Which organism type is harmed by the absence of oxygen?

• A. Facultative anaerobe
• B. Aerotolerant anaerobe
• C. Obligate aerobe
• D. Obligate anaerobe

Answer: (D)

The key idea is that some microbes must have oxygen to grow, because they rely on aerobic respiration to meet their energy needs. Obligate aerobes require oxygen for energy production; without it, their cells can’t generate enough ATP and growth halts or cells die. In contrast, facultative anaerobes can switch to fermentation or anaerobic respiration when oxygen isn’t available, so they’re not harmed by its absence. Aerotolerant anaerobes don’t use oxygen but tolerate its presence, so absence isn’t particularly harmful. Obligate anaerobes, on the other hand, are harmed by the presence of oxygen, not by its absence. Therefore, the type harmed by the absence of oxygen is the one that must have oxygen to grow—obligate aerobes.