A cross-species comparison of excitation energy transfer in red-lineage photosystem I

The Photosystem I supercomplex (PSI) is a central component of photosynthesis in plants and algae. Its antenna subunits absorb the light energy and transfer it through an interconnected pigment network to the reaction center (RC), where it is stored as chemical energy. Although the excitation energy transfer (EET) processes in photosystems have been well characterized for individual organisms, evolutionary variation and inconsistent modeling settings have obscured cross-species comparisons, and it remains unclear which common structural features sustain EET efficiency. To address this, we combined kinetic modeling and rate theories to simulate EET processes in multiple red-lineage PSIs, each consisting of hundreds of chlorophylls, using the same set of experiment-derived parameters. We performed network analysis to probe how their EET processes respond to structural variation across distinct species. The exciton becomes increasingly delocalized as it approaches the core region of the PSIs, creating a strong exciton-energy gradient (EEG) that funnels it to the RC. However, the EEG is effective only over a limited distance range from the RC, encompassing small PSIs almost entirely but not the larger ones. In large PSIs, the pigments farther from the RC are weakly coupled, thereby hampering EET. Large PSIs, therefore, adopt strategies to maintain the efficiency of long-range EET alongside the EEG effect, such as variations in pigment identity in the peripheral region of the antenna. Such effects also broaden the energy range of light absorption. By comparing red-lineage PSIs on an equal footing, we present a general simulation framework to understand the response of EET to structural rearrangements in the pigment network.