Structure of Photosystem I-FCP from giant kelp uncovers drivers of antenna evolution across the red lineage

Brown algae and other red-algae-derived organisms such as diatoms 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, reveals new drivers of antenna conservation and diversification, and sheds light on evolutionary relationships between organisms of the red lineage.