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Cyanobacteria (Oxygenic Phototrophs)
The cyanobacteria (the earlier blue-green algae), or the blue-green bacteria, represent a group of
photosynthetic, mostly photolysis-mediated oxygen-evolving monerans (prokaryotes). These are the
only organisms able to perform oxygenic photosynthesis that can also fix nitrogen. These organisms
are amongst the oldest organisms known dating back to the early Precambrian period 3.6 x 109 years
ago and probably played a crucial role in the evolution of higher plants
Cyanobacterial thallus ranges from unicellular, colonial to filamentous; multiseriate branched
filamentous condition is the highest level of organization attained by them. Because of their close
architectural, physiological and biochemical similarities with bacteria, especially the gram-negative
ones, the cyanobacteria have been placed under a separate division, namely, 'The Cyanobacteria' in
Bergey's Manual of Determinative Bacteriology (8th edition, 1974), which has been accepted and used
widely as the standard reference for bacterial toxonomy. This manual, however, has recognized the
Kingdom-Monera of Whittaker, but has called it Kingdom-Prokaryotae because of the prokaryotic
nature of all the monerans. The cyanobacteria possess various distinguishing characters, which can
be summarized as under.
(a) Cyanobacteria can grow in diverse habitats, but one striking feature in their occurrence and
predominance in habitats alternating between photoaerobic and photoanaerobic conditions can be
correlated with their preference for low oxygen tension and low redox-potential. These properties stem
from their recently discovered dual-capacity of oxygenic photosynthesis and facultative anoxygenic
photosynthesis.
(b) The cyanobacteria possess various morphologically distinctive structures, e.g., akinetes and
heterocysts.
(c) The main cell wall constituent of cyanobacteria is peptidoglycan.
(d) The cyanobacterial cytoplasm is traversed extensively by flattened vesicular structures called
thylakoids or lamellae, the photosynthetic sites.
(e) The principal photosynthetic pigment of all cyanobacteria is chlorophyll a. Besides, they possess bcarotene and other accessory pigments, namely, phycobiliproteins. The phycobiliproteins are
phycocyanin (PC), allophycocyanin (AP), allophycocyanin B (APB), and phycoerythrin.
(f) Most filamentous cyanobacteria show a gliding motility at some stage of development; they lack
flagella.
Classification
Rippka et al. (1979) have proposed a modern scheme of cyanobacteria classification taking mainly
their physiology, cell constituents and DNA characteristics into consideration. They have created five
sub-groups called 'sections'.
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System of classification of cyanobacteria
Section
Cell type
I
Unicells or
aggregates
II
Unicells or
aggregates
III
IV
V
Filaments;
unbranched
trichomes
with only
vegetative
cells
Filaments
can form
heterocysts;
no true
branching
Filaments
can form
heterocysts
and true
branches
Mode of
reproduction
Range Recognized Genera
of G +
C%
Binary fission or 35-71 Synechocoous,
budding
Synechocyatis,
Gloeobacter,
Gloeotheca, Gloeocapsa,
Chamesiphon
Multiple fission to 38-47 Dermocarpa,
form baeocytes
Xenococcus,
Myxosarchina,
Binary fission in a 40-67
single plane
Hormogonia form; 38-47
binary fission in a
single plane
Hormogonia,
Akinetes, 42-46
Hormocysts;
binary fission in
more than one
plane.
42-46
Chroococcidiopsis
Spirulina, Lyngbya,
Oscillatoria,
Pseudoanabaena,
Phormidium,
Plectonema
Anabaena, Nodularia,
Nostoic,
Cylindrospermum
Scytonema, Calothrix,
Tolypothrix, Rivularia
Chogloeopsis,
Fischerella,
mastigocladus
Other properties
Almost always
nonmotile
Usually some
baeocytes are
motile
Usually motile
Often motile; may
form akinetes
Greatest morphological complexity
and differentiation
in cyanobacteria
Ultrastructure of Cyanobacterial Cell
(i) Mucilage Sheath
The cells and filaments of most cyanobacteria are generally surrounded by a mucilaginous sheath
whose thickness, pigmentation, consistency, and nature is greatly influenced by environmental factors.
