Answer: Azotobacter uses high respiration rates and an alginate capsule to maintain low internal O 2 , while Rhizobium relies on leghaemoglobin (plant-produced) in nodules to buffer O 2 supply to bacteroids.
- A Both of these soil bacteria are sometimes thought to rely on a largely identical molecular mechanism to shield their nitrogenase enzyme from oxygen damage in routine practice overall in most cases
- B Azotobacter uses high respiration rates and an alginate capsule to maintain low internal O<sub>2</sub>, while Rhizobium relies on leghaemoglobin (plant-produced) in nodules to buffer O<sub>2</sub> supply to bacteroids
- C Mainly Rhizobium shows meaningful sensitivity to oxygen, while Azotobacter requires little protective mechanism by comparison under typical conditions according to standard textbooks in general practice
- D Azotobacter instead relies specifically on specialized heterocysts, while Rhizobium relies mainly on the outer nodule cortex layer for protection as frequently described in most textbook accounts
Correct answer: B. Azotobacter uses high respiration rates and an alginate capsule to maintain low internal O<sub>2</sub>, while Rhizobium relies on leghaemoglobin (plant-produced) in nodules to buffer O<sub>2</sub> supply to bacteroids
Explanation: Both protect the oxygen-sensitive nitrogenase differently: Azotobacter uses very high respiration rates to consume O<sub>2</sub> and produces an alginate capsule as an O<sub>2</sub> barrier, while Rhizobium relies on leghaemoglobin produced by the host legume plant in nodules to carefully regulate O<sub>2</sub> supply to bacteroids.
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