A spatial atlas of the seaweed CO ₂ -fixation machinery reveals a unique Rubisco condensation mechanism

Seaweeds (macroalgae) are important primary producers that sustain food webs in coastal ecosystems. Most algae accelerate inorganic carbon assimilation by actively concentrating CO ₂ in a Rubisco-rich specialized organelle called the pyrenoid. However, the molecular composition of this pathway is unknown in seaweeds. Here, we investigated the intracellular localization of 160 proteins associated with CO ₂ acquisition in the green seaweed Ulva (Sea lettuce). We assign 68 proteins to different pyrenoid subdomains and identify a consensus Ulva Rubisco binding motif revealing the molecular logic of the Ulva pyrenoid. We reveal Seaweed Ulva Pyrenoid Assembly 1 (SUPA1) as the core pyrenoid assembly factor. We show that Rubisco condensation is driven by a unique mechanism: the helical folding of SUPA1 motifs upon Rubisco binding, combined with steric hindrance that halves the available Rubisco binding sites from eight to four. Our data gives an unprecedented sub-cellular spatial understanding on seaweed carbon fixation and provides insights into the evolution of this important pathway in the global carbon cycle.