Aggregation-Induced Excitation-Energy Quenching in Fucoxanthin Chlorophyll <i>a</i>/<i>c</i>-Binding Proteins from the Diatom <i>Phaeodactylum tricornutum</i>

Light-harvesting complexes (LHCs) are vital for photosynthesis, capturing light energy and transferring it to photosystems I and II. In diatoms, fucoxanthin chlorophyll (Chl) <i>a</i>/<i>c</i>-binding proteins (FCPs) function as unique LHCs. In this study, we examined the spectral properties of untreated and aggregated FCP complexes (Untreated-FCP and Aggregated-FCP, respectively) from the diatom <i>Phaeodactylum tricornutum</i>. Fluorescence quantum yields and excitation-energy transfer pathways were evaluated using absolute fluorescence spectroscopy and fluorescence decay-associated (FDA) spectra. Aggregation of FCPs significantly enhanced excitation-energy quenching, with a marked decrease in fluorescence quantum yield from 37.6% in Untreated-FCP to 4.8% in Aggregated-FCP. The FDA spectra of Aggregated-FCP showed prominent fluorescence decays with relatively high amplitudes with time constants of 310 ps and 1.6 ns, reflecting distinct alterations in excitation-energy transfer among Chls upon aggregation. These changes were accompanied by long-wavelength shifts and broadening of the fluorescence-emission spectra, characteristics typically observed in aggregated LHCs in land plants. Our results suggest that the structural rearrangement of pigment molecules, driven by changes in Chl-Chl and Chl-Car interactions, underlies the observed excitation-energy quenching upon aggregation. This study provides key insights into the quenching mechanisms of diatom FCPs, offering broader implications for understanding energy regulation in photosynthetic systems.