Nam, O.; McKenzie, C.; Dowle, A.; Dowson, M.; Barrett, J.; Mackinder, L.

Diatoms are central to the global carbon cycle. At the heart of diatom carbon fixation is an overlooked organelle called the pyrenoid, where concentrated CO2 is delivered to densely packed Rubisco. Diatom pyrenoids fix approximately one-fifth of global CO2 but virtually nothing is known about this organelle in diatoms. Using large-scale fluorescence protein tagging and affinity purification-mass spectrometry we generate a high-confidence spatially-defined protein-protein interaction network for the diatom pyrenoid. Within our pyrenoid interactome are 10 proteins with no known function. Six form a static shell encapsulating the Rubisco matrix of the pyrenoid, with the shell critical for pyrenoid structural integrity and potentially acting as a CO2 diffusion barrier. Although no conservation at a sequence level, the diatom pyrenoid shares some structural similarities to prokaryotic carboxysomes. Collectively, our results support the convergent evolution of pyrenoids across the two main plastid lineages and uncovers a major structural and functional component of global CO2 fixation.