Most types of bacteria have specific metabolic oxygen requirements. Whether a bacterium requires oxygen or not has a large effect on its physiology. For example, aerobic bacteria, which require oxygen for their metabolic processes, must be able to counter the highly reactive oxygen species such as singlet oxygen (1o2) or a superoxide radical (o2−) that result from reactions within the cell. Bacteria that lack this capability are generally considered anaerobic. Since anaerobic bacteria utilize molecules other than oxygen as their final electron acceptor, they must possess adaptations to facilitate their survival in environments without oxygen. A majority of bacterial species lie somewhere in between these two polar opposites in that they are facultative anaerobes or able to utilize oxygen but also capable of surviving without it. Some particularly specialized facultative anaerobes are microaerophilic and grow only in low concentrations of oxygen. What are the enzymes and their reactions that allow bacteria to grow in the presence of oxygen