It is considered that these microorganisms secrete the mucilage through pores present in their cell
walls
(iii) Plasma membrane
A typical cyanobacterial plasma membrane is 70A thick. It is selectively permeable, lacks sterole such
as cholesterol, and consists of a high proportion of protein to phospholipid (typically 2: 1) like other
monerans. It generally fuses with the photosynthetic lamellae and gives rise to inward foldings in the
cytoplasm; the foldings are called lamellosomes or mesosomes (Fig. 4.10). The latter are mostly
similar in functions to mesosomes occurring in other monerans.
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Anaeba Possessing Heterocyst and Akinete
7.Cell wall
1. Photosynthetic Lamella
8. Cell Membrane
2. Ribosome
9. Cyanophycin Granule
3. Beta-Granule
10. DNA
4. Alpha-Granule
11. Protein Body
5. Polyphosphate Granule
12. Reserve Food
6. Sheath
13. Lamellosome
The Cytoplasm
The cytoplasm of cyanobacterial cell, like that of bacteria, is incredibly boring. It lacks eukaryotic
organelles such as chloroplasts, mitochondria, endoplasmic reticulum, Golgi bodies. But it possesses
photosynthetic apparatus, ribosomes, and a large number of subcellular inclusions such as glycogen or
a-granules, polyphosphate bodies, polyhedral bodies, cyanophycin granules, and the genetic material
Photosynthetic apparatus. In place of the chloroplasts of photosynthetic eukaryotes, cyanobacteria
have flattened vesicular structures called thyllakoids or lamellae, which resembles the individual
thyllakoids of the true chloroplasts of photosynthetic eukaryotes. The lamellae or thyllakoids are both
physiologically or structurally complex and possess photosynthetic pigments. As described earlier, the
principal pigment of all cyanobacteria is chlorophyll a. In addition, there are b-carotene and other
accessory pigments, namely, phycobiliproteins. The phycobiliproteins are phycocyanin (PC),
allophycocyanin (AP), allophycocyanin-B (APB), and phycoerythrin.
By possessing phycocyanin and phycoerythrin accessory pigments, the cyanobacteria resemble with
red-algae. However, the necessary pigments of these organisms are generally organized into organelles
called phycobilisomes and trap light energy of lower wavelengths, which cannot be absorbed by
chlorophyll a, and pass it on to the chlorophyll in the diagram. This is the reason why cyanobacteria,
like green algae, can exploit deeper waters where the quality and quantity of illumination is
inappropriate for the photosynthetic plants.
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Energy Shunting of Phycoerythrin and Phycocyanin
(b) Ribosomes. These are the sites of protein synthesis. Cyanobacterian ribosomes occur freely In the
cytoplasm and are identical to those of bacteria in being 70S ribosomes.
(c) Glycogen or alpha-granules. Glycogen or alpha-granules are the sites for storage of excess
photosynthetic products. The latter is used as energy source in darkness or when CO2 supply is
limiting.
(d) Polyphosphate bodies. These are the spherical structures formed as a result of the aggregation of
high molecular weight linear polyphosphates. These subcellular inclusions are also called
metachromatin granules or volutin granules and serve as phosphate stores and are consumed during
periods of phosphate starvation. These structures develop mostly in those cyanobacteria, which grow
in a phosphate-rich environment.
(e) Polyhedral bodies. All cyanobacteria store their ribulose I, 5-bisphosphate carboxylase (RUBP
carboxylase) enzyme in structures referred to as polyhedral bodies.
(f) Cyanophycin granules. Cyanobacteria growing in nitrogen-rich environment produce structures
called cyanophycin granules, which are made up of arginine and aspartic acid.
(g) Genetic material. The genetic material of cyanobacteria is made up of naked DNA fibrils found
dispersed in the central region of the cytoplasms. Like other monerans, they lack membrane-¬bound
organized nucleus. The exact number of genomes per cell is not yet known; it has recently been
reported that Agmenellum contains 2, 3 or more copies of its genetic material. The molecular weight
of the cyanobacterial genome is considered to range from 2.7 to 7.5 x 109 daltons.
