Figure 1.

Major sources of ${\text{O}}_{\text{2}}^{-*}$ in natural waters. (A) Abiotic, photochemical oxidation of natural organic matter (NOM) with concomitant reduction of O₂. (B) Biological reduction of O₂ in the extracellular milieu via cell surface enzymes. (C) Biological reduction of oxidized labile redox-active compounds (LRAC_ox) in the extracellular milieu to form reduced labile redox-active compounds (LRAC_red) that subsequently reduce O₂ to ${\text{O}}_2^{-*}.$ (D) Biological release of LRAC_red into the extracellular milieu that subsequently reduce O₂ to ${\text{O}}_2^{-*}.$ (E) Diffusion of LRAC_red produced under suboxic conditions into more oxygenated waters. (F) The ${\text{O}}_2/{\text{O}}_2^{-*}$ couple readily exchanges electrons with a range of other labile redox-active compounds in the extracellular milieu. Decay pathways for ${\text{O}}_2^{-*}$ are not shown.