A Possible Mechanism for Aggregation-Induced Chlorophyll Fluorescence Quenching in Light-Harvesting Complex II from the Marine Green Alga Bryopsis corticulans

The light-harvesting complex II of a green alga Bryopsis corticulans (B-LHCII) is peculiar in that it contains siphonein and siphonaxathin as carotenoid (Car). Since the S₁state of siphonein and siphonaxathin lies substantially higher than the Q_ystate of chlorophyll a (Chl a), the Chl a(Q_y)-to-Car(S₁) excitation energy transfer is unfeasible. To understand the photoprotective mechanism of algal photosynthesis, we investigated the influence of temperature on the excitation dynamics of B-LHCII in trimeric and aggregated forms. At room temperature, the aggregated form showed a 10-fold decrease in fluorescence intensity and lifetime than the trimeric form. Upon lowering the temperature, the characteristic 680 nm fluorescence (F-680) of B-LHCII in both forms exhibited systematic intensity enhancement and spectral narrowing; however, only the aggregated form showed a red emission extending over 690-780 nm (F-RE) with pronounced blueshift, lifetime prolongation, and intensity boost. The remarkable T-dependence of F-RE is ascribed to the Chl-Chl charge transfer (CT) species involved directly in the aggregation-induced Chl deactivation. The CT-quenching mechanism, which is considered to be crucial for B. corticulans photoprotection, draws strong support from the positive correlation of the Chl deactivation rate with the CT state population, as revealed by comparing the fluorescence dynamics of B-LHCII with that of the plant LHCII.