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Which of the following is a free-living aerobic nonphotosynthetic nitrogen fixer?

1. Rhizobium
2. Azotobacter
3. Azospirillum
4. Nostoc

Correct answer: Azotobacter

Solution

Azotobacter is a free-living, aerobic, nonphotosynthetic bacterium capable of fixing atmospheric nitrogen. Rhizobium is symbiotic, Azospirillum is typically associative, and Nostoc is photosynthetic.

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