The Thermal-Stable LH1–RC Complex of a Hot Spring Purple Bacterium Powers Photosynthesis with Extremely Low-Energy Near-Infrared Light

<i>Blastochloris (Blc.) tepida</i> is a hot spring purple nonsulfur phototrophic bacterium that contains bacteriochlorophyll (BChl) <i>b</i>. Here, we present a 2.21 Å cryo-EM structure of the thermostable light-harvesting 1-reaction center (LH1-RC) complex from <i>Blc. tepida</i>. The LH1 ring comprises 16 circularly arranged αβγ-subunits plus one αβ-subunit that surround the RC complex composed of C-, H-, L-, and M-subunits. In a comparative study, the <i>Blc. tepida</i> LH1-RC showed numerous electrostatic and hydrophobic interactions both within the LH1 complex itself and between the LH1 and the RC complexes that are absent from the LH1-RC complex of its mesophilic counterpart, <i>Blc. viridis</i>. These additional interactions result in a tightly packed LH1-RC architecture with a reduced accessible surface area per volume that enhances the thermal stability of the <i>Blc. tepida</i> complex and allows the light reactions of photosynthesis to proceed at hot spring temperatures. Moreover, based on high-resolution structural information combined with spectroscopic evidence, the unique photosynthetic property of the <i>Blc. tepida</i> LH1-RC─absorption of energy-poor near-infrared light beyond 1000 nm─can be attributed to strong hydrogen-bonding interactions between the C3-acetyl C═O of the LH1 BChl <i>b</i> and two LH1 α-Trp residues, structural rigidity of the LH1, and the enhanced exciton coupling of the LH1 BChls of this thermophile.