Abstract

Brown algae and other red-algae-derived organisms are major contributors to global CO2 fixation via photosynthesis. To understand the photosynthetic function of brown algae, we obtained the structure of giant kelp Macrocystis pyrifera photosystem I (PSI) with a fucoxanthin-chlorophyll-protein (FCP) antenna and compared it to known structures from the red-algal lineage. We identified differences in M. pyrifera's antenna composition, architecture, and chlorophyll networks, as well as a pronounced variation in transmembrane hydrophobic thickness across the PSI-FCP supercomplex, with implications for photochemical function. Our work lays the foundation to understand kelp's high photosynthetic productivity and reveals drivers of antenna conservation and diversification for the red lineage.