Structures of PSI-FCPI from Thalassiosira pseudonana in high light provide convergent evolution and light-adaptive strategies in diatom FCPIs

Diatoms achieve great survival success in the fluctuating oceanic environment, rely on fucoxanthin chlorophyll a / c -binding proteins (FCPs) to complete light harvesting and quenching, which provide about 20% primary productivity on earth. We report two cryo-electron microscopic structures of photosystem I (PSI) with 13 or 5 FCPIs respectively at 2.78 Å and 3.20 Å resolution from Thalassiosira pseudonana under high light conditions. 8 Lhcr FCPIs are found detached from the PSI-13FCPI supercomplex under high light conditions, remaining 5 FCPIs are stably combined with the PSI core including Lhcr3, RedCAP, Lhcq8, Lhcf10, and FCP3 subunits. The specific pigment network in this centric diatom T. pseudonana demonstrates a higher proportion of Chlorophylls a , diadinoxanthins, and diatoxanthins but fewer fucoxanthins compared with the huge PSI-FCPI from another centric diatom Chaetoceros gracilis , thus exhibiting more efficiency in energy transfer and dissipation among FCPI antennas. These results reveal the assembly mechanism of several types of peripheral FCPIs and corresponding light-adaptive strategies in T. pseudonana , as well as the convergent evolution of the diatoms PSI-FCPI structures.