First glimpses into the molecular kaleidoscope of pyrenoid diversity.
A significant proportion of global photosynthetic carbon fixation relies on the pyrenoid, a biomolecular condensate found in the chloroplast of most unicellular algae, where the CO2-fixing enzyme Rubisco is exposed to saturating concentrations of the gas. In this review, we highlight recent advances in our understanding of the molecular basis of diverse pyrenoids. Phase separation of phylogenetically distant Rubiscos is mediated by convergently evolved linker proteins, with an emerging theme of pyrenoid condensation being organized via Rubisco-binding motifs. To minimize CO2 leakage out of the pyrenoid, starch sheaths and protein shells have evolved to surround the pyrenoid in various algal lineages. Crucially, the pyrenoid is a biomolecular condensate with an increasingly well-defined function that has evolved multiple times over the past billions of years. The emerging similarities and differences of these various pyrenoids will inform our appreciation of phase separation in biology and empower engineering efforts aimed at enhancing photosynthetic CO2 assimilation.