Photosynthesis, necessary for the maintenance of life keeps the oxic ocean in disequilibrium; O2 and other byproducts of photosynthesis such as organic matter, CH4, CO, and H2 co-exist. Thus, the real ocean cannot be characterized conceptually or operationally by a unique pε. The redox components of the atmosphere and of the ocean are nearly in a steady state composition. Oxygen is the atmospheric oxidant most influential in regulating with its redox partner H2O the redox level of aerobic water. Biological mediation of most redox processes in natural waters (and of the four-electron reduction of O2) justifies the modeling - as was done by Sillén - of the idealized counterpart of the aerobic ocean (atmosphere, water, sediment), i. e., of a hypothetical equilibrium ocean, in terms of O2 (g) + 4H+ + 4e-⇄2H2O1.