Question

Free- living nitrogen- fixing aerobic bacterium is
(a) Rhodospirillum
(b) Anabaena
(c) Nostoc
(d) Beijerinckia
(e) Rhizobium

Answer 1

Hint: They are aerobic and free- living microbes present in the soil, which play a significant role in the nitrogen cycle in nature, binding atmospheric nitrogen, which is inaccessible to plants, and releasing it within the sort of ammonium ions into the soil.  It is usually motile, oval or spherical bacteria that form thick- walled cysts.

Complete answer:
Nitrogen-fixing bacteria, microorganisms capable of transforming atmospheric nitrogen into fixed nitrogen (inorganic compounds usable by plants). More than 90 percent of all nitrogen fixation is done by these organisms, thus they play a vital role in the nitrogen cycle.
Free- living nitrogen- fixing aerobic bacteria is Beijerinckia. It has an abundance of nitrogenase enzymes capable of nitrogen reduction. They can be distinguished from other nitrogen-fixing bacteria by cell morphology and some physiological characteristics. They are typically rod-shaped cells with round ends containing polar lipoid bodies.
Beijerinckia species show great acid tolerance, being able to grow and fix dinitrogen at pH 3- 4. On agar media (nitrogen-free, glucose mineral age), they usually produce highly raised colonies of very tenacious and elastic slime, and on liquid media, they turn the whole medium viscous.

So, the correct answer is ‘Beijerinckia’.

Additional Information:
- Rhodospirillum is a free-living nitrogen-fixing anaerobic bacteria.
- Anabaena & Nostoc are symbiotic nitrogen-fixing cyanobacteria.
- Rhizobium is symbiotic nitrogen-fixing aerobic bacteria but fixes nitrogen in anaerobic conditions.


Note: The nitrogen fixation process is affected by enzymes called nitrogenases. These enzymes contain iron. Nitrogen within the atmosphere is very stable and nonreactive. Atmospheric nitrogen is molecular dinitrogen, a closely nonreactive molecule that's metabolically useless.