(h) Plasmids. All the naturally occurring plasmids in cyanobacteria are phenotypically cryptic. They
are covalently closed circular DNAs and their genetic compositions and complete function is not yet
known. However, plasmid-mediated transfer of genetic material has been reported in certain
cyanobacteria
Reproduction
Like bacteria, the cyanobacteria also reproduce asexually and the commonest mode of reproduction in
them in transverse binary fission (see bacteria). In addition, there are certain specialized structures
such as akinetes, hormogonia, hormocysts and spores, which are partly involved in the process of
reproduction. So far as the sexual reproduction in its true sense is concerned, it is absent in them and
the requirements of sexuality are considered to be met by some alternative pathways referred to as
parasexual-pathways
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(i) Akinetes
Most filamentous cyanobacteria develop perennating (dormant structures) in adverse condition. These
structures are larger than the vegetative cells, are equipped with thick walls, and are called akinetes
(Fig. 4.12). When favourable conditions return, they germinate and produce new filaments.
(ii) Hormogonia
All filamentous Cyanobacterium reproduce by fragmentation of their filaments (trichomes) at more or
less regular intervals to form short pieces each consisting of 5-15 cells. These short pieces of filaments
are called hormogonia. The latter show gliding motility and develop into new full-fledged filaments.
(iii) Hormocysts
Some cyanobacterial produce hormocysts, which are multicellular structures having a thick and
massive sheath. 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.
(iv) Spores
Nonfilamentous cyanobacteria generally produce spores such as endospores, exospores and nanocysts
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 nanocysts is that in endospore formation the parent cell concomitantly
enlarges in size, whereas in nanocysts formation there is no such enlargement of the cell.
Parasexuality in cyanobacteria
The knowledge of cyanobacterial genetics is relatively new and was pioneered by Kumar (1962) who
obtained penicillin and streptomycin resistant strains of Anacystis nidulans, crossed them, and
successfully demonstrated the appearance of a third type of recombinant strain resistant to both the
antibiotics. However, the mechanisms of genetic recombination in cyanobacteria are thought to be the
same as those of bacteria
The existence of the process of transformation in cyanobacteria was established experimentally in
1979 by Doolottle. Stevens and Porter in 1980 has successfully demonstrated this process in
Agmenellum quadruplicatum. The transforming principle was shown to be DNA. Some
cyanobacteriologists, however, have found that the transformation is mediated in some cases by
complexes of DNA and RNA.
The first report on conjugation in a Cyanobacterium, namely, Anacysts nidulans was by Kumar and
Ueda (1984). The frequency of conjugation is very low (about 1 in 106 cells) and cells conjugate by
means of a conjungation tube.
The knowledge of transduction in these microorganisms at present is restricted to some preliminary
reports. However, the occurrence of different cyanophages, e.g., LIP 1-7, SM-1, N-1 and infection of
several cyanobacteria by them prompts one to imagine that the virus-mediated method of genetic
transfer (transduction) in cyanobacteria will be conclusively established in the near future
Heterocyst in Cyanobacteria
As stated earlier, the cyanobacteria are the only organisms able to perform oxygenic photosynthesis
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that can also fix nitrogen; many, but not all, are vigorous nitrogen fixers. The coexistence of the
processes of oxygenic photosynthesis and intrinsically anaerobic nitrogen fixation process in a single
organism presents an obvious paradox because nitrogenase, the key enzymes, is rapidly and
irreversibly inactivated by an exposure even to low partial pressure of oxygen. However, the nitrogen
fixing cyanobacteria produce a specialized type of cell, the heterocyst, within which nitrogen is fixed
Filamentous forms of cyanobacteria such as Anabaena form large, distinctive cyst-like cells, the
heterocysts at intervals along the trichome (filament), . The latter develop from normal vegetative cells
particularly in conditions deficient in NH3+ or NO3-, and are considered to be the site of nitrogen
fixation. The conversion of atmospheric nitrogen to ammonia takes place under highly anaerobic
conditions, that only the heterocysts are able to provide. For instance, the oxygen-evolving part of the
photosynthetic mechanism (photosystem II) is blocked in heterocysts, and the remaining
photosynthetic machinery becomes geared to provide energy for the reduction of nitrogen to NH3+.
