B4.2.2—Oxygen tolerance

Oxygen tolerance divides organisms into obligate aerobes, obligate anaerobes and facultative anaerobes according to whether oxygen must be present, must be absent or may vary.

Obligate aerobes require oxygen and cannot grow without it. Obligate anaerobes are harmed or killed by oxygen and occupy anoxic environments. Facultative anaerobes grow with or without oxygen, changing metabolic pathway as conditions change.

Examples: Mycobacterium tuberculosis is an obligate aerobe; many strict anaerobes grow only in oxygen-free sediments or tissues; Escherichia coli is facultative and can tolerate both oxic and anoxic conditions.

Across a sediment gradient, an obligate aerobe is restricted to the oxygenated surface, an obligate anaerobe to deeper anoxic layers, and a facultative anaerobe may occur in both.

Anaerobe does not automatically mean killed by oxygen: that describes an obligate anaerobe. Facultative anaerobes tolerate oxygen absence but often grow differently when oxygen is available.