The Magic of Dioxygen -- Light-Dependent Production of Dioxygen in Photosynthesis -- Production of Dioxygen in the Dark: Dismutases of Oxyanions -- Respiratory Conservation of Energy with Dioxygen: Cytochrome c Oxidase -- Transition Metal Complexes and the Activation of Dioxygen -- Methane Monooxygenase: Functionalizing Methane at Iron and Copper -- Metal Enzymes in "Impossible" Microorganisms Catalyzing the Anaerobic Oxidation of Ammonium and Methane.