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Chapter: 11th Botany : Chapter 1 : Living World

Cyanobacteria (Blue Green Algae)

Stromatolites reveals the truth.

Cyanobacteria (Blue Green Algae)

How old are Cyanobacteria ?

Stromatolites reveals the truth.

Stromatolites are deposits formed when colonies of cyanobacteria bind with calcium carbonate. They have a geological age of 2.7 billion years. Their abundance in the fossil record indicates that cyanobacteria helped in raising the level of free oxygen in the atmosphere.

Cyanobacteria are popularly called as 'Blue green algae' or 'Cyanophyceae'. They are photosynthetic, prokaryotic organisms. According to evolutionary record Cyanobacteria are primitive forms and are found in different habitats. Most of them are fresh water and few are marine (Trichodesmium and Dermacarpa) Trichodesmium erythraeum a cyanobacterium imparts red colour to sea (Red sea). Species of Nostoc, Anabaena lead an endophytic life in the coralloid root of Cycas, leaves of aquatic fern Azolla and thallus of hornworts like Anthoceros by establishing a symbiotic association and fix atmospheric nitrogen. Members like Gloeocapsa, Nostoc, Scytonema are found as phycobionts in lichen thalli.


Salient features


            The members of this group are prokaryotes and lack motile reproductive structures.


            The thallus is unicellular in Chroococcus, Colonial in Gloeocapsa and filamentous trichome in Nostoc.


            Gliding movement is noticed in some species(Oscillatoria).


            The protoplasm is differentiated into central region called centroplasm and peripheral region bearing chromatophore called chromoplasm.


            The photosynthetic pigments include c-phyocyanin and c-phycoerythrin along with myxoxanthin and myxoxanthophyll.


The reserve food material is Cyanophycean starch.


            In some forms a large colourless cell is found in the terminal or intercalary position called Heterocysts. They are involved in nitrogen fixation.


            They reproduce only through vegetative methods and produce Akinetes (thick wall dormant cell formed from vegetative cell), Hormogonia (a portion of filament get detached and reproduce by cell division), fission, Endospores.


            The presence of mucilage around the thallus is characteristic feature of this group. Therefore, this group is also called Myxophyceae


            Sexual reproduction is absent.


            Microcystis aeruginosa, Anabaena flos-aquae cause water blooms and release toxins and affect the aquatic organism. Most of them fix atmospheric nitrogen and are used as biofertilizers (Example: Nostoc, Anabaena). Spirulina is rich in protein hence it is used as single cell protein. The thallus organisation and methods of reproduction is given in Figure 1.17


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