Trichome of Anabaena possessing Heterocyst and Akinetic
1. Vegetative Cells
2. Heterocyst
3. Akinete
4. Thick wall
5. Cytoplasm Devoid of Photosystem II
6. Cyanophycin Plug
7. Thicker Mucilaginous Coat
Heterocyst:
1. Heterocysts are distinctive and specialized cells which occur in certain filamentous
genera of blue-green algae.
2. These are round , large in size and possess a thick wall and one or two polar nodules
on the side of attachment with the negative cell or cells.
3. Mucilage envelops except at the region of polarity.
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4.
5.
6.
7.
The cell contents are homogenous and yellowish green in appearance.
They occur singly in terminal or intercalary position in a filament.
Newly formed vegetative cell gets transformed into a heterocyst.
Vegetative enlarges and cell wall become many layered . Except carotenoides other
pigments are lost.
8. granular bodies disappear , the photosynthetic lamella become and form a complex
reticulation .
9. Under certain environmental conditions , the protoplast of the heterocyst may
rejuvenate and germinate.
10. Heterocyst function differently under different environmental conditions. They reported
to play role in sporulation.
11. Heterocyst can transform in to akinetes .
12. They may provide a suitable place for fragmentation of the filaments.
13. It is also reported to be a store house of enzymes and they play role in nitrogen fixation.
Akinetes ( Resting Spores ):
1.These Are oblong or spherical and large in size than vegetative cells.
2. They do not contain photosynthetic pigment.
3. Contain abundant food granules.
4.They have thick wall ; the outer wall shows ornamentation and yellow or brown in colour.
5. Thickness of wall is due to deposition of material during enlargement .
6.Akinetes may occur singly or in long chains.
7. They can with stand desiccation and extremes of temperatures.
8.They germinate after a period of rest.
Nutrition:
Blue-green algae are obligatory autotrophic . A few members of Nostoc can grow heterotrophically.
Similarities between blue-green algae and Bacteria.
1. The nucleus in both lack double membrane. DNA fibrils are dispersed in the central region of
the cell . Histones are absent .
2. Blue-green algae and photosynthetic bacteria possess thylakoids which are not membrane
bound.
3. True sexuality is absent .No meiosis
4. Cytoplasmic streaming is absent .Cell organelles such as endoplasmic reticulum ,
mitochondria, golgi bodies and membrane bound vacuoles are absent.
5. photosynthetic lamellae or thylakoides perform photosynthesis , respiration , oxidative
phosphorylation and nitrogen fixation.
6. Cell wall possess a characteristic muco eptide , a l- aminopimelic acid and muramic acid
component.
7. Both have a highly refined and complex biochemical property of fixation of elementary
nitrogen and oxidation of H2S .
8.
9. Morphological resemblance are noticed between Beggiatoa and Oscillatoria in structure,
method of movement and deposition of sulphur droplets in the cells.
10. Hormogonia like structure occur in thiothrix , Sheath and false branches are known to be
present in the filaments of Cladothrix.
11. Both are susceptible to phages ;cyanophages and bacteriophages respectively.
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12. Fossile evidence show that the unicellular forms of the two groups were first cellular
organism to have evolved on the surface of earth some 3 billion years before.
Cyanobacteria Promient Characters
1.Possess two photosystems; PS-I and PS-II
2.Photosynthetic pigments are Chlorophyll-a Phycobiliproteins and carotenoides.
3.Electron source is water . Reduced compounds for few species.
4.Respiration is aerobic when growing photosynthetically.
5. Some strains can fix Nitrogen in the dark.
Differences between Blue-green algae and Bacteria
Blue-green Algae
1) Majority of them are photosynthetic.
Bacteria
Majority of them are saprophytic , parasites or
chemo synthetic.
2) Flagella are absent.
Flagella are present.
3) Endogenous spore formation is absent.
Present
4) Chlorophyll-a is main photosynthetic Bacteriochlorophyll is main photosynthetic
pigment.
pigment.
