Figure 1.

Processes involved in the production, recycling, and consumption of O₂ in modern environments. Reactions responsible for net O₂ production (reactions 1–4) are shown above, and those that recycle or consume O₂ (reactions 5–9) are below the dotted line. Abiotic reactions are shown to the left and biotic reactions to the right of the dashed lines. Note: intermediate reactions and electrons are not shown. This overview highlights the reactions reviewed here and is not exhaustive, e.g. most abiotic and biotic O₂-consuming redox reactions are not shown.