6.2.3: Oxygen

Dioxygen or simply oxygen \(\left(\text{O}_{2}\right)\) serves as an environmental control in at least two ways. First, it is used as an electron acceptor by aerobic organisms. Thus, the supply of oxygen influences where aerobes may be active. Secondly, oxygen is also a source of stress for cells. Oxygen toxicity for some cells can occur because cells contain oxygen-sensitive molecules, and the cells may lack the protective enzymes that detoxify reactive oxygen species (ROS) generated by oxygen metabolism. These ROS include the superoxide anion \(\left(\text{O}_{2}^{2-}\right)\), hydrogen peroxide \(\left(\text{H}_{2}\text{O}_{2}\right)\), and hydroxyl radical \((\text{OH})\) (Fridovich, 1999).

As described by Madigan et al. (2003), there are three main categories of microorganisms based on their response to oxygen availability (Fig. \(6.2\)). Aerobes are capable of using oxygen in environments with atmospheric levels of oxygen. Microaerophiles are microorganisms that can use oxygen only when it is present at low levels relative to the atmosphere. And lastly, anaerobes are microorganisms that cannot use oxygen.

Among these classes, we can further subdivide aerobes and anaerobes (Madigan et al., 2003). Aerobes can be facultative aerobes, which can grow with or without oxygen, or obligate aerobes, which can only grow aerobically. Similarly, anaerobes can be aerotolerant anaerobes, which can tolerate and grow in the presence of oxygen, or obligate anaerobes, which are inhibited or killed by oxygen exposure.

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