Cyanobacteria, also known as blue-green algae, utilize not only chlorophyll-based photosynthesis but also a light-driven protein called “rhodopsin” to harness solar energy. In this study, researchers identified a novel type of cyanobacterial rhodopsin, named Cyanorhodopsin-II (CyR-II), through metagenomic and structural analyses. CyR-II functions as a light-driven outward proton pump that expels protons (H⁺) from the cell upon light activation. Furthermore, CyR-II is divided into two subtypes based on light absorption properties: YCyR-II, which absorbs yellow light (570 nm), and GCyR-II, which absorbs green light (550 nm). These types are adapted to different environments such as terrestrial soils and marine biofilms, respectively. The study also suggests that these rhodopsins have spread among cyanobacterial lineages via horizontal gene transfer (HGT). While the role of chlorophyll diversification in cyanobacterial environmental adaptation has been widely recognized, this study highlights the importance of rhodopsins as an alternative light-harvesting strategy that may have contributed to their expansion into diverse habitats.

https://academic.oup.com/ismej/article/18/1/wrae175/7850928