Q.Name two genera of nitrogen-fixing cyanobacteria (blue-green algae) commonly used to enrich paddy-field soils, and explain how they add organic matter besides fixing nitrogen.

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. …

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. …

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 con …