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 light and transfer the excitation energy through an interconnected pigment network to the reaction center (RC), where it is converted to chemical energy. Although the excitation energy transfer (EET) in photosystems has 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 PSI complexes, 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 PSI complexes. This, together with a radial site-energy bias, creates a strong exciton energy gradient (EEG) that funnels the exciton to the RC. However, the EEG is effective only over a limited distance range from the core region. In large PSI complexes, the pigments farther from the RC are weakly coupled, and the efficiency of long-range EET is maintained by additional features that complement the EEG, such as the pigment composition in the peripheral regions of the antenna, which also broadens the energy range of light absorption. By comparing red-lineage PSI complexes with a common parameter set, we provide a framework for relating EET to structural rearrangements in the pigment network.