Answer: Through the glyoxylate cycle under aerobic, high-sugar, low-pH conditions.
- A Through the glyoxylate cycle under aerobic, high-sugar, low-pH conditions
- B Through fermentation under anaerobic conditions at neutral pH
- C Through beta-oxidation of fatty acids under aerobic conditions
- D Through the pentose phosphate pathway under nitrogen-rich conditions
Correct answer: A. Through the glyoxylate cycle under aerobic, high-sugar, low-pH conditions
Explanation: Aspergillus niger produces citric acid through an overflow of the TCA (Krebs) cycle under specific conditions: high sugar concentration, low pH (2-3), nitrogen and manganese limitation, and vigorous aeration (aerobic conditions).
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