Which type of microbes are found in paddy fields and act as biofertil…

The correct answer is Option (3) → Cyanobacteria

Biofertilisers are living organisms that add nutrients to the soil. The main ones are bacteria, fungi and cyanobacteria. Each works in its own way, for example by fixing nitrogen from the air or by helping roots take up phosphorus.

Option (1) Viruses — Incorrect. Viruses cannot live or fix nitrogen on their own, and they are not used as biofertilisers. Some viruses (baculoviruses) are used in farming, but as biocontrol agents that kill insect pests.

Option (2) Fungi — Incorrect. Fungi do act as biofertilisers, but in a different way. Fungi such as Glomus form mycorrhiza with plant roots and pass phosphorus from the soil to the plant. The book does not name fungi as the biofertiliser of paddy fields.

Option (3) Cyanobacteria — Correct. Cyanobacteria (blue-green algae) are autotrophic microbes, and many of them fix atmospheric nitrogen. They grow well in the water of paddy fields, where they are an important biofertiliser.

Option (4) Protozoa — Incorrect. Protozoa are single-celled, animal-like organisms. They cannot make their own food or fix nitrogen, so they do not enrich the soil. Some protozoa cause diseases, such as malaria and amoebiasis.

NCERT Extract:
Microbes as Biofertilisers – “… 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. Currently, in our country, a number of biofertilisers are available commercially in the market and farmers use these regularly in their fields to replenish soil nutrients and to reduce dependence on chemical fertilisers.”