Jan 7, 2013

Cyanobacteria or Blue Green Algae


Blue green algae contain a blue pigment called phycocyanin and a red pigment called phycoerythrin. The simplest mixture of chlorophyll and blue green pigment in some species produces the blue green color that gives the entire group its common name. But those species that contain red pigments appear red, purple brown or even black. Examples: Spirulina, Anabaena, Rivularia, Osdilatoria, Nostoc.

a

b

c
                                Blue-green algae (a) Glocapsa (b) Anabaena (c) Oscillatoria

NOSTOC

Habitat: Fresh water e.g. ponds, ditches and other pools of water and wet soil. It is filamentous in from but occurs in colonies. The colony floats in water like a ball and is surrounded by a strong mucilage sheath. It can be seen by naked eyes. They’re several centimeters thick.

Structure: It occurs as a jelly like mass. The mass consists of many slender, long and short filaments. The filaments are interwoven. The filament under microscope looks like a chain of beads. Each filament is un-branched. Its mucilage sheath is covering each filament. The individual chain of cells is called Trichome. Heterocysts are transparent round or barrel shaped cells found in the trichome. It may be terminal (at the end of the filament) or intercalary (in between the two cells).

Nostoc a blue-green alga 
Cells of trichome: Each cell of trichome consists of two regions:
(a)   Chromoplasm is the outer peripheral zone with photosynthetic membrane.
(b)   Central body is the inner zone with nuclear material.

Reproduction:

(a) Sexual reproduction is absent.

(b) Asexual reproduction: It takes place by hormogonia, akinetes and spore formation.

Hormogonia: The trichome breaks at the point of heterocyst. The pieces are called hormogonia. Each hormogonium divides and forms a new filament (trichome)

Akinete: Certain cells of the filament may become enlarged. The walls become thick. They contain reserve food and DNA. These are the resting stages called akinetes. After resting stage, the wall of the akinete ruptures and a short filament of cells, is released.

Spore formation: It is not common. Sometimes the heterocyst forms endospores. The nuclear material divides and then the cytoplasm of the heterocyst divides within the parent cell wall and many spores called endospores are formed. Each spore forms a new Nostoc filament.

Main Features of Cyanobacteria

Taxonomic position: It is included in kingdom Monera by modern biologists.

Habitat: Any damp place; salt water, fresh water, in moist soil, on damp rocks, tree trunks, hot springs (with temperature. up to 85°C).

Mode of Life: May be symbiotic or epiphytic.

Forms of life: May be (a) Unicellular and solitary, (b) In the form of colonies, (c) In the form of filaments attached end to end.

Are prokaryotes: (a) Nucleus not organized i.e. nuclear membrane is absent. (b) The chromosomes do not have protein combined with DNA. (c) Membrane bound organelles e.g. mitochondria, Golgi complex etc are absent.

Cell wall: It contains muramic acid (found only in prokaryote). The cell wall is often surrounded by mucilaginous sheath.

Genetic material: Circular strand of DNA.

Ribosomes: Many ribosomes are present in the cytoplasm.

Photosynthesis: Release oxygen during photosynthesis. Takes place in the extensive system of membrane. It is located in the outer zone of the cytoplasm.

Their photosynthetic system closely resembles that of eukaryotes because they have chlorophyll “a” and photosysterm II. They use water as n electron donor and generate oxygen during photosynthesis. Cyanobacteria use phycobilins as accessory pigments. Photosynthetic pigments and electron transport chain components are located in thylakoid membrane linked with particles called phyobilisomes. Phycocyanin (a blue pigment) is their predominant phycobilin and CO2 in them is assimilated through Calvin cycle.

Reproduction

(I) sexual reproduction: It is absent.

(ii) Asexual reproduction: Takes place by: (a) Cell division e.g. by unicellular form. (b) Fragmentation: It is the breaking of plant body into small pieces of fragments. It takes place at weak points next to heterocyst forming hormogonia. The new filaments are produced by continued cell division of hormogonia. (c) Spores (d) Akinete.


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