Answer: Anabaena and Nostoc.
- 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.
Sewage passes through physical removal (primary), then aerobic biological treatment where microbes form flocs that consume organic matter (secondary); the settled activated sludge is then digested anaerobically to produce biogas - a deliberate switch from aerobic to anaerobic conditions between stages.
Concept context
Microorganisms in food, medicine, sewage treatment, biocontrol, and biofertilizers