Anaerobic
Organisms that do not take up molecular oxygen from the growth environment are described as living anaerobically.  Some species of microorganisms that live anaerobic lifestyles are only able to thrive under conditions where the oxygen tension of the growth environment is extremely low; they are killed by the intermediate reactive oxygen species generated when oxygen is partially reduced.  ATP production in anaerobic organisms is achieved either by fermentation or anaerobic respiration.