What are organisms that only respire in the absence of oxygen called?

Organisms that only respire in the absence of oxygen are classified as obligate anaerobes. These organisms have evolved to thrive in environments devoid of oxygen and utilize anaerobic respiration or fermentation for energy production. Their metabolic processes rely on substrates that do not require oxygen for the breakdown of organic molecules. In many cases, obligate anaerobes can be sensitive to oxygen, as it may be toxic to them due to the formation of reactive oxygen species that they cannot effectively neutralize.

Understanding the different types of respiratory pathways is crucial in biology. For instance, facultative aerobes can switch between aerobic respiration in the presence of oxygen and anaerobic respiration when oxygen is scarce. Obligate aerobes, on the other hand, require oxygen for their survival, relying exclusively on aerobic respiration. Oligo-aerobes tolerate only low levels of oxygen, which they utilize for respiration but can also survive in anaerobic conditions, unlike obligate anaerobes that cannot survive with oxygen at all.