Two types of fucoxanthin-chlorophyll-binding proteins I tightly bound to the photosystem I core complex in marine centric diatoms

Abstract Intact fucoxanthin (Fucox)-chlorophyll (Chl)-binding protein I-photosystem I supercomplexes (FCPI-PSIs) were prepared by a newly developed simple fast procedure from centric diatoms Chaetoceros gracilis and Thalassiosira pseudonana to study the mechanism of their efficient solar energy accumulation. FCPI-PSI purified from C. gracilis contained 252 Chl a , 23 Chl c , 56 Fucox, 34 diadinoxanthin + diatoxanthin, 1 violaxanthin, 21 s-carotene, and 2 menaquinone-4 per P700. The complex showed a high electron transfer activity at 185,000 μmol mg Chl a − 1  · h − 1 to reduce methyl viologen from added cytochrome c 6 . We identified 14 and 21 FCP proteins in FCPI-PSI of C. gracilis and T. pseudonana , respectively, determined by N-terminal and internal amino acid sequences and liquid chromatography–tandem mass spectrometry (LC–MS/MS) analyses. PsaO and a red lineage Chl a / b -binding-like protein (RedCAP), Thaps3:270215, were also identified. Severe detergent treatment of FCPI-PSI released FCPI-1 first, leaving the FCPI-2-PSI-core complex. FCPI-1 contained more Chl c and showed Chl a fluorescence at a shorter wavelength than FCPI-2, suggesting an excitation-energy transfer from FCPI-1 to FCPI-2 and then to the PSI core. Fluorescence emission spectra at 17 K in FCPI-2 varied depending on the excitation wavelength, suggesting two independent energy transfer routes. We formulated a model of FCPI-PSI based on the biochemical assay results.