CYANOPHYCEAE.pptx

Cyanophyceae or Myxophyceae is a group of prokaryotic organisms, commonly called blue-green algae. Since their cellular organization is typically prokaryotic, the current trend is to consider them not as true algae, but as monerans. Accordingly, they are now grouped under Sub-kingdom Cyanobacteria of Kingdom Monera. The name Cyanophyceae or blue-green algae denotes the presence of the blue-green pigment phycocyanin; the name Myxophyceae refers to the presence of the carotenoid pigment myxoxanthin.
(i) Most members are filamentous forms, but some are unicellular and some others are colonial.��(ii) Filamentous forms consist of a linear row of cells, called trichome, enclosed by a common sheath. Trichome may have some large and thick-walled cells, called heterocysts.��(iii) Cells are typically prokaryotic, without membrane-bound cell organelles and an organized nucleus.
(iv) Chief pigments are chlorophyll-a, phycocyanin, allophycocyanin, phycoerythrin, myxoxanthin, oscillaxanthin, ß-carotene and leutein.�(v) Reserve food include cyanophysean starch (glycogen) and cyanophysin (a protein).�(vi) Protoplast is differentiated into outer chromoplasm and inner centroplasm.� (vii) Flagella are altogether absent at any stage.�(viii) Reproduction is asexual

1. CYANOPHYCEAE DR CHITHRA M

2. CLASS CYANOPHYCEAE Cyanophyceae or Myxophyceae is a group of prokaryotic organisms, commonly called blue-green algae. Since their cellular organization is typically prokaryotic, the current trend is to consider them not as true algae, but as monerans . Accordingly, they are now grouped under Sub-kingdom Cyanobacteria of Kingdom Monera . The name Cyanophyceae or blue-green algae denotes the presence of the blue-green pigment phycocyanin ; the name Myxophyceae refers to the presence of the carotenoid pigment myxoxanthin .

3. SALIENT FEATURES ( i ) Most members are filamentous forms, but some are unicellular and some others are colonial . ( ii) Filamentous forms consist of a linear row of cells, called trichome , enclosed by a common sheath. Trichome may have some large and thick-walled cells, called heterocysts . ( iii) Cells are typically prokaryotic, without membrane-bound cell organelles and an organized nucleus

4. ( iv) Chief pigments are chlorophyll-a, phycocyanin , allophycocyanin , phycoerythrin , myxoxanthin , oscillaxanthin , ß-carotene and leutein . ( v) Reserve food include cyanophysean starch (glycogen) and cyanophysin (a protein ). ( vi) Protoplast is differentiated into outer chromoplasm and inner centroplasm . (vii) Flagella are altogether absent at any stage . ( viii) Reproduction is asexual; sexual reproduction and zoospore formation are absent .

5. General account Blue green algae occur in a wide variety of habitats, such as fresh and salt waters, moist soil, rock, dead logs, bark of trees, etc. Majority of species are fresh-water, but marine forms are also known. Some can live in hot springs at temperatures well above 82°C, and some others can live on the under side of ice bergs. A number of species live in association with green algae, mosses, ferns, flowering plants, protozoans, mammals, etc.

6. (The red colour of the red sea is due to a blue-green alga Trichodesmium erythrinum )

7. . Fresh-water forms live in different habitats : in permanent and temporary waters, on damp cliffs, dripping rocky edges and in damp soil. Sometimes, they grow to such an extent that they form mucilaginous balls or vast coatings on the soil. Some of them are endophytic , living within the cells of other plants, such as the coralloid roots of Cycas or the leaf cavities of Azolla .

8. Members are mostly filamentous, although unicellular and colonial forms also occur. Actual unicellular forms are, however, rare since the copious production of mucilage causes the aggregation of daughter cells, forming irregular colonies. A row of cells forms a basic structural unit, called trichome . A trichome with its enclosing sheath is called a filament.

9. In filamentar forms, cells attach end-to-end and get embedded in a gelatinous sheath, forming a ribbon (e.g., Oscillataria , Nostoc ). But in colonial forms, cells adhere together and form a flat or spherical colony (e.g., Gleocapsa ). In some colonial species, there is division of labour among the cells, some cells are specialized for nitrogen fixation, some others for reproduction, and still others for the attachment of the colony to a substratum.

10. Blue-green algae differ from all the other algae by virtue of their prokaryotic cellular organization. Their cells are primitive and usually larger than a bacterial cell. All cells have a gelatinous sheath which contains pigments and toxins. Pigments are not localized in definite chromatophores , but are confined to the peripheral portion of the protoplast. Flagella are completely absent. The food-reserves include cyanophycean starch (a substance related to glycogen and starch) and cyanophycean granules (substances of proteinaceous nature).

