Division : Thallophyta Class : Cyanophyceae Order : Nostocales Family : Nostocaceae
Genus : Nostoc
Nostoc is a colonial prokaryotic cynophycean or BGA , found in both aquatic and terrestrial habitats. The plants may be found free floating or attached to the solid substratum. Nostoc forms shining mucilaginous colonies, popularly known as moon spit or star jelly.
The plant body is simple, unbranched and filamentous. A trichome enclosed in a sheath is called filament . All the vegetative cells of the trichome are similar in structure which may be spherical , barrel shaped or cylindrical. At intervals it bears large thick walled cells called heterocyst which joins the vegetative cells.
The typical cell of the Nostoc posses cell wall, cell membrane and protoplast. Cell wall is covered on the outside by mucilaginous sheath. Plasma membrane is selectively permeablein nature and surrounds the protoplast . Protoplast is differentiated into cytoplasm and nuclear body (genophore).
Protoplast is commonly differentiated into outer colored chromoplasma and colorless centroplasm. The color of chromoplasm is due to the presence of photosysthetic thyllakoids. To the membrane of thyllakoids are attached the minute vesicles known as phycobilisome which posses three pigments: phycocyanin (blue) and phycoerythrin (red). The major cytoplasmic organelles like are present. Nuclear body consist of double strand of naked DNA which lies variously in the cytoplasm.
Sexual reproduction is absent and it reproduces vegetatively by means of fragmentation . Asexaully it reproduces by spoire like hormogones, akinets and heterocyst. The hormogones are short segments of trichome comes out from the old colonies and forms the heterocyst at one or both ends and later forms mucilage and develops the young colony. Akinets are thick walled restiong spore, which after death and decay of parental filament, germinates and produce new colony. Heterocyst is large and thick walled cells which in fact fix the atmospheric nitrogen but some times it helps in reproduction by forming hormogones and endospores. The endospore are formed by the cleavage of protoplast of heterocyst whcih on germination gives rise to the new colony of Nostoc.
Economic importance :
1. Colonies of Nostoc are rich in nitrogen substance and are edible in nature.
2. Nostoc increases the fertility of soil by fixing atmospheric nitrogen.
3. The blue green algae add organic matter to the soil and increase fertility.
4. Cyanobacteria perform the oxygenic photosynthesis and they are the chief producers of the biosphere.
5. Some blue green algae are used as antibiotics.
6. Species of Cyanobacteria can grow on the walls and roof of building during rainy seasons causing discoloration , corrosion and leakage.
7. Some of the blue green algae are poisonous to fish and domesticated animals.
8. The thick blooms caused by some blue green algae bring about depletion of oxygen resulting in large scale death of fish and other aquatic animals.