Which among the following is the aquatic fern which is an excellent biofertiliser?

Azolla

The correct answer is Option (1)- Azolla

Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria, etc.

In paddy fields, cyanobacteria serve as an important biofertiliser. Blue green algae also add organic matter to the soil and increase its fertility.

Azolla pinnata is a freshwater floating fern. It exists in association with the filamentous nitrogen fixing cyanobacteria called Anabaena azollae.

The fern provides shelter to the cyanobacteria while the latter helps in nitrogen fixation and supplies it to the fern. The death of the plant having Azolla causes degradation followed by the release of nitrogenous compounds to the nearby plants.