5) Hydroxyl ions are used as hydrogen donor Hydroxyl ions are not used and oxygen is not
and oxygen is evolved.
evolved.
6) Contain certain unsaturated fatty acids.
Do not contain.
7) Heterocyst present.
Heterocyst absent.
Classification:
The most commonly followed classification is the one proposed by Fristch ( 1945).
a) Without hormogonia
Order-I Chrococcales.
1) Unicellular or colonial
2) Multiplication by cell division and by endospores
Ex: Chrococcus, Gleocapsa
Order-II Chamaesiphonales
1) Unicellular or colonial epiphytes or lithophytes exhibiting marked polarity.
2) Multiplication by endospores or exospores.
Ex: Dermocapsa , Chamaesiphon.
Order-III Pleurocapsales
1)Heterotrichous filamentous types
2) Heterocyst absent
3) Multiplication by endospores , hormogonia absent.
Ex: Pleurocapsa, Hyella.
With hormogonia.
Order-IV Nostacales
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1) Non –heterotrichous filamentous type showing false branching.
2) Herocyst present
3) Multiplication by hormogonia, hormocysts and akinetes.
Ex: Nostoc , Oscillatoria
Order-V Stigonematales.
1) Heterotrichous filamentous type with true branching .
2) Herocyst present
3) Ex: Stigonema, Haplosiphon.
Important Characteristics of Blue-Green Algae
Order
General Shape
Reproduction &
Growth
Heterocyst
Other
properties
Rep.Genera
Chroococcales
Unicellular rods or
cocci:nonfilamentous
aggregates
Binary
fission,budding
Absent
Almost always
nonmotile
Gleocapsa
Chamaesiphon
Pleurocapsales
Unicelluar rods or
cocci;may be held
together in aggregates
Multiple fission to
form baeocytes
Absent
Only some
baeocytes are
motile
Plerocapsa
Dermocarpa
oscillatoriales
Filamentous,
unbranched trichome
with only vegetative
cells
Binary fission in a
single
plane,fragmentation
Absent
Usually motile
Oscillatoria
Spirulina
Nostacales
Filamentous ,
unbranched trichome
may contain
specialised cells
Binary fission in a
single plane ,
fragmentation to
form hormogonia
Present
Often motile ,
may produce
akinetes
Filamentous trichomes
either with branches or
composed of more
than one row of cells
Binary fission in
more than one plane ,
hormogonia formed
Present
Stigonematales
May produce
akinetes
,greatest
morphological
complexity in
cyanobacteria
Nostoc
Anabaena
Stigonema
Fishcherella
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CYANOPHYCEAE
HORMOGONALES (NOSTOCALES)
SCYTONEMATACEAE
SCYTONEMA
Occurrence: Generally found as intricately fused filamentous masses either in purely aquatic
or semi terrestrial conditions. Species of Scytonema like S. simplex are completely submerged
forming the undergrowths of ponds and. lakes. Some species are found attached to aquatic plants.
Terrestrial species like S. oscillatum form mats on wet rocks, damps barks of trees etc. Some species
of Scytonema are also known to occur as "Phycobionts" (algal partners! in Lichens.
Structure: The plant body is a filament and is surrounded by mucilage sheath, like in other
blue green algae. The. sheath is lamellated and of golden yellow or brown color. 'Ihe filaments
possess either intercalary or terminal heterocysts.
The genus is characterized by its unique type of branching. This is known as ,"false branching" or
"Gemminate branching".
The filament breaks up near a heterocyst and the broken end protrudes out of the sheath as a branch.
If both the free ends protrude out as branches there will be two branches and it is known-as
"gemminate branching". If only one develops into a branch, it is known as "false branching". Later
the branches develop their
own mucilage sheath.
Cause of Branching: Branching is the result of the following causes:
(1) By the degeneration of Intercalary cell: When one intercalary cell
dies, the free ends of cells continue growth resulting in gemminate
branching.
(2) By the development of separation discs: An intercalary cell in the
filament becomes dark in color and thin walled losing its contents.
Gradually it becomes biconcave in shape and degenerates ultimately. Soon
after,
branching follows, producing a pair of branches .
.
