The Significance of O₂ for Biology

Life has prevailed on Earth for ∼3.5 billion years and has been subjected to a variety of selection pressures during this period: temperature, light, nutrients, and O 2 concentrations. The latter point may come as a surprise as molecular oxygen is seemingly a quite innocuous gas. However, the availability of O 2 provides a plentiful supply of chemical oxidant for biology – multicellular organisms rely heavily on O 2 for energy conversion in respiration. This review covers an introduction into the role of O 2 for the staging of life, reasons why it has accumulated in the atmosphere, and biological strategies for reacting O 2. The main biochemical consumers of oxygen in the biosphere, cytochrome c oxidase, alternative oxidase, plastid terminal oxidase, and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) proteins, are also outlined in function.