Q.Differentiate between symbiotic and free-living (asymbiotic) nitrogen-fixing bacteria, giving one example of each.

Anabaena and Nostoc are cyanobacteria that fix atmospheric nitrogen in symbiotic association with certain plants.

Several cyanobacteria (blue-green algae) can fix atmospheric nitrogen (N2) into usable ammonia using the enzyme nitrogenase, housed in specialised cells called heterocysts. Some do this in symbiotic association with a host plant:

• Anabaena azollae lives symbiotically within the leaf cavities of the aquatic fern Azolla, which is used as a biofertiliser in paddy fields.

• Nostoc forms a symbiotic association with the coralloid roots of Cycas.

Anabaena is a classic example of a symbiotic, nitrogen-fixing cyanobacterium, living in the leaf cavities of the aquatic fern Azolla.

Certain cyanobacteria (blue-green algae) are capable of fixing atmospheric nitrogen (N₂) into forms plants can use (like ammonia), and some do this in symbiosis with a host plant:

• Anabaena lives symbiotically within the leaf cavities of the water fern Azolla. The cyanobacterium fixes atmospheric nitrogen and enriches the soil/water with usable nitrogen compounds, while the fern provides shelter and nutrients to the cyanobacterium.
• This Anabaena–Azolla association is widely used as a natural biofertiliser in rice (paddy) fields, since Azolla grows readily in flooded paddies and its associated Anabaena continuously fixes nitrogen, reducing the need for chemical nitrogenous fertilisers.

(Another commonly cited example is Nostoc, found in symbiotic association with the coralloid roots of Cycas.)

The heterocyst — the specialised, thick-walled cell of filamentous cyanobacteria where nitrogen fixation takes place.

Heterocysts are specialised cells in cyanobacteria that provide the oxygen-free environment nitrogenase needs to fix atmospheric nitrogen.

In filamentous, nitrogen-fixing cyanobacteria (blue-green algae) such as Nostoc and Anabaena, certain cells along the filament differentiate into heterocysts — larger cells with thick walls that exclude oxygen and lack the oxygen-evolving photosystem II. This creates the anaerobic (low-oxygen) internal environment required by the enzyme nitrogenase, which is extremely oxygen-sensitive, to convert atmospheric nitrogen (N2) into ammonia that the organism can use. This makes heterocyst-forming cyanobacteria valuable natural biofertilisers in agriculture, especially in paddy fields.