Abstract. Cyanobacteria rely on bicarbonate (HCO3-) as the primary inorganic carbon (Ci) source for photosynthesis in aquatic environments. To use this Ci

The valency of intrinsically disordered proteins underpins liquid-liquid phase separation (LLPS), yet how this parameter shapes condensate function and cellular fitness remains poorly understood. Here we exploit the algal pyrenoid—a minimal, two component LLPS system—to directly link condensate properties to physiological performance. Pyrenoid assembly is driven by a disordered, multivalent Linker protein that binds Rubisco at symmetry-related surface sites, with the number of binding motifs (″stickers″) varying across species. Using Chlamydomonas reinhardtii , we systematically tuned sticker number from two to nine and examined effects on Rubisco condensation, pyrenoid architecture and CO2 fixation. Three stickers were sufficient for condensation in vitro, but at least four were required for pyrenoid assembly in vivo. Cryo-electron tomography and single-molecule tracking revealed that increasing sticker number enhances Rubisco packing and mobility, while time-resolved imaging and competition assays demonstrated that sticker number governs the kinetics of pyrenoid formation and determines cellular fitness under fluctuating carbon conditions. Our findings establish sticker number as an evolutionary tuning parameter that balances condensate formation, dynamics, and function, providing a quantitative framework for linking the molecular grammar of phase separation to biological fitness. ### Competing Interest Statement The authors have declared no competing interest. EPSRC, EP/W024063/1

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