Galileo spacecraft data obtained from Jupiter's ice-covered satellite Europa suggest the existence of a subsurface water ocean. The formation of this ocean is a direct consequence of early igneous and hydrothermal processes that may have facilitated oxidation through H2 and CH4 escape. These processes could have led to a moderately alkaline sulfate-carbonate ocean and a moderately oxidized magnetite-bearing silicate mantle. When the ocean formed, low-temperature chemical disequilibria involving oceanic sulfate, bicarbonate, and ferrous minerals in underlying rocks could have provided multiple energy sources for chemotrophic organisms. Potential metabolic processes include oxidation of ferrous iron, sulfides, native sulfur, methane, hydrogen, and organic compounds, as well as reduction of water, sulfate, bicarbonate and carbonate ions (methanogenesis and acetogenesis), native sulfur, and ferric iron. If they occurred, these reactions would have provided close coupling for biogeochemical cycles of S, C, and Fe in the ocean. Periodic supplies of fresh rocks and/or aqueous fluids at the oceanic floor could drive these cycles throughout the satellite's history, including the present epoch. Signs of chemical disequilibria in the ocean would indicate chemical sources of energy for metabolism but should not be considered as indicators of life. In turn, observational signs of low-temperature redox equilibration among sulfur and carbon species would be suggestive of ancient life.