Question

Free-living bacteria that can fix $N_2$ in the soil are
a. Azotobacter
b. Beijerinckia
c. Clostridium
d. All the above

Answer 1

Hint: Nitrogen fixation is the process by which the atmospheric nitrogen is converted to ammonia or other nitrogenous compounds by some nitrogen-fixing bacteria, this is because, the atmospheric nitrogen is a dinitrogen molecule, where it cannot be utilized for metabolism, this dinitrogen form is converted to other nitrogenous compounds which can be easily utilized by the bacteria.

Complete answer:
Nitrogen fixation is very important because the nitrogen cycle helps the nitrogen in the atmosphere to be fixed on earth, where the organic compounds are useful for the production of various nitrogen containing products like amino acids, nitrogen bases, etc.

This nitrogen fixation Is mainly done by nitrogen-fixing bacteria, these bacteria are mainly of two types which include, symbiotic and non-symbiotic bacteria.

Symbiotic bacteria are one which is in mutualism with plants, and some of the examples of symbiotic bacteria are, Rhizobium associated with leguminous, Frankie associated with dicots, and Azospirillium which is associated with cereal grass.

Whereas another type of bacteria, which is not associated with plants and they are free-living, the examples are cyanobacteria or blue-green algae, Anabaena, and nostoc. And some genera like Azotobacter, beijerinckia, and clostridium, all of these are free-living bacteria and do nitrogen fixation.

Hence, the correct answer is option (D).

Note: In this nitrogen fixation, bacteria convert the atmospheric nitrogen into ammonia, by the enzyme nitrogenase, where this ammonia is freely utilized by plants, and this ammonia is further converted to nitrites and later into nitrates.