The protoplast of vegetative cells are either homogenous or granulose filled with numerous pseudovacuoles.
The heterocysts are intercalary and are larger than vegetative cells.
Reproduction:
Vegetative reproduction by fragmentation and Hormogonia formation.
Asexual reproduction by Akinetes.
It is very hard to distinguish Anabaena from Nostoc. Nostoc canbe identify by a firm gelatinous envelope in which trichome are always much contorted, whereas Anabaeana does not have contorted trichome. Anbaena has a watery gelatinous sheath and never form colonies of definite form.
No comments:
Post a Comment