Protein-independent light harvesting governed by structural heterogeneity
Chlorosomes are the largest known photosynthetic light-harvesting antennas, yet unlike protein-based antennas, they lack protein scaffolds that organize pigment molecules and instead contain self-assembled tubular and lamellar bacteriochlorophyll aggregates. How these antennas achieve directional and efficient energy transfer has remained unresolved. By applying ultrafast transient absorption spectroscopy to individual wild-type and mutant chlorosomes, we resolved six kinetic components obscured by ensemble averaging and assigned each to an excitation process associated with either lamellar or tubular aggregates. Lamellar aggregates tend toward exciton localization and expand light-harvesting capacity, whereas tubular aggregates retain a relatively delocalized excitonic character and serve as the primary energy donors to the baseplate. Structural heterogeneity therefore emerges as a functional feature that balances light-harvesting capacity with robust directional energy transfer. These findings reveal a division of labor among self-assembled pigment aggregates for efficient light harvesting without protein scaffolds.