Robust light-harvesting properties upon low-light acclimation in purple bacteria

Abstract

Purple bacteria convert solar energy into biochemical energy with high quantum efficiency across diverse environments. Under low light, many species increase the number of antenna complexes and replace their primary light-harvesting complex 2 (LH2) with a blue-shifted variant, LH3. The structural basis of the blue shift and its influence on the dynamics of solar energy conversion have remained unclear. Here, we integrated cryogenic electron microscopy, ultrafast spectroscopy, and quantum dynamics simulations to compare LH2 and LH3 from Rhodoblastus acidophilus strain 7750. Our analyses revealed that hydrogen bonding dynamically tunes the transition energy, introducing a previously unreported excitation energy equilibrium between bacteriochlorophyll rings in LH3. This energy redistribution opened new inter-complex pathways, enabling 68% faster energy transport to maintain high conversion efficiency even with the larger antenna. Collectively, these results establish structural modifications as a tunable knob to optimize both absorption and transport for robust light harvesting under fluctuating conditions.