Question

The common nitrogen fixer organisms in paddy fields is
(A) Azospirillum
(B) Oscillatoria
(C) Frankia
(D) Rhizobium

Answer 1

Hint: It is a non-fermentative, gram-negative bacterium. When added to the soil, it multiples in millions and can supply 20-40 kg per hectare of nitrogen per season.

Complete answer: Azospirillum is the popular nitrogen-fixer found in paddy fields. It is a microaerophilic, gram-negative, non-fermentative, nitrogen-fixing bacteria. It also has the ability to synthesize phytohormones, especially indole-3-acetic acid and gibberellins, and helps to promote proliferation of roots. When a small portion of it is applied to soil, it multiplies in million and can provide up to 20-40 kg of nitrogen per hectare per season.
1. scillatoria is an important cyanobacteria, as it can perform photosynthetic activities. It does not have a role in nitrogen fixation.
2. Frankia is a gram-positive Bacteria present on plant roots. It deals with the heavy metal resistance that may be found in the degraded soil.
3. Rhizobium is the bacteria that live in symbiotic association with the leguminous plant root nodules. Nitrogen fixation cannot be performed independently. Thus the rhizobium needs a host plant for fixing nitrogen.

Additional information: Nitrogen fixation is a process of converting molecular nitrogen in the air into ammonia or into other nitrogen compounds in soil.
Atmospheric nitrogen is molecular nitrogen, a compound that is fairly unreactive and is metabolically inaccessible to all except a few microorganisms.
Biological nitrogen fixation transforms ammonia into molecular nitrogen and is metabolized by most organisms.

Thus, the right answer is option A i.e., Azospirillum

Note: Azospirillum is a free-living nitrogen-fixing bacterium that can be fixed nitrogen independently but Rhizobium requires a plant host for fixing nitrogen.