CO2fixation is commonly limited by inefficiency of the CO2-fixing enzyme Rubisco. Eukaryotic algae concentrate and fix CO2in phase-separated condensates called pyrenoids, which complete up to one-third of global CO2fixation. 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 fromChlorellaand demonstrate that it binds a conserved site on the Rubisco large subunit. We show theChlorellalinker phase separatesChlamydomonasRubisco and that despite their separation by ∼800 million years of evolution, theChlorellalinker can support the formation of a functional pyrenoid inChlamydomonas. This cross-species reactivity extends to plants, with theChlorellalinker able to drive condensation of some native plant Rubiscosin vitroandin planta. Our results represent an exciting frontier for pyrenoid engineering in plants, which is modelled to increase crop yields.