The anaerobe is defined by Webster as an anaerobic organism. Anaerobic itself is defined as
1a: living, active, occurring, or existing in the absence of free oxygen <anaerobic respiration>
b : of, relating to, or being activity in which the body incurs an oxygen debt <anaerobic exercise>
2 : relating to or induced by anaerobes

Anaerobes were some of the first life forms to appear on Earth some 3.8 billion years ago (give or take 0.2 billion years). It is believed that the metabolism at the time was based on sulfur compounds from the vents deep in the ocean. The Archaean Period ended around 3 billion years ago (give or take 0.5 billion) with the appearance of the cyanobacteria which was able to photosynthesize and produced oxygen as a byproduct of the process in an event known as the "Oxygen Holocaust".

Probably the most well known anaerobe is called 'yeast'. While yeast can preform cellular respiration aerobically (in the presence of O2) once the oxygen is depleted it will continue to respire anaerobically and produce alcohol and CO2 as part of fermentation and anaerobic respiration.

Actually, yeast is known as a facultative anaerobe because it is capable of growing in both aerobic and anaerobic conditions. It grows best in the presence of oxygen when it is able to use oxygen as part of Krebs cycle and produce 38 ATP per glucose molecule compared to glycolysis (converting glucose into two molecules of pyruvic acid and 2 ATP). Later, the pyruvic acid is converted into ethanol as part an anaerobic equivalent to chemiosmosis (which requires oxygen).

Within the medical field, anaerobes are an important part of life, not because of their use but rather because 5-10% of all infections within clinical environments are from anaerobes. Within the mouth, this type of bacteria is what hides behind plaque and causes cavaties and tooth infections. Anaerobes often thrive in the acidic pH and the low oxygen areas associated with abscesses and deep injuries.

Recently, anaerobic bacteria have become of much more interest to xenobiologists studying such planets as Mars and the moons Europa and Titan where there is little oxygen around.

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