Question

Free living bacteria that can fix N${}_2$ in soil are

• A. Azotobacter
• B. Beijerinckia
• C. Clostridium
• D. All the above

Answer 1

Answer: (D)

All the above

Nitrogen-fixing bacteria are microorganisms that are capable of transforming atmospheric nitrogen into fixed nitrogen (inorganic compounds which can be used by the plants).
Two kinds of nitrogen-fixing bacteria are recognized. They are:

1. Free-living (non-symbiotic) bacteria: cyanobacteria (or blue-green algae), Anabaena and Nostoc and genera such as Azotobacter, Beijerinckia, and Clostridium.

2. Mutualistic (symbiotic) bacteria: Rhizobium, associated with leguminous plants; Frankia, associated with certain dicotyledonous species; and certain Azospirillum species, associated with cereal grasses.

Thus, the correct answer is option (D).