Biochemical Characterization of Photosystem I Complexes Having Different Subunit Compositions of Fucoxanthin Chlorophyll A/c-binding Proteins in the Diatom Chaetoceros Gracilis

Diatoms are dominant phytoplankton in aquatic environments and have unique light-harvesting apparatus, fucoxanthin chlorophyll a/c-binding protein (FCP). Diatom photosystem I (PSI) interacts with specific FCPs (FCPI); however, it remains unclear how PSI cores receive excitation energy from FCPI. To analyze the energy transfer dynamics, it is necessary to isolate both PSI cores and PSI-FCPI complexes. In this study, we prepared three PSI complexes, which are PSI-FCPI membrane fragments, detergent-solubilized PSI-FCPI supercomplexes and PSI core-like complexes, from the marine centric diatom, Chaetoceros gracilis, and examined their biochemical properties. Both the PSI-FCPI membrane fragments and supercomplexes showed similar subunit compositions including FCPI, whereas the PSI complexes were devoid of most FCPI subunits. The purity and homogeneity of the two detergent-solubilized PSI preparations were verified by clear-native PAGE and electron microscopy. The difference of pigment contents among the three PSI samples was shown by absorption spectra at 77 K. The intensity in the whole spectrum of PSI-FCPI membranes was much higher than those of the other two complexes, while the spectral shape of PSI complexes was similar to that of cyanobacterial PSI core complexes. 77-K fluorescence spectra of the three PSI preparations exhibited different spectral shapes, especially peak positions and band widths. Based on these observations, we discuss the merits of three PSI preparations for evaluating excitation energy dynamics in diatom PSI-FCPI complexes.