Aerobic microorganisms require the presence of oxygen for growth. Molecular oxygen functions in the respiratory pathway of the microbes to produce the energy necessary for life. Bacteria, yeasts, fungi, and algae are capable of aerobic growth.

The opposite of an aerobe is an anaerobe. An anaerobe does not require oxygen, or sometimes cannot even tolerate the presence of oxygen.

There are various degrees of oxygen tolerance among aerobic microorganisms. Those that absolutely require oxygen are known as obligate aerobes. Facultative aerobes prefer the presence of oxygen but can adjust their metabolic machinery so as to grow in the absence of oxygen. Microaerophilic organisms are capable of oxygen-dependent growth but cannot grow if the oxygen concentration is that of an air atmosphere (about 21% oxygen). The oxygen content must be lower.

Oxygen functions to accept an electron from a substance that yields an electron, typically a substance that contains carbon...