What does it mean when an organism can exist without oxygen?

When an organism can exist without oxygen, it is referred to as anaerobic. Anaerobic organisms do not require oxygen for their metabolic processes and may even find oxygen toxic. They derive energy through anaerobic respiration or fermentation, which allows them to thrive in environments lacking oxygen, such as deep underwater sediments or in the gastrointestinal tracts of animals.

In contrast, aerobic organisms require oxygen for survival and rely on aerobic respiration. Facultative organisms can adapt to both aerobic and anaerobic environments, switching their metabolic processes based on the availability of oxygen. Obligate organisms are strictly defined in their oxygen needs, either requiring oxygen (aerobes) or completely avoiding it (anaerobes). Understanding these classifications helps to clarify the varying adaptations organisms have developed in response to their environments.