If an organism switches from aerobic respiration to fermentation due to oxygen depletion, this is an example of metabolic shift observed in which group?

• A. Obligate aerobes
• B. Facultative anaerobes
• C. Aerotolerant anaerobes
• D. Obligate anaerobes

Answer: (B)

The main idea here is metabolic flexibility in response to oxygen availability. Organisms that are facultative anaerobes can use aerobic respiration when oxygen is present, yielding more ATP, and switch to fermentation when oxygen runs out to keep glycolysis going and regenerate NAD+. This switch allows energy production to continue even without oxygen, though less efficiently. Obligate aerobes rely exclusively on oxygen-using pathways and don’t switch to fermentation, so they’re limited by oxygen absence. Aerotolerant anaerobes don’t adjust with oxygen levels and mainly rely on fermentation regardless of O2. Obligate anaerobes thrive in and require anaerobic conditions and don’t switch to fermentation in response to oxygen depletion because they are already adapted to absence of oxygen.