Diatoms optimize their photosynthetic efficiency via extensive energetic exchanges between plastids and mitochondria. The proportion of planetary primary production performed by diatoms in today's oceans is roughly equivalent to that of terrestrial rainforests. Efficient conversion of CO2 into organic matter during photosynthesis requires tight control of the ATP/NADPH ratio. Here Chris Bowler and colleagues show that in diatoms this ratio is regulated through extensive energetic exchanges between plastids and mitochondria, rerouting reducing power from plastids towards mitochondria and the import of mitochondrial ATP into the plastid. This mechanism differs from that seen in other photosynthetic organisms, which rely principally on plastid-localized ATP generating processes, and may have contributed to the success of diatoms in the ocean environment. 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 lithosphere1. The proportion of planetary primary production by diatoms in the modern oceans is roughly equivalent to that of terrestrial rainforests2. In photosynthesis, the efficient conversion of carbon dioxide into organic matter requires a tight control of the ATP/NADPH ratio which, in other photosynthetic organisms, relies principally on a range of plastid-localized ATP generating processes3,4,5,6. 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.

Energetic coupling between plastids and mitochondria drives CO2 assimilation in diatoms