Question

Cyanobacteria fixing atmospheric nitrogen are
I. Oscillatoria and Anabaena
II. Azospirillum and Nostoc
III. Azotobacter and Rivularia
IV. Nostoc and Ocillatoria

• A. I & IV
• B. I & II
• C. II & IV
• D. II & III

Hint:

Nitrogen fixation in cyanobacteria occurs in specialized cells called heterocysts. Remember key examples: Anabaena, Nostoc, and optionally Oscillatoria. Non-cyanobacterial nitrogen fixers like Azospirillum and Azotobacter should not be confused in cyanobacteria MCQs.

Answer 1

The Correct Option is A

Cyanobacteria are prokaryotic photosynthetic organisms capable of nitrogen fixation. Anabaena and Nostoc form heterocysts which fix atmospheric N₂. Oscillatoria does not fix N₂ typically, but in some MCQs it is considered a nitrogen fixer. Azospirillum and Azotobacter are free-living nitrogen-fixing bacteria, not cyanobacteria. Thus, I & IV (Oscillatoria and Anabaena, Nostoc and Oscillatoria) are correct.