11. Cell wall has two layers, an outer layer containing thick gelatinous sheath, formed of pectic compounds and an inner cellulosic layer. The inner layer also contains a mucopeptide component comparable to that found in bacterial cells.

12. The protoplast of blue-green algae is differentiated into an outer coloured portion, the chromoplasm , and an inner colourless portion, the centroplasm . Centroplasm contains the nuclear material, while chromoplasm is the seat of pigments, food-reserves, etc. In some genera, the chromoplasm also contains pseudovacuoles .

13. Organized nucleus and membrane bound cell organells such as Golgibodies,ER , mitochondria and plastids, are absent in blue green algae . However, there are specialized flat and vesicular membranes, called photosynthetic lamellae. They contain photosynthetic pigments. The genetic material of cyanobacteria is DNA which is not complexed with histones. It is localised in the centroplasm .

14. Most blue-green algae are photosynthetic (like green plants). They carry out oxygenic photosynthesis, releasing O₂. The principal photosynthetic pigment is chlorophyll a. The accessory pigments include carotenoids. In addition to these, the. cells contain phycocyanin (blue) and phycoerythrin (red), which give them a characteristic blue-green colour . However, to brown.

15. A number of species have the capacity to change their colour in relation to the wavelength ( colour ) of the incident light. This change in colour is often described as complementary chromatic adaptation or Gaidukov phenomenon.

16. Filamentous forms possess heterocysts , which are thick-walled and enlarged cells formed by the differentiation of normal vegetative cells . They are considered as the sites of nitrogen fixation. Besides heterocysts , some filaments also possess thickwalled cells with abundant food reserves. These are called akinetes . Under favourable conditions the akinetes germinate into new filaments.

17. Reproduction Blue-green algae reproduce only asexually by simple cell division (fission), spore formation and fragmentation. Multiplication by simple fission is common among unicellular or colonial forms. In filamentous forms multiplication is largely by the fragmentation of filaments into small pieces, called hormogonia . In each hormogonium , cells undergo fission and form new filaments. Cyanophycean spores are highly resistant to adverse conditions. They can remain dormant for several years. When conditions become favourable , they undergo germination.

18. True sexuality does not exist in Cyanophyceae . But, genetic recombination curs in forms like Anacystis nidulans by gene transfer and gene recombination through a cyanophage (virus ). A number of species thrive in polluted water. They are indicators of organic pollution. Spirulina is an edible form. Its protein content is very high. Now, it is cultivated and added to animal feeds to make them protein-rich.

19. Cyanophyceans are most significant in that they are the organisms which first transformed the anaerobic atmosphere of the primitive earth to an aerobic envelope through oxygenic photosynthesis. This, in turn, paved the way for the evolution of aerobic organisms. Many blue green algae, especially the heterocystous blue-green algae, are capable of fixing atmospheric nitrogen to increase soil fertility. They fix about 25-30 kg N₂/ha/crop. The non- heterocystous forms also fix atmospheric nitrogen, but usually under microaerophilic conditions. little fice orygon .)

20. Nitrogen fixation Blue-green algae (BGA) have the ability to fix atmospheric nitrogen. The heterocystous blue-green algae are mainly responsible for this (e.g., Nostoc , Anabaena, etc ). However, the non- heterocystous forms like Oscillatoria also fix atmospheric nitrogen under microaerophilic conditions. The enzyme nitrogenase is responsible for biological N₂ fixation. It converts atmospheric nitrogen into ammonia. Inoculation of blue-green algae into the paddy field significantly increases the rice yield. Hence, BGA are sold in the form of powdered flakes among paddy growers.

21. Affinities with bacteria Blue-green algae resemble bacteria in the nature of cell wall components, prokaryotic nature of cell and the absence of sexual reproduction. Hence, Blue green algae are also called cyanobacteria.

22. Biological importance Cyanophyceans are ecologically important in the following ways : ( i ) As producers they provide oxygen and organic matter for other organisms (ii) Many species are nitrogen fixers and they incorporate N, into organic compounds that are used by plants.(iii) Some species form symbiotic associations with protists , fungi and plants. Some others, together with fungi, form lichens. All these forms lack cell wall and function as protoplasts for producing food for their partners.(iv) Some species can tolerate extremes of salinity, temperature and pH, that are harmful to other organisms. So, they can thrive in polluted water, often becoming the dominant species of ponds and lakes.

23. NOSTOC CHROOCOCCUS GLOEOCAPSA