Read the following passage carefully and answer the given questions.
Biofertilisers
Biofertilisers are organisms that enrich the nutrient quality of the soil and their main sources are bacteria, fungi and cyanobacteria. You have studied about the nodules on the roots of leguminous plants formed by the symbiotic association of Rhizobium. These bacteria can fix atmospheric nitrogen into organic forms. Other bacteria can also fix atmospheric nitrogen while free-living in the soil (examples Azospirillum and Azotobacter). Fungi are also known to form symbiotic associations with plants (mycorrhiza). Many members of the genus Glomus form mycorrhiza. Plants having such associations show other benefits also, such as resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in plant growth and development. 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 in 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.
Which one of the following is not a symbiotic microbe?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Azotobacter
Option (1) Rhizobium — A symbiotic microbe. Rhizobium lives in the root nodules of leguminous plants. It fixes atmospheric nitrogen for the plant, and the plant gives it food and shelter. Both partners gain.
Option (2) Mycorrhiza — A symbiotic microbe. Mycorrhiza is the symbiotic association of a fungus, such as Glomus, with plant roots. The fungus gives the plant phosphorus, and the plant gives the fungus food.
Option (3) Azotobacter — Not a symbiotic microbe. Azotobacter is a bacterium that fixes nitrogen while free-living in the soil. It does not need to live with any plant, so it is not symbiotic.
Option (4) Lichens — A symbiotic microbe. A lichen is a close partnership of an alga and a fungus. The alga makes food by photosynthesis, and the fungus gives shelter and absorbs water and minerals.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.