The pyrenoid: the eukaryotic CO(2)-concentrating organelle

The pyrenoid is a phase-separated organelle that enhances photosynthetic carbon assimilation in most eukaryotic algae and the land plant hornwort lineage. Pyrenoids mediate approximately one-third of global CO(2) fixation, and engineering a pyrenoid into C(3) crops is predicted to boost CO(2) uptake and increase yields. Pyrenoids enhance the activity of the CO(2)-fixing enzyme Rubisco by supplying it with concentrated CO(2). All pyrenoids have a dense matrix of Rubisco associated with photosynthetic thylakoid membranes that are thought to supply concentrated CO(2). Many pyrenoids are also surrounded by polysaccharide structures that may slow CO(2) leakage. Phylogenetic analysis and pyrenoid morphological diversity support a convergent evolutionary origin for pyrenoids. Most of the molecular understanding of pyrenoids comes from the model green alga Chlamydomonas (Chlamydomonas reinhardtii). The Chlamydomonas pyrenoid exhibits multiple liquid-like behaviors, including internal mixing, division by fission, and dissolution and condensation in response to environmental cues and during the cell cycle. Pyrenoid assembly and function are induced by CO(2) availability and light, and although transcriptional regulators have been identified, posttranslational regulation remains to be characterized. Here, we summarize the current knowledge of pyrenoid function, structure, components, and dynamic regulation in Chlamydomonas and extrapolate to pyrenoids in other species.