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Pyrenoids are subcompartments of algal chloroplasts that increase the efficiency of Rubisco-driven CO₂ fixation. Diatoms fix up to 20% of global CO₂, but their pyrenoids remain poorly characterized. Here, we used in vivo photo-crosslinking to identify pyrenoid shell (PyShell) proteins, which we localized to the pyrenoid periphery of model pennate and centric diatoms, Phaeodactylum tricornutum and Thalassiosira pseudonana. In situ cryo-electron tomography revealed that pyrenoids of both diatom species are encased in a lattice-like protein sheath. Single-particle cryo-EM yielded a 2.4-Å-resolution structure of an in vitro TpPyShell1 lattice, which showed how protein subunits interlock. T. pseudonana TpPyShell1/2 knockout mutants had no PyShell sheath, altered pyrenoid morphology, and a high-CO₂ requiring phenotype, with reduced photosynthetic efficiency and impaired growth under standard atmospheric conditions. The structure and function of the diatom PyShell provide a molecular view of how CO₂ is assimilated in the ocean, a critical ecosystem undergoing rapid change.

Shimakawa G., Demulder M., Flori S., Kawamoto A., Tsuji Y., Nawaly H., Tanaka A., Tohda R., Ota T., Matsui H., Morishima N., Okubo R., Wietrzynski W., Lamm L., Righetto RD., Uwizeye C., Gallet B., Jouneau PH., Gerle C., Kurisu G., Finazzi G., Engel BD.*, Matsuda Y.* (2024). Diatom pyrenoids are encased in a protein shell that enables efficient CO2 fixation. Cell. doi: 10.1016/j.cell.2024.09.013