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Nostoc Fresh Water Algae (Characteristics + Morphology + Reproduction)

Nostoc Fresh Water Algae – Characteristics, Morphology and Methods of Reproduction

Nostoc also known as fresh water algae genus of cyanobacteria is found in the fresh water, on the damp soil, even as 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 fresh water they are found free floating balls or attached to some submerged object.

Characteristics of Fresh Water Algae

The nostoc specifically belongs to blue green algae (cyanobacteria). They are not be confused with bacteria. They are even found on the tree trunks forming mucous or leathery out growths. It secretes an extraordinary amount of the gelatinous substances and forms jelly like lumps in which a large amount of trichomes are held. These jelly-like substances are more or less rounded and attains large size 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.

nostoc
Nostoc 3D Structure

Systematic Position

  • Class: Cyanophyceae
  • Order: Nostocales
  • Family: Nostocaceae
  • Genus: Nostoc

Morphology of Nostoc

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-structure
Nostoc Structure: A, Colony in mucilaginous envelop; B, Colony with heterocysts and akinetes (arthrospores); C, gelatinous colony D, enlarged single cell

Cell Structure

Each cell has well developed cell wall consists of an inner thin cellular layer a medium pectic layer and an outer mucilage layer. The protoplasm consists of colored peripheral chromoplasm and central colorless cytoplasm containing the nucleus like material known as central body. Plastids are absent while several small granules are present pseudo-vacuole is additionally found in some cases. Reserve food material is glycogen like substance and not starch. Additionally, oil drops may also be found. It is capable of fixing free nitrogen mitochondria, endoplsamic reticulum and Golgi bodies are also absent.

Reproduction

Sexual reproduction is not reported from this genus. Asexual reproduction take place by vegetative and asexual (spore formation) means.

Colony Fragmentation: The large colony accidental breaking may grow into smaller separate colonies.

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Nostoc sp: Hormogone and their germination

Hormogones: This is the most common method of reproduction. Hormogones are produced by 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 envelop or they may show wriggling movement come out of the gelatinous envelop to develop, into a new colony. Very often hormogone may develop into trichomes without being released from the colony. This may result in the large number of trichomes in the colony.

anthrospores
Anthrospores and Their Germination

 

Resting Spores (Akinetes or Anthrospores): They are occasionally formed during unfavorable conditions by accumulating excessive amount of food material and development of thick-walls 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 Function: 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 germiling which escape either by the rupture of wall or by gradual dissolution and widening of pore. The germiling germinate to produce a new plant.

Endospores: Occasionally in Nostoc commune and Nostoc microscopicum, the heterocyst contents divide to produce endospore which on liberation give rise to new filaments.

heterocyst-and-germination-of-heterocyst
Heterocyst and Germination of Heterocyst
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