Which one of the following organisms is NOT free living?

Correct option is D

The correct answer is (D) Frankia

Explanation:
• Frankia is a genus of nitrogen-fixing bacteria that lives in a symbiotic relationship with actinorhizal plants (like Casuarina and Alder).
• Unlike free-living bacteria that fix nitrogen independently in the soil, Frankia induces the formation of root nodules where it lives and provides nitrogen to the host plant in exchange for carbohydrates.
• While it can exist in the soil, its primary nitrogen-fixing role is performed within the host plant environment.
• Azotobacter (A) is a well-known aerobic free-living nitrogen fixer. Cyanobacteria (B) like Anabaena and Nostoc can be free-living or symbiotic, but many are prominent free-living fixers. Rhodospirillum (C) is a free-living anaerobic bacterium.

Information Booster:
• Rhizobium is the most famous symbiotic nitrogen fixer, associated with legume crops.
• Nitrogen fixation is the process of converting atmospheric $N_2$ into ammonia ($NH_3$) using the enzyme nitrogenase.
• Actinorhizal plants (associated with Frankia) are important for reforesting poor soils and stabilizing sand dunes.

Additional Knowledge:
• Option A (Azotobacter): Used as a biofertilizer for non-legume crops like wheat and maize.
• Option B (Cyanobacteria): Often used in rice fields to increase nitrogen availability.