Photosystem I enters into the spotlight: Plants rely on large complexes of proteins, chlorophyll, and other cofactors to turn light into chemical energy. Qin et al. present the crystal structures of photosystem I (PSI) and the light-harvesting complex I (LHCI) supercomplex from pea plants (see the Perspective by Croce). The well-resolved structure of the outer antenna complexes and their interaction with the PSI core provide a structural basis for calculating excitation energy transfer efficiency. Moreover, the organization and orientation of chlorophyll and carotenoid cofactors within and between PSI and LHCI hint at energy transfer and photoprotection mechanisms. Science , this issue p. 989 ; see also p. 970

All the hues, even the blues: Photosynthetic organisms must balance maximizing productive light absorption and protecting themselves from too much light, which causes damage. Both tasks require pigments—chlorophylls and carotenoids—which absorb light energy and either transfer it to photosystems or disperse it as heat. Wang et al. determined the structure of a fucoxanthin chlorophyll a/c–binding protein (FCP) from a diatom. The structure reveals the arrangement of the specialized photosynthetic pigments in this light-harvesting protein. Fucoxanthin and chlorophyll c absorb the blue-green light that penetrates to deeper water and is not absorbed well by chlorophylls a or b. FCPs are related to the light-harvesting complexes of plants but have more binding sites for carotenoids and fewer for chlorophylls, which may help transfer and disperse light energy. Science , this issue p. eaav0365

A light-harvesting array in diatoms: Photosynthetic organisms use huge arrays of pigments to draw light energy into the core of photosystem II. The arrangement of these pigments influences how much energy reaches the reaction center. Pi et al. determined the structure of photosystem II from a diatom in complex with an antenna of fucoxanthin–chlorophyll a/c binding proteins (FCPs) (see the Perspective by Büchel). The specialized pigments in this complex allow microalgae to harvest light within a wide range of the visible spectrum. The FCPs are arranged in a pattern analogous to light-harvesting complexes in plants. Science , this issue p. eaax4406 ; see also p. 447

Photosystem I (PSI) and II (PSII) balance their light energy distribution absorbed by their light-harvesting complexes (LHCs) through state transition to maintain the maximum photosynthetic performance and to avoid photodamage. In state 2, a part of LHCII moves to PSI, forming a PSI-LHCI-LHCII supercomplex. The green alga Chlamydomonas reinhardtii exhibits state transition to a far larger extent than higher plants. Here we report the cryo-electron microscopy structure of a PSI-LHCI-LHCII supercomplex in state 2 from C. reinhardtii at 3.42 Å resolution. The result reveals that the PSI-LHCI-LHCII of C. reinhardtii binds two LHCII trimers in addition to ten LHCI subunits. The PSI core subunits PsaO and PsaH, which were missed or not well-resolved in previous Cr-PSI-LHCI structures, are observed. The present results reveal the organization and assembly of PSI core subunits, LHCI and LHCII, pigment arrangement, and possible pathways of energy transfer from peripheral antennae to the PSI core.

Diatom is an important group of marine algae and contributes to around 20% of the global photosynthetic carbon fixation. Photosystem I (PSI) of diatoms is associated with a large number of fucoxanthin-chlorophyll a/c proteins (FCPIs). We report the structure of PSI-FCPI from a diatom Chaetoceros gracilis at 2.38 Å resolution by single-particle cryo-electron microscopy. PSI-FCPI is a monomeric supercomplex consisting of 12 core and 24 antenna subunits (FCPIs), and 326 chlorophylls a, 34 chlorophylls c, 102 fucoxanthins, 35 diadinoxanthins, 18 β-carotenes and some electron transfer cofactors. Two subunits designated PsaR and PsaS were found in the core, whereas several subunits were lost. The large number of pigments constitute a unique and huge network ensuring efficient energy harvesting, transfer and dissipation. These results provide a firm structural basis for unraveling the mechanisms of light-energy harvesting, transfer and quenching in the diatom PSI-FCPI, and also important clues to evolutionary changes of PSI-LHCI.