The pyrenoid at a glance.

Ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) is the central CO2-fixing enzyme of the biosphere. It is present in virtually all photosynthetic organisms, including cyanobacteria, algae and land plants. Rubisco catalyses the carboxylation step of the Calvin-Benson-Bassham (CBB) cycle that underpins the light-independent 'dark' reactions of oxygenic photosynthesis. However, Rubisco exhibits a low catalytic rate and wasteful oxygenation activity, a constraint rooted in its ancient evolutionary origin. To overcome this specificity limitation, many photosynthetic lineages have evolved CO2-concentrating mechanisms (CCMs) that increase the local concentration of CO2 relative to O2 around Rubisco. In algae, these biophysical CCMs actively deliver CO2 to the dense, phase-separated Rubisco matrix of chloroplast microcompartments called pyrenoids. Here, we outline the molecular principles that define pyrenoids, their remarkable diversity arising from convergent evolution and the fundamental role of Rubisco condensation. Using case studies from well-established model organisms, we highlight current insights and future directions of pyrenoid and CCM research.