The pyrenoid is a carbon-fixing organelle in algae that undergoes LLPS owing to multivalent interactions between Rubisco and Essential Pyrenoid Component 1 (EPYC1). Rubisco and the linker protein EPYC1, are both necessary and sufficient to phase separate and form liquid droplets. The phase-separated Rubisco is functional. Droplet composition is dynamic and components rapidly exchange with the bulk solution. Rubisco has eight binding sites for EPYC1, while EPYC1 has four binding sites for Rubisco. Modeling suggests that such systems will exhibit a magic number effect where certain numbers of particles form an unusually stable state. The magic number effect manifests when the valency of one partner is an integral multiple of the valency of the second and the binding sites of the two partners can be saturated. This magic number effect could impact the phase diagram in many biological contexts and is predicted to give rise to unexpectedly sharp phase transition (PMID:30951647) If each repeat of EPYC1 binds Rubisco, then EPYC1 could link multiple Rubisco holoenzymes together to form the pyrenoid matrix. Multiple Rubisco binding sites on EPYC1 could arrange Rubisco into the hexagonal closely packed or cubic closely packed arrangement observed in recent cryoelectron tomography studies of the Chlamydomonas pyrenoid. EPYC1 and Rubisco could interact in one of two fundamental ways: (i) EPYC1 and Rubisco could form a codependent network, or (ii) EPYC1 could form a scaffold onto which Rubisco binds. Importantly, the 60-aa repeat length of EPYC1 is sufficient to span the observed 2- to 4.5-nm gap between Rubisco holoenzymes in the pyrenoid, and a stretched-out repeat could potentially span the observed 15-nm Rubisco center-to-center distance (PMID:27166422).