Energy is the most important requirement for life. It can be acquired by harvesting electrons flowing naturally from reduced to oxidized inorganic compounds (lithotrophy) or organic compounds (organotrophy), or by light-mediated processes such as photosynthesis. Patterns of energy utilization have played a dominant role in the evolution of life and its diversification into intricately rich and complex ecosystems. A supply of reduced chemical species on the ocean floor supports chemoautotrophic life that is confined to the ocean abyss, whereas light at the surface propels the flow of photosynthetically produced electrons throughout most of the oceanic biosphere. Re-energized by light, these electrons redistribute their energy among myriad anabolic and catabolic processes, relentlessly turning over and maintaining most of the marine biomass. This phenomenon involves a system of energy and electron exchange that extends from photoautotrophic organisms at the ocean surface to all ocean biota, including heterotrophic communities below the ocean floor. This article briefly discusses the role of this system in maintaining microbial life in the ocean.