Diatoms are one of the most ecologically successful classes of photosynthetic marine eukaryotes in the contemporary oceans. Over the past 30 million years, they have helped to moderate Earth's climate by absorbing carbon dioxide from the atmosphere, sequestering it via the biological carbon pump and ultimately burying organic carbon in the lithosphere(1). The proportion of planetary primary production by diatoms in the modern oceans is roughly equivalent to that of terrestrial rainforests(2). In photosynthesis, the efficient conversion of carbon dioxide into organic matter requires a tight control of the ATP/NADPHratio which, in other photosynthetic organisms, relies principally on a range of plastid-localized ATP generating processes(3-6). Here we show that diatoms regulate ATP/NADPH through extensive energetic exchanges between plastids and mitochondria. This interaction comprises the re-routing of reducing power generated in the plastid towards mitochondria and the import of mitochondrial ATP into the plastid, and is mandatory for optimized carbon fixation and growth. We propose that the process may have contributed to the ecological success of diatoms in the ocean.