Oxygen and Oxygen Toxicity: The Birth of Concepts

Abstract

Molecular dioxygen (O₂) is an essential element of aerobic life, yet incomplete reduction or excitation of O₂ during aerobic metabolisms generates diverse oxygen-containing reactive species, commonly known as reactive oxygen species (ROS). On the one hand, ROS pose a serious threat to aerobic organisms via inducing oxidative damage to cellular constituents. On the other hand, these reactive species, when their generation is under homeostatic control, also play important physiological roles (e.g., constituting an important component of immunity and participating in redox signaling). This article defines oxygen and the key facts about oxygen, and discusses the relationship between oxygen and the emergence of early animals on Earth. The article then describes the discovery of oxygen by three historical figures and examines the birth of the concepts of oxygen toxicity and the underlying free radical mechanisms. The article ends with a brief introduction to the emerging field of ROS-mediated redox signaling and physiological responses.

Keywords: Oxygen; Oxygen toxicity; Reactive oxygen species; Redox signaling.

FIGURE 1. Historical figures who discovered oxygen

Carl Wilhelm Scheele, Joseph Priestley, and Antoine-Laurent Lavoisier are generally credited with the discovery of oxygen (source: Library of Congress; www.loc.gov).

FIGURE 2. Oxygen and emergency of animals on Earth

The increase of oxygen in the atmosphere 2.4–2.1 billion years ago, commonly known as the Great Oxidation Event (GOE), is believed to lead to the emergence of earliest animals on Earth. On the other hand, oxygen toxicity is thought to exert evolutional pressure contributing to the diversification of animals. Organisms that could not accommodate to the challenge of oxygen toxicity evolved into anaerobes. This scheme is based on Ref. .

FIGURE 3. Free radical mechanisms of oxygen toxicity

On the one hand, oxygen is essential for aerobic life. On the other hand, utilization of oxygen also inevitably results in the formation of reactive oxygen species (ROS). The ROS, when their formation is abnormally increased, may cause oxidative damage to cellular constituents, which constitutes the molecular basis of oxygen toxicity.

FIGURE 4. Reactive oxygen species (ROS) in physiology

The involvement of ROS in physiology is two-fold: On the one hand, ROS derived from O₂ contribute to killing of pathogenic microorganisms by phagocytic cells, thereby promoting the survival of the host. On the other hand, ROS act as second messengers to activate cell signal transduction, leading to desired physiological responses.