Structural insights into assembly and environmental adaptation of photosystem I from a lichen green alga.

Photosystem I (PSI) of terrestrial plants typically contains an antenna system smaller than that of aquatic algae, presumably an adaptation to terrestrial high-light conditions. Moreover, green plants face multiple other stresses including drought when colonizing terrestrial environments and have evolved effective protective mechanisms. However, how exactly PSI antenna size is reduced and the precise nature of these photoprotective strategies remain to be elucidated. Lichens, composed of a mycobiont and a phycobiont, represent a symbiotic life form that thrives under terrestrial-like stress, with their phycobionts evolving in parallel with initial land-colonizing plants. Here, we report four structures of PSI complexes from a lichen phycobiont, Diplosphaera chodatii, at resolutions ranging from 3.0-Å to 4.1-Å, revealing the structural plasticity, the unique pigment composition and arrangement of DcPSI complexes. Our results provide insights into protective mechanisms of D. chodatii and potential structural strategies for reducing PSI antenna size to adapt to terrestrial habitats.