Nostoc is a genus of photosynthetic cyanobacteria or blue-green algae. They are mainly found as free-living colonies in freshwater, bottoms of lakes, or attached to rocks and tree trunks. The species under this genus perform photosynthesis owing to the presence of chlorophyll in them. They are also capable of nitrogen fixation and present as an endosymbiont to fungi.

Alt-text: Nostoc sp. under light microscope
Systematic Classification
Domain: Bacteria
Kingdom: Monera (not used in modern classification)
Phylum: Cyanobacteria
Class: Cyanophyceae
Order: Nostocales
Family: Nostocaceae
Genus: Nostoc
Name of Some Common Species
- Nostoc rivulare
- Nostoc carneum
- Nostoc entophytum
- Nostoc commune
- Nostoc flagelliforme
- Nostoc punctiforme
Structure of Nostoc
- Nostoc are filamentous and unbranched cyanobacteria.
- Numerous filaments are embedded in a gelatinous mass, forming a colony.
- Each filament, called a trichome, consists of a chain of spherical, cylindrical, or barrel-shaped cells.
- Each cell has a thick cell wall made of peptidoglycan. The cytoplasm is differentiated into an outer region containing peripherally arranged chromoplasts (with pigments phycocyanin, phycoerythrin, and chlorophyll) and an inner clear cytoplasm.
- Some cells within the filament are differentiated into heterocysts, specialised for nitrogen fixation.
- Trichome remains surrounded by a protective, mucilaginous sheath of polysaccharides and proteins, absorbing and retaining water.
- The colonies vary in shape, size, and colour (mostly bluish-green, sometimes red-brown).
- Extracellular pigments such as nostocine and scytonemin protect the cells from ultraviolet radiation.
- Hormogonia are short free filaments formed by fragmentation and help in asexual reproduction.

Reproduction in Nostoc
Sexual reproduction is not reported in Nostoc. Asexual reproduction occurs by the following methods:
- Binary fission: Cells divide into two identical daughter cells.
- Fragmentation: Trichomes break into smaller fragments called hormogonia, which grow into new filaments.
- Akinetes: Thick-walled resting cells formed during unfavourable conditions that germinate when conditions improve.
Economic Importance of Nostoc
- Nostoc acts as a natural nitrogen fixer and is used as a biofertiliser.
- Some species are rich in proteins and vitamin C and are consumed as food in parts of Asia.
- Certain species show antibacterial and antiviral properties.
- Nostoc is a potential biofuel resource due to high biomass production.
- It is used in bioremediation as it can absorb heavy metals and toxic pollutants.
Frequently Asked Questions (FAQs)
Q1. Does Nostoc produce oxygen during photosynthesis?
Yes. Nostoc contains chlorophyll and performs oxygenic photosynthesis, releasing oxygen.
Q2. How does Nostoc fix atmospheric nitrogen?
Nostoc fixes nitrogen using specialised cells called heterocysts, which contain nitrogenase enzymes that convert atmospheric nitrogen into ammonia.
Q3. Does Nostoc form symbiotic relationships?
Yes. Nostoc forms symbiotic associations with liverworts, hornworts, ferns, and some flowering plants.