Refers to either an environment that contains molecular oxygen gas (O₂); an organism or tissue that requires oxygen for its metabolism; or a chemical or biological process that requires oxygen. Aerobic organisms use molecular oxygen in respiration, releasing carbon dioxide (CO₂) in return. These organisms include mammals, fish, birds, and green plants, as well as many of the lower life forms such as fungi, algae, and sundry bacteria and actinomycetes. Many, but not all, organic decomposition processes are aerobic; a lack of oxygen greatly slows these processes.