Nostoc : Monera

Terrestrial species of Nostoc can be found in damp soil. Few species of Nostoc are also found in the rice field in which they increase the soil fertility by fixing atmospheric free nitrogen. 

Occurrence

Mostly Nostoc is found in fresh water. Few species are also found in marine water. Terrestrial species of Nostoc can be found in damp soil. Few species of Nostoc are also found in a rice field in which they increase the soil fertility by fixing atmospheric free nitrogen.

Structure

Trichome:

A single unbranched filament of Nostoc is called trichome. Trichome is covered with mucilage sheath. A trichome consist of vegetative cells and heterocyst joined end to end and form a beaded chain-like appearance.

Vegetative cells:

Vegetative cells are thin walled smaller in size and round or oval in shape. They contain photosynthetic pigment and they are colourful. They take part in photosynthesis.

Heterocyst:

Heterocyst is thick walled larger in size and round or oval in shape. They are colourless without photosynthetic pigment. They don't take part in photosynthesis. They help in reproduction and nitrogen fixation. Heterocyst may be terminal or intercalary in position.

Colony

Several trichomes aggregate together and get surrounded by common mucilage sheath and colonies are formed. Colonies are round or ball-like structure diameter of which may vary from few mm to 8 or 9 cm.

 

Cell structure of Nostoc

Cell structure of Nostoc consists of following parts;

Mucilage sheath

It is the outermost covering layer. This sheath may be thin, thick or lamella. In aquatic species, mucilage sheath is thin and in terrestrial species mucilage sheath is thick. It works as a protective layer, absorbs water and retains it.

Cell wall

A cell wall is thick, the rigid structure made up of four layers in which the second layer is made up of peptidoglycan or murein. Cell wall provides the structural framework and provides support and protection to the cell.

Cell membrane

The cell membrane is semipermeable thin membrane present inside the cell wall.

Cytoplasm

Cytoplasm is differentiated into two regions outer colourful chromoplast and inner colourless cytoplasm.

Chromoplast:

This is the outer region of cytoplasm, chromoplast is colourful because there is the presence of photosynthetic pigments Chlorophyll A, C- phycocyanin(blue), C- phycoerythrin(red), B-carotenes(orange), Xanthophyll(brown). Chromoplasm contains 70s type ribosomes, Gas vacuole, food reserved materials like cyanophycean starch, cyanophycean protein granule, and oil droplets. Single membrane bound lamellar thylakoids are present which are the site of photosynthesis.

Cytoplasm

Centroplasm is the inner colourless region of cytoplasm. Centroplasm contains the nuclear body. The nucleus is of incipient type i.e. without nuclear membrane and nucleolus. DNA is double stranded, naked which means without histone protein. DNA carries all genetic information.

Reproduction in Nostoc

Nostoc is reproduced only by vegetative and asexual methods. The sexual reproduction is completely absent.

Vegetative reproduction

Nostoc reproduces vegetatively by the following methods;

Fragmentation:

Old trichome becomes very large and irregular due to which it gets to break up into short fragments. These short fragments of trichome divide vegetative cells and develop into new trichome.

Hormogones:

Hormogones are the short fragment of trichomes. Hormogones are developed in the region of heterocyst. Then they came out of the trichome due to some movement. They divide vegetative cells and developed heterocyst and again get surrounded by mucilage sheath. In this way, new trichome is formed.

Asexual reproduction

Nostoc reproduces asexually by following methods;

Akinetes:

In unfavourable conditions, vegetative cell stores enough food materials and get enlarged in size. They secrete thick wall around and undergo into resting period. In return of favourable condition, akinetes germinate and give rise to new trichome.

Heterocyst:

In some species of Nostoc, heterocyst cell divides transversely and form (2-4) celled hormogones. These hormogones come out by bursting the thick wall of heterocyst and germination occurs to give rise to new trichome.

Endospores:

In few species of Nostoc, heterocyst cell undergoes irregular cell division and forms thin-walled spores which are called endospores. Endospores come out by bursting thick wall of heterocyst and germination occurs and new trichome is formed.