Microbes In Human Welfare

Cyanobacteria such as Anabaena, Nostoc, and Oscillatoria are free-living, photosynthetic, nitrogen-fixing microbes that add organic nitrogen to the soil, and in paddy (rice) fields Anabaena is especially valuable as a biofertilizer that boosts soil fertility. It carries specialised cells called heterocysts in which atmospheric nitrogen is fixed into usable forms, and it is often associated with the water fern Azolla, further enhancing nitrogen supply to rice crops. Rhizobium is incorrect because it is not free-living in this role; it is a symbiotic bacterium that fixes nitrogen only inside the root nodules of leguminous plants, not as a paddy-field cyanobacterium. Glomus is wrong because it is a mycorrhizal fungus that helps plants absorb phosphorus, not a nitrogen-fixing cyanobacterium. Trichoderma is incorrect since it is a fungal biocontrol agent against plant pathogens rather than a biofertilizer cyanobacterium. As outlined in NCERT Class 12, Chapter 10 (Microbes in Human Welfare), cyanobacteria add organic matter and nitrogen to nutrient-poor soils. As a biological consistency check, only Anabaena is simultaneously a free-living photosynthetic cyanobacterium and a paddy-field nitrogen-fixing biofertilizer, confirming it as the correct answer.