Algal carbon concentrating drives fatty acid biosynthesis beyond photosynthesis.

Algal photosynthesis contributes to half of global carbon fixation and is enhanced by CO2-concentrating mechanisms (CCMs), which depend on the pyrenoid organelle in eukaryotic species. Despite its importance, the coordination between photosynthesis and other inorganic-carbon using pathways like fatty acid biosynthesis (FAS) remains underexplored. Here, we investigate this interplay using the green alga Chlamydomonas reinhardtii. We find that subunits of the acetyl-CoA carboxylase, key in FAS, form protein condensates at the pyrenoid periphery under limiting CO2 and redistribute throughout the chloroplast stroma under high CO2. The condensate structures form via liquid-liquid phase separation and are located in the pyrenoid's outer layer, which contains the bicarbonate-generating enzymes. Pyrenoid function-deficient mutants display impaired FAS, supporting a model where CCMs supply carbon to FAS. Our results unveil a moonlighting function of the pyrenoid and have implications for global carbon cycle, algal biotechnology, and the transfer of pyrenoids into crops for photosynthesis improvements.