Which of the following is a symbiotic nitrogen fixer?
Show Hint
Free-living = Azotobacter (aerobic), Clostridium (anaerobic).
Associative = Azospirillum.
- Clostridium
- Bradyrhizobium
- Azospirillum
- Azotobacter
The Correct Option is B
Solution and Explanation
Nitrogen fixation is the conversion of atmospheric $N_2$ into ammonia.
Microbes that do this are either "free-living" (in soil) or "symbiotic" (associated with a plant host).
Step 2: Detailed Explanation:
• Bradyrhizobium: This is a genus of slow-growing Gram-negative soil bacteria.
They form symbiotic relationships with specific legumes, such as soybeans (Glycine max), by forming root nodules.
Within the nodule, they provide fixed nitrogen to the plant in exchange for carbohydrates.
• Comparison with other options:
- Clostridium: This is an anaerobic, free-living nitrogen-fixing bacterium.
- Azotobacter: This is an aerobic, free-living nitrogen-fixing bacterium.
- Azospirillum: This is an associative nitrogen fixer; it lives in the rhizosphere (root zone) and may enter root tissues but does not form true nodules.
• Importance: Symbiotic nitrogen fixers like Rhizobium and Bradyrhizobium are the most efficient nitrogen fixers in agriculture.
Step 3: Final Answer:
Bradyrhizobium is the correct answer as it is the only true symbiotic nodule-former in the list.
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