Oxygen—Its Toxicity and Defense against It

Abstract

Life originated in an oxygen-free atmosphere. The first organisms are thus considered to have been strictly anaerobic; they could not live in the presence of oxygen. As the atmosphere became oxygenic, aerobic organisms evolved. This occurred because aerobic respiration provided organisms with mechanisms to make more effecient use of nutrients. Most of the organisms on the present earth are aerobes. Thus, the strong oxidizing power of oxygen is beneficial, but it can also be deleterious to organisms. The very reason that allows oxygen to serve as an efficient ultimate electron acceptor in respiration, its strong oxidizing effect, poses a threat to the survival of organisms. The organisms on the present earth live rather precarious lives in the presence of oxygen, and this life has been made possible by the development of elaborate defensive mechanisms to act against oxygen toxicity. We will here survey both the toxic effects of oxygen and the major defense mechanisms.