Spectral tuning and excitation-energy transfer by unique carotenoids in diatom light-harvesting antenna

The light-harvesting antennae of diatoms and spinach are composed of similar chromophores; however, they exhibit different absorption wavelengths. Recent advances in cryoelectron microscopy have revealed that the diatom light-harvesting antenna fucoxanthin chlorophyll a/c-binding protein (FCPII) forms a tetramer and differs from the spinach antenna in terms of the number of protomers; however, the detailed molecular mechanism remains elusive. Herein, we report the physicochemical factors contributing to the difference in light absorption between the light-harvesting antennae of diatoms and spinach based on spectral calculations using an exciton model. Spectral analysis reveals the significant contribution of unique fucoxanthins (fucoxanthin-S) in FCPII to the diatom-specific spectrum, and further analysis determines their essential role in the excitation-energy transfer to chlorophyll. The findings of this study demonstrate that diatoms employ fucoxanthin-S to harvest energy under the ocean in the absence of long-wavelength sunlight and can provide significant information on the survival strategies of photosynthetic organisms to adjust to their living environment.