Clarification of photosynthesis mechanism in purple sulfur bacteria that live in highly saline and alkaline environments

On February 14, a research group led by Professor Kazutoshi Tani of the Center for Computational Sciences at the University of Tsukuba announced their research results showing that the structure of a protein complex, which plays an important role in photosynthesis performed by purple sulfur bacteria living in highly saline and alkaline environments, underlies their improved energy conversion efficiency. Such bacteria are expected to find applications in environmental protection, such as effectively utilizing solar energy and treating hydrogen sulfide-containing wastewater. The results were published in the international journal Nature Communications on February 6.

Unlike photosynthesis by plants, photosynthesis by purple sulfur bacteria, which are adapted to habitats in extreme environments, does not produce oxygen. Instead, these bacteria use hydrogen sulfide to efficiently convert solar energy into chemical energy (redox potential difference). It is known that these bacteria have evolved uniquely with an emphasis on efficiency to perform photosynthesis stably, even in extreme environments. In particular, the purple sulfur bacterium Halorhodospira (Hlr.) halophila is believed to perform this function through the strong association and integration between a protein complex specialized for light-harvesting (LH2) and a protein complex responsible for light-harvesting and energy conversion (LH1-RC). However, their interaction is weak, and the mechanism of forming the LH1-LH2 co-complex remained unknown.

In this study, the research group analyzed the protein complex from Hlr. halophila, a purple sulfur bacterium isolated from an extreme environment in a hypersaline lake in the Libyan Desert, whose total salinity exceeded 35% at pH 10.7. They observed the sample solution containing LH2 and LH1-RC from Hlr. halophila at the amino acid level and found that the LH1-LH2 and LH1-RC complexes were formed. The smallest unit of the LH1 structure consisted of an unusual polypeptide chain, and the LH1 structure encircled the LH2 subunit or reaction center (RC). The light energy transfer efficiency of the LH1-LH2 complex was almost 100%, indicating that this protein complex structure enhances the energy conversion capacity. The structural features of the LH1-LH2 co-complex may have allowed the bacterium to adapt to physiologically challenging environments.

These findings suggest that the minimal LH2 complex transfers excitation energy as much as possible to the LH1-RC core complex, effectively harvesting light. Tani said, "Contrary to conventional wisdom, the coexistence of the newly discovered LH1-LH2 complex in addition to LH1-RC in Hlr. halophila was demonstrated, surprising me with the cleverness of nature. Moving forward, I hope to continue observing interesting natural phenomena that positively defy the common wisdom."

Journal Information
Publication: Nature Communications
Title: A distinct double-ring LH1-LH2 photocomplex from an extremophilic phototroph
DOI: 10.1038/s41467-024-55811-9