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.
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 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.
A filament of single trichome
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.
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 consists of following parts;
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.
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.
The cell membrane is semipermeable thin membrane present inside the cell wall.
Cytoplasm is differentiated into two regions outer colourful chromoplast and inner colourless cytoplasm.
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.
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.