Red states modulate energy localization between the antenna and trap in PSI.

The core antenna of the photosystem I (PSI) complex of oxygenic photosynthesis operates at near perfect quantum efficiency, making it one of the most efficient energy conversation devices in nature. The presence of low energy or red chlorophylls (chls) is nearly universal in this core antenna, and these red chls are known to extend the light harvesting capacity and control the rate of energy migration in PSI. Surprisingly, the significance of the structural position of the red chls in PSI remains unknown. In this study, we introduced mutations that generate specific red sites in the PSI core antenna, consistent with those observed in natural species. Using CryoEM, we characterized the structural changes at these red sites. Using transient absorption spectroscopy, we measured the effects of red chlorophyll at specific sites on energy transfer and trapping in PSI to identify their functional significance. While one site funneled energy to the reaction center, the other slowed equilibration in the antenna. These opposing effects indicate different functions are associated with the different red chlorophyll location, reconciling previous seemingly incompatible suggestions as to their functional role.