Cryo-EM Structure of Photosystem I from mangroves in their natural environment
Photosystem I (PSI) is a multi-subunit pigment-protein supercomplex responsible for light driven electron transport in photosynthesis. Its structural plasticity under natural environmental stress remains poorly understood. Here, we present a 2.1 [A] cryo-EM structure of the PSI-LHCI supercomplex from Avicennia marina, a halotolerant, dominant mangrove species native to the Arabian Gulf that thrives in harsh coastal environments. Unlike structures derived from laboratory grown specimens, our analysis captures PSI directly from wild-grown plant leaves, offering an in-situ snapshot of plant adaptation. The core architecture comprising twelve protein subunits and four Lhca antenna proteins coordinating 157 chlorophylls, 36 carotenoids and 24 lipids is highly conserved relative to model angiosperms yet exhibits subtle shifts in antenna core coupling and pigment organization. Red-shifted chlorophylls in Lhca3, conserved among far red adapted plants, suggest an ancestral photoprotective mechanism retained in high light, high-salinity environments. Lipid mediated stabilization at interfaces likely counteracts salinity-induced membrane fluidity changes enhancing resilience under fluctuating light and salt stress. Comparative analysis with PSI structures from other vascular plants highlights both evolutionary conservation and distinct ecological tuning of LHCI organization and cofactor binding in mangroves. These insights reveal molecular strategies enabling A. marina to maintain efficient photosynthesis under extreme conditions, offering strategies for engineering photosystems in crops.