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Nostoc also known as the freshwater algae genus of cyanobacteria is found in the freshwater, on the damp soil, even as an endophyte in the tissues of other plants such as Anthoceros, or as an algal component of some lichens.
This genus is represented by 29 species. In freshwater, they are found free-floating balls or attached to some submerged object.
The nostoc specifically belongs to blue-green algae (cyanobacteria). They are not confused with bacteria. They are even found on the tree trunks forming mucous or leathery outgrowths.
It secretes an extraordinary amount of the gelatinous substances and forms jelly-like lumps in which a large number of trichomes are held. These jelly-like substances are more or less rounded and attain large sizes which may be as large as a hen’s egg.
It is also found in paddy fields where it helps in atmospheric nitrogen fixation. Common species are Nostoc rivulare, N. caneum, N. entophytum, N. commune.
Systematic Position Of Nostoc
- Class: Cyanophyceae
- Order: Nostocales
- Family: Nostocaceae
- Genus: Nostoc
Morphology Of Nostoc | The Heterocysts
Each colony contains gelatinous material in which numerous filaments are embedded. The filaments are not branched and consist of a row of bead-like cells.
The filaments may contain, one or more than one enlarged, empty-looking cells known as heterocysts. The filament is not a straight structure but is twisted and folded on itself to form a spherical structure.
Each filament is enveloped by a hyaline or colored gelatinous sheath of an exceedingly firm consistency. Many close sheaths usually unite together to form thick masses of jelly in which filaments are embedded.
Nostoc Cell Structure
Each cell has a well-developed cell wall consisting of an inner thin cellular layer a medium pectic layer and an outer mucilage layer.
The protoplasm consists of colored peripheral chromoplast and central colorless cytoplasm containing the nucleus-like material known as the central body.
Plastids are absent while several small granules are present pseudo-vacuole is additionally found in some cases.
Reserve food material is a glycogen-like substance and not starch. Additionally, oil drops may also be found. It is capable of fixing free nitrogen mitochondria, endoplasmic reticulum and Golgi bodies are also absent.
Reproduction In Nostoc Algae
Sexual reproduction is not reported in this genus. Asexual reproduction takes place by vegetative and asexual (spore formation) means.
The large colony’s accidental breaking may grow into smaller separate colonies.
This is the most common method of reproduction. Hormogones are produced by the breaking of filament or the decay of any vegetative cell or at heterocysts.
The heterocysts may be terminal or intercalary and arranged singly or in series.
The hormogones may be retained by the gelatinous envelope or they may show wriggling movement coming out of the gelatinous envelope to develop, into a new colony.
Very often hormogone may develop into trichomes without being released from the colony. This may result in a large number of trichomes in the colony.
Resting Spores (Akinetes Or Anthrospores)
They are occasionally formed during unfavorable conditions by accumulating an excessive amount of food material and developing thick-wall spores.
Spore formation first starts in between the heterocysts and proceeds towards it.
Quite frequently they are arranged on either side of heterocysts. A mature akinete possesses a thick wall which is 2 to 3 layered thick.
The wall may be carved or smooth and highly granular protoplasm and can flourish well even during dry conditions.
The Akinetes may be spiracle, oblong, cylindrical, and have food reserve in the form of cyanophycin starch grains.
With the return of favorable conditions, they began to germinate by liberating their contents through a pore. Thus, a newly formed filament forms a new colony.
Heterocyst is a known means of reproduction in Nostoc commune, in which case each germinates to produce a new plant.
The contents of heterocysts divide into two and then four-cell germling which escapes either by the rupture of the wall or by gradual dissolution and widening of the pore. The germling germinates to produce a new plant.
Occasionally in Nostoc commune and Nostoc microscopicum, the heterocyst contents divide to produce endospore which on liberation give rise to new filaments.