Shun Arai

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 either lamellar or tubular aggregates. Lamellar aggregates expand light-harvesting capacity, whereas tubular aggregates serve as the primary energy donors to the baseplate. Such structural heterogeneity is therefore not merely suppressed but tuned to balance light-harvesting capacity with robust energy delivery. These findings reveal a division-of-labor strategy among pigment aggregates for efficient light harvesting without protein scaffolds.

Photosynthetic light harvesting proceeds rapidly and robustly through precisely arranged pigment molecules. However, the molecular arrangements are not uniform because they exhibit structural heterogeneity among individual complexes and across spatial regions and undergo dynamic fluctuations. Such static and dynamic disorder can substantially perturb excitation dynamics. Here, we report a highly sensitive transient absorption microscope that integrates single-objective absorption microscopy, balanced detection, and lock-in amplification, enabling the quantitative analysis of heterogeneity and temporal fluctuations in excitation dynamics. By analyzing individual chlorophyll-derivative aggregates mimicking photosynthetic light-harvesting antennas, we demonstrated that two kinetic components with nearly identical time constants can be resolved based on differences in their time-constant distributions. We further quantified the photophysical properties of each component, including the absorbance change, fluorescence intensity, fluorescence efficiency, and fluorescence peak intensity ratio. These results establish an analytical framework for excitation dynamics that leverages not only the mean values of time constants but also their distribution profiles.

Photosynthetic light harvesting proceeds rapidly and robustly through precisely arranged pigment molecules. However, the molecular arrangements are not uniform because they exhibit structural heterogeneity among individual complexes and across spatial regions, and undergo dynamic fluctuations. Such static and dynamic disorder can substantially perturb excitation dynamics. Here, we report a highly sensitive transient absorption microscope that integrates single-objective absorption microscopy, balanced detection, and lock-in amplification, enabling quantitative analysis of heterogeneity and temporal fluctuations in excitation dynamics. By analyzing individual chlorophyll-derivative aggregates mimicking photosynthetic light-harvesting antennas, we demonstrated that two kinetic components with nearly identical time constants can be resolved based on differences in their time-constant distributions. We further quantified the photophysical properties of each component, including absorbance change, fluorescence intensity, fluorescence efficiency, and fluorescence peak intensity ratio. These results establish an analytical framework for excitation dynamics that leverages not only the mean values of time constants but also their distribution profiles.