Previous Year Question

Blue-Green Algae are not true algae; they are actually Cyanobacteria. They are photosynthetic prokaryotic organisms, and many species can convert free atmospheric Nitrogen (N₂) into biologically usable nitrogen compounds.

This process is called Biological Nitrogen Fixation.

Several Cyanobacteria such as Nostoc, Anabaena, Aulosira and Calothrix are capable of Nitrogen fixation.

In many filamentous Cyanobacteria, this function is carried out in specialised thick-walled cells called Heterocysts. Heterocysts maintain a low-Oxygen environment because the Nitrogenase enzyme required for Nitrogen fixation is highly sensitive to Oxygen.

The basic result of Nitrogen fixation can be understood as:

Atmospheric N₂ → Ammonia/usable Nitrogen compounds

Thus, these organisms also play an important role in improving soil fertility.

Blue-Green Algae are particularly important in paddy fields. The Anabaena–Azolla association is a well-known example that provides biological nitrogen input in rice cultivation and is used as a biofertilizer.