​​Diatoms are very abundant oceanic organisms that contribute to 20% of the daily planetary carbon assimilation. This function is mediated by their pyrenoid structure that concentrate the CO₂ fixing enzyme Rubisco inside the PyShell, a densely layer of protein that modulates gas exchanges with the environment. By concentrating Rubisco pyrenoids located within the diatom chloroplasts increase the efficiency of CO₂ assimilation in an oxygen-free environment. Despite their importance the underlying molecular mechanisms that allow diatoms to efficiently assimilate CO₂ via their pyrenoids remain poorly understood.

​In this work a French, Japanese and Swiss consortium combine state of the art in vivo photo-crosslinking imaging and photophysiology to characterise the pyrenoid shell (PyShell) layer. This proteinaceous ordered assembly is localized at the pyrenoid periphery of Diatoms. Using in situ cryo-electron tomography (cryo-ET) single particle cryo-ET and Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) researchers reveal that the PyShell encased in a lattice-like protein sheath -instead of a lipid membrane. Disruption of the PyShell protein sheath by targeted mutagenesis leads to a fragmented pyrenoid structure, high-CO₂ requiring-photosynthesis and reduced cell growth. Recombinant PyShell proteins self-assembled into helical tubes enabling the researchers to determine a 3.0 Å-resolution PyShell structure and fit it into the in vivo structure.

​Overall the structure and function of the diatom PyShell provides new molecular insights for how CO₂ is assimilated in the ocean a crucial biome that is on the front lines of climate change.