(3) By the formation of Loop: It is also due to the formation of loops resulting in branching of
Scytonema. The filament bulges out and forms a loop. One of the terminal cells in the loop degenerates
and this results 'in two pseudobranches.
(4) By the Heterocysts: The trichome breaks up at the point of' heterocyst and branching follows the
usual pattern.
Cell structure: Cells are cylindrical or rectangular in shape. Heterocysts are found in between the
vegetative cells. The terminal cell is usually hemisperical
There is no sexual reproduction:
Asexual reproduction is brought about by the formation of hormogonia. A hormogonium is formed
by the formation of two separation discs. Each one consists of
several cells and germinates into a new filament.
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Classification
Kingdom: Bacteria
Division: Cyanobacteria
Class: see taxonomic note
Order: Nostocales
Family: Nostocaceae
Genus: Nostoc
Species:
Nostoc calcicola, N. commune, N. cycadae, N. desertorum, N. edaphicum, N. ellipsosporum, N.
entophytum, N. flagelliforme, N. indistinguenda, N. lichenoides, N. linckia, N. muscorum, N.
paludosum, N. piscinale, N. punctiforme, N. sphaericum.
Nostoc is a genus of fresh water cyanobacteria that forms spherical colonies composed of filaments of
moniliform cells in a gelatinous sheath. When on the ground, a Nostoc colony is ordinarily not seen;
but after a rain it swells up into a conspicuous jellylike mass, which was once thought to have fallen
from the sky, whence the popular names, fallen star and star jelly. It is also called witches' butter
(not to be confused with the fungus Tremella mesenterica). Michael Quinion of the World Wide Words
newsletter says that it is known in Welsh as pwdre sêr, or rot of the stars.[1]
Nostoc can be found on moist rocks, at the bottom of lakes and springs, and rarely in marine habitats.
It may also grow symbiotically within the tissues of plants, such as the aquatic fern Azolla (mosquito
fern) or hornworts, providing nitrogen to its host. These bacteria contain photosynthetic pigments in
their cytoplasm to perform photosynthesis.
Description and Significance
Nostoc is a diverse genus of simple algae, belonging to the blue-green group. They are found in
gelatinous colonies, composed of filaments called "trichomes" surrounded by a thin sheath. They are
common in both aquatic and terrestrial habitats. These organisms are known for their unusual ability to
lie dormant for long periods of time and abruptly recover metabolic activity when rehydrated with
liquid water. The bacteria's ability to withstand freezing and thawing cycles make them well-adapted
to living in extreme environments, such as the Arctic and Antarctica. They can fix atmospheric
nitrogen, making them good candidates for environments with low nitrogen rates. Nostoc, first
discovered in the 19th century, is one of the most widespread phototrophic bacteria in the world. As a
nitrogen fixer, these bacteria may provide plants with important nutrients and therefore can be used
agriculturally. In 1988 a terrestrial species, Nostoc commune, was found to harbor a previously
unidentified UV-A/B absorbing pigment. This protective pigment has enabled them to survive not
only while under hydration-related stress, but in areas of extreme UV radiation as well.
Genome Structure
Nostoc's genetics are worth studying because of the genus' unique adaptations which allow them to
survive and even thrive in extreme environments. Also, a better understanding of soil-dwelling
nitrogen fixers such as Nostoc may help advance fertilizer production and benefit agriculturalists.
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Cell Structure and Metabolism
Nostocs are photosynthesizers which use cytoplasmic photosynthetic pigments rather than
chloroplasts in their metabolic process. They are single-celled, and lack a nucleus or other internal
membrane systems; their cytoplasm is composed 70%-85% of water. The cells do not possess flagella,
but are motile by a swaying motion. Division is by binary fission; some branching may occur. The
cells form filamentous structures known as trichomes, which in turn make up colonies encased by a
thin sheath; these colonies may be mat-like or spherical and are either micro- or macroscopic-spherical colonies may reach sizes of up to 2.6 kg wet weight.
* * * * * * * ***
Anabaena
Class : Cyanophyceae
Order: Nostacales
Family: Nostacaceae
Genus : Anabaena
Occurrence: Commonly found in aqatic environments . They grow luxiantly in the rice fields .many
species are plankonic in lakes and ponds and impart a blue-green coloration to water.
