Strains of the cyanobacterium Acaryochloris marina exhibit diverse far-red light-harvesting properties during chlorophyll d-based photosynthesis. Here, we show that differences in light absorption among A. marina strains arise exclusively from Photosystem I (PSI) and reflect variation in multiple low-energy chlorophyll states. Time-resolved fluorescence reveals different combinations of low-energy states among strains, generating a continuum of spectral phenotypes. Cryo-EM structures of PSI at ~1.8 angstrom resolution reveal similar low-energy states arising from distinct pigment environments, demonstrating that red-shifted absorption is not governed by a single conserved motif. Phylogenetic analyses show that spectral tuning evolved through modular variation and reassortment of PSI components. These results indicate that distinct pigment configurations can converge on similar low-energy states, extending light harvesting near the energetic limit of oxygenic photosynthesis.
Mehra, H. S., Magdaong, N. C. M., Flesher, D. A., Shen, G., Ulrich, N. J., Brininger, C. M., Niedzwiedzki, D. M., Miller, S. R., Gisriel, C. J.
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