- A Anabaena and Nostoc
- B Spirulina mainly
- C Chlorella mainly
- D Oscillatoria mainly
Correct answer: A. Anabaena and Nostoc
Explanation: Anabaena and Nostoc are cyanobacteria found free-living in paddy fields where they fix atmospheric nitrogen and act as natural biofertilizers.
Concept context
Microorganisms in food, medicine, sewage treatment, biocontrol, and biofertilizers