Anaerobic

The term anaerobic refers to living processes (usually the release of energy from nutrients) that take place in the absence of molecular oxygen. The earliest organisms, the prokaryotic bacteria, lived in an oxygen deficient atmosphere and extracted energy from organic compounds without oxygen (that is, by anaerobic respiration). Most organisms alive today extract their energy from nutrients aerobically (in the presence of oxygen) although a few respire. Aerobic respiration releases a lot of energy from nutrients, whereas anaerobic respiration releases relatively little energy.