The first photosynthetic mutant in diatoms targets the subunit γ of plastidial ATP synthase and reveals a key role of thylakoid electrochemical proton gradient in photosynthesis regulation and heterotrophic metabolism

Diatoms are major phytoplanktonic algae with secondary endosymbiotic plastids that differ in cellular and regulatory traits from those of the green lineage. Here we exploited the heterotrophic growth ability of Cyclotella cryptica to create the first diatom photosynthetic mutant by CRISPR-Cas inactivation of the nucleus-encoded ATP synthase subunit γ. These mutants showed impaired phototrophic capacity and altered thylakoids morphology. In absence of γ-ATP, protons that accumulate in the thylakoid lumen slow down the cytochrome b ₆ f complex, thus keeping the electron carriers downhill oxidized. These results, reversible when the proton gradient is suppressed, demonstrate the existence of a photosynthetic control in diatoms. At variance with the wild type, γ ATP synthase mutants cannot grow heterotrophically in darkness nor in the light when photosystem II is inhibited. This requirement of heterotrophic growth on photosynthetic electron transfer or on the presence of plastidial ATP synthase suggests that the proton motive force (pmf) is a central integrator of the metabolic interaction between photosynthesis and heterotrophy. Our results establish C. crytica as a robust model for analysis of photosynthetic function, regulation and metabolic integration in organisms with secondary plastids.