Question

Select the mismatch.
(A) Rhizobium - Alfalfa
(B) Frankia - Alnus
(C) Rhodospirillum - Mycorrhiza
(D) Anabaena - Nitrogen fertilizer

Answer 1

Hint: Diazotrophs consist of BGA and Bacteria. There are two types of BGA i.e. free-living and Symbiotic while bacteria are divided into three types which include free-living, symbiotic, and loose symbiosis. Rhizobium is an example of symbiotic bacteria whereas Rhodospirillum is a free-living bacteria. Anabaena is an example of a free-living BGA.

Complete answer:
> Rhizobium: They are free-living bacteria that are found in soil. Rhizobium is symbionts that can fix atmospheric nitrogen. Rhizobium has a symbiotic relationship with the root legumes nodules of leguminous plants such as Garden pea, Alfafa, Sweet clover, etc. They are unable to fix nitrogen independently and hence, they need a plant host for the same. These bacteria convert atmospheric nitrogen into nitrogenous compounds such as amino acids or salts of ammonia.
> Frankia: Frankia is known as a microbe that produces a nitrogen-fixing nodule on the roots of non-leguminous plants. They depict symbiotic relationships with plants belonging to the genus Alnus. These bacteria are gram-positive in nature that are known to form root nodules in alder trees.
> Rhodospirillum: Rhodospirillum is photosynthetic bacteria. Photosynthesis is non oxygenic and they require hydrogen ions for the same. They take hydrogen ions from inorganic sulfur compounds or organic compounds. Rhodospirillum is purple non-sulfur bacteria. They are basically gram-negative bacteria. They do not show any association with Mycorrhiza. The mycorrhizal association is a symbiotic association that is observed between fungus and a plant.

Therefore the correct option is (C) Rhodospirillum – Mycorrhiza.

Note:
Nitrogen fixation is done by two methods, symbiotically and asymbiotically. Rhizobium and Frankia live symbiotically and do nitrogen fixation whereas Rhodospirillum occurs in free soil and does the nitrogen fixation i.e. asymbiotically.