General Characters

• Mostly cyaobacteria are aquatic and some are terrestrial.
• The organisms are prokaryotic. They may be unicellular, colonial.
• Cell wall is composed of cellulose.
• Flagella are absent. Mesosomes are absent like bacteria.
• They contain photosynthetic pigments, so they are autotrophs.
• No sex organs are produced and sexual reproduction is absent.
• They reproduce asexually.
• One of the important members of cyanobacteria is Nostoc

Nostoc:

They are found in fresh water, paddy field. The body is filamentous which is simple, unbranched, and covered by gelatinous sheath. Filament without gelatinous sheath is called trichome. Trichome is made up of small spherical or oval cells.

At frequent interval there are some specialized large elongated and thick walled cells called heterocyst. The heterocyst consists of two polar nodules. The heterocyst helps to fix atmospheric nitrogen. Heterocyst is intercalary (in between two vegetative cells) but sometimes it is terminal (at the end of the filament)

Reproduction:

• Nostoc does not develop sex organs. It reproduces by asexual method.
• Fragmentation: when the colony breaks into small piece or fragment new colony is formed from each fragment.
• Hormogonia: The filament breaks into point of heterocysts. Each piece is called horomogonia and form that new colony is formed.
• Heterocysts: The heterocyst itself germinates and produces new filament.
• Akinetes: Under favorable condition somatic cells store food materials, which become thick walled called akinetes. They germinate to form new filament and colony.

Economic importance of Cyanobacteria:

• Cyanobacteria are the source of food for aquatic animals.
• They also fix atmospheric nitrogen in crop growing field.
• They are nutritious and decrease the alkalinity of the soil.
• Some species are toxic to aquatic animals cause poison.
• Accumulation of cyanobacteria in water source contaminates and pollutes water.