Answer: Azospirillum.
- A Rhizobium
- B Nostoc
- C Azospirillum
- D Anabaena
Correct answer: C. Azospirillum
Explanation: Azospirillum is a free-living nitrogen-fixing bacterium found in the rhizosphere (root zone) of grasses and cereals, enhancing their nitrogen supply.
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