Question

Which of the following is a nitrogen fixing organism?
A. Aspergillus
B. Rhizobium
C. Both A and B
D. None of the above

Answer 1

In agricultural soils, even those in dry locations, biological $N_2$ fixation is the primary source of nitrogen. The principal $N_2$-fixing systems are symbiotic systems, which can improve the fertility and productivity of low-N soils significantly. Rhizobium -legume symbioses have gotten the most interest and have been thoroughly investigated.

Complete answer:
Rhizobium is a genus of nitrogen-fixing Gram-negative soil bacteria. Rhizobium species create a nitrogen-fixing endosymbiotic relationship with the roots of (mostly) legumes and other flowering plants.

Nitrogen fixing bacteria are microorganisms that convert nitrogen gas from the atmosphere into solid nitrogen compounds that plants can use in the soil. They can be found in the soil or in plant roots.

**Hence option B is the correct answer:Rhizobium. **

Free-living (non-symbiotic) bacteria and symbiotic bacteria are the two types of nitrogen fixers. Rhizobia are soil bacteria that fix nitrogen after establishing themselves inside legume root nodules.

The bacteria invade plant cells within root nodules, where they use the enzyme nitrogenase to convert ambient nitrogen to ammonia and then deliver organic nitrogenous compounds to the plant, such as glutamine or ureides. The bacteria, in turn, get organic substances produced by photosynthesis from the plant. All of the rhizobia, of which the genus Rhizobium is an example, have a mutually beneficial interaction. Phosphorus can also be solubilized by Rhizobium.

Note:-
Rhizobium is a soil bacteria that aids in nitrogen fixation in leguminous plants. It creates nodules by attaching itself to the roots of leguminous plants. These nodules fix nitrogen from the air and convert it to ammonia, which the plant can use for growth and development. Plants that fix nitrogen do not get nitrogen from the air on their own. They actually require the assistance of a common bacteria known as Rhizobium.