A promiscuous mechanism to phase separate eukaryotic carbon fixation in the green lineage

CO ₂ fixation is commonly limited by inefficiency of the CO ₂ -fixing enzyme Rubisco. Eukaryotic algae concentrate and fix CO ₂ in phase-separated condensates called pyrenoids, which complete up to one-third of global CO ₂ fixation. Condensation of Rubisco in pyrenoids is dependent on interaction with disordered linker proteins that show little conservation between species. We developed a sequence-independent bioinformatic pipeline to identify linker proteins in green algae. We report the linker from Chlorella and demonstrate that it binds a conserved site on the Rubisco large subunit. We show the Chlorella linker phase separates Chlamydomonas Rubisco and that despite their separation by ∼800 million years of evolution, the Chlorella linker can support the formation of a functional pyrenoid in Chlamydomonas . This cross-species reactivity extends to plants, with the Chlorella linker able to drive condensation of some native plant Rubiscos in vitro and in planta . Our results represent an exciting frontier for pyrenoid engineering in plants, which is modelled to increase crop yields.