Biology · Microbes in Human Welfare · NEET
They are free-living. This means they live freely in the soil on their own and do not live inside any plant. They fix atmospheric nitrogen without a partner. Rhizobium is the opposite - it is symbiotic and must live inside legume root nodules. NEET often asks you to sort these into the correct group, so remember: Azospirillum and Azotobacter = free-living; Rhizobium = symbiotic.
Rhizobium lives inside root nodules of legumes (symbiotic) and fixes nitrogen only with its plant host. Azotobacter lives freely in the soil (free-living / non-symbiotic) and fixes nitrogen on its own, no nodule needed. Both add nitrogen to the soil, so both are biofertilisers - but only Rhizobium needs a legume partner.
No. Root nodules are made only by symbiotic fixers like Rhizobium inside legume roots. Azospirillum and Azotobacter are free-living, so they do not make nodules. This is a common trap - do not link the word 'nodule' with these two.
They are biofertilisers, not biocontrol agents. A biofertiliser enriches soil nutrients (here, by fixing nitrogen). A biocontrol agent kills pests or pathogens (like Bacillus thuringiensis, Trichoderma, Baculovirus). NEET 2019 used exactly this mix-up: Azospirillum was placed in a wrong 'biocontrol' group to test you.
Azospirillum and Azotobacter are ordinary bacteria (free-living soil bacteria). Cyanobacteria such as Anabaena, Nostoc and Oscillatoria are a separate group of nitrogen-fixing photosynthetic microbes. All fix nitrogen, but they belong to different groups - do not call Azotobacter a cyanobacterium.
Which of the following organisms cannot fix nitrogen? A. Azotobacter B. Oscillatoria C. Anabaena D. Volvox E. Nostoc. Choose the correct answer from the options given below:
Select the correct group of biocontrol agents.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because they add nutrients to the soil naturally. By fixing atmospheric nitrogen into a form plants can absorb, they raise soil fertility without chemical fertiliser. NEET lists them with Rhizobium and cyanobacteria as biofertilisers.
Free-living means the bacterium lives independently in the soil and fixes nitrogen on its own, without living inside a plant. It is the opposite of symbiotic fixers like Rhizobium that must live inside root nodules.
Remember Azospirillum and Azotobacter as the two free-living aerobic soil bacteria that fix nitrogen. Pair them against Rhizobium (symbiotic) and cyanobacteria like Anabaena and Nostoc (photosynthetic fixers).
Azotobacter is a well-known aerobic (oxygen-using) free-living fixer. For NEET the key point is simply that both are free-living soil bacteria acting as biofertilisers; deep oxygen detail is not usually tested.