Symbiotic members are found in association with Azolla.
Thallus Structure: A thallus may consists of single filament with or without individual envelop . In the
planktonic forms , it may consists of many filaments entangled and enclosed in a delicate
mucilagenous matrix of indefinite shape . The sheath surronding trichomes are always hyaline and
diffluent, narrow or broad.
Mostly thalli are free floating in water or form a mucilagenous stream on the surface of moist soil.
Filaments are usually straight . Planktonic species may be coiled or irregularly contorted .Filaments
are uniserate , in which the cells are arranged end to end.
Trichomes are uniformly broad throughout. The cells are cylindrical but spherical cells are also present
in some species.
Pseudovacuoles are present in planktonic forms.
The protoplasm is either homogenous or granular and cell contents are blue-green , gray or variously
colored. Heterocysts are also found.Heterocyst are usually of the same shape and slightly larger than
vegetative cells.In many species a restricted number of akinetes occur adjacent to heterocysts. In other
species , they develop in long chains and away fron heterocysts.
Reproduction :
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a) By Hormogonia:Hormogonia are frequently formed due to breaking up of trichomes in to
smaller pieces at the region of heterocysts. On germination , they develop heterocyst and grow
into long filaments. They are motile in many species.
b) By akinetes : in many species the number and position of akinetes is a specific character i.e;
they may be one or two or more in chains .Further , they may be on one or both the sides of a
heterocyst or may be disposed away from it. Akinetes are generally cylindrical but may be
spherical .On germination , akinetes form a few celled hormogonia.
Nitrogen deficiency appears to be one factor leading to production of akinetes.
c) By germination of heterocysts:In a few species , heterocyst germinate to form a few celled
hormogonia . The pigments reappear and cell contents transform into a germling.
Anabaena possess motile stage in the vegetative pase and the non motile phase is merely a response to
adverse conditions.
Fixation of nitrogen has been convingly demonstrated in species of A.cylindrica,
A. ambigua, A. naviculodes
Stigonema
Order:Stigonamatales
Family:Stigonemataceae
Genus:Stigonema
Occurence: Stigonema is widely distributed on sub aerial situations including moist rocks and tree
trunks . Some species grow on moist soils and some other as free floating forms in fresh water
reservoirs . The algae often grows mixed with mosses and smaller plants . Some species are known to
dwell as the algal component in certain lichens.
Thallus Structure:
Thallus possess a filamentous and hetertrichous habit. The filaments are variuosly curved and
irregularly branched . Many species have densely interwined filaments and form a thin cushion like
growth. The branches may arise from all sides of the main axis and may branch and rebranch
repeatedly . The branches are true branch and lateral which retain the continuity with the main
filament.
In inition of a true branch, the cell divides parallel to the axis of the filament . The upper cell as grows,
is accommodated within simultaneously growing sheath and it does not come out . in the young
filaments , the sheath remains close to the trichome while in mature filaments it becomes wide , firm
and mucilagenous . The sheath may remain hyaline and homogenous or mor e commonly it may
become lamellated and develop yellow brown or black colour. In early stage , the filaments may be
uniserate but later stage they become multiserrate due to longitudinal divisions of cells.
The cells are characteristically spherical but in young axis and branches they are flattened due to
mutual pressure. In old filaments the lie at a distance fro one another and these cells may be surronded
by their individual sheath. In mature filaments ,adjacent sister cells are connected with each other by
pit connections.
Heterocysts are present both in the main filament as well as in the branches . They may be terminal or
intercalary but lateral in position
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The filament exhibit apical growth .In many species , as the filament becomes older , the pit
connections are withdrawn , cells divide to form independent pockets with lamellated envelop.
Reproduction:
In the multiserate species ,ther are two kinds of true lateral branches . Some branches are of limited
growth , multiserate and are purely vegetative . Others are short with club-shaped apieces , uniserate
and produce a few celled hormogonia .
The hormogonia groe independently and form new filaments .
Fixation of elementary nitrogen has been recorded in Stigonema informe

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