Abstract
<jats:p> Pyrenoids are CO <jats:sub>2</jats:sub> -fixing organelles responsible for approximately one-third of global CO <jats:sub>2</jats:sub> fixation. The pyrenoids of many algae are surrounded by a starch sheath proposed to perform the critical function of slowing leakage of concentrated CO <jats:sub>2</jats:sub> out of the pyrenoid. How the cell shapes starch granules into curved starch plates that encase the pyrenoid to enable efficient CO <jats:sub>2</jats:sub> fixation is currently unknown. Here, we elucidate how starch transitions from granules into a fully formed pyrenoid starch sheath using confocal microscopy of the model green alga <jats:italic>Chlamydomonas reinhardtii</jats:italic> . We observe that after initiation, starch granules grow circumferentially along the surface of the pyrenoid matrix to encapsulate it. We show that the starch branching enzyme SBE1 localizes to the pyrenoid and is essential for this circumferential starch granule growth. Our data suggest that SBE1 promotes the circumferential growth of pyrenoid-associated starch granules by branching starch at the matrix-granule-stroma interface. Our findings advance the understanding of pyrenoid starch sheath assembly and, more broadly, starch-shaping mechanisms. </jats:p>