Title: Photochemistry beyond the red-limit in chlorophyll f-photosystems

Photosystems I and II convert solar energy into the chemical energy that powers life. Both photosystems use chlorophyll-a photochemistry, absorbing almost the same color of light and thus accessing comparable amounts of energy. This energy is considered the “red-limit” for oxygenic photosynthesis. Here we report that a trait that is common in cyanobacteria extends the photochemical red-limit, with both photosystems using ~110 meV less energy. This ~45 nm upshift of the photochemically active chlorophylls may represent an extended red-limit relevant to stable environments. Studies of these unprecedented low-energy photosystems, with their farred photochemistry but largely red antenna, could solve key long-standing mechanistic and energetic questions in photosynthesis, some of which are relevant to the hunt for life on planets in other solar systems. Understanding the bioenergetics and resilience of these systems could provide design principles and feasibility assessments for projects aimed at improving photosynthetic efficiency by engineering long-wavelength photosystems. One Sentence Summary: Long-wavelength chlorophylls in key positions in Photosystem I and II reflect an extension of the photochemical red-limit for photosynthesis.