Cyanobacteria are well known as oxygenic photosynthetic organisms that harvest sunlight through chlorophyll. However, they also use another light-harvesting system: rhodopsins, light-driven proteins that capture solar energy.

In this study, we identified a new type of cyanobacterial rhodopsin, named Cyanorhodopsin II (CyR-II), through metagenomic and structural analyses. CyR-II functions as a light-driven outward proton pump, expelling protons (H⁺) from the cell upon light activation.

Interestingly, CyR-II consists of two distinct subtypes based on their absorption properties:

• YCyR-II, which absorbs yellow light (~570 nm) and is adapted to terrestrial soils.
• GCyR-II, which absorbs green light (~550 nm) and is found in marine biofilms.

Our analyses also suggest that CyR-II spread across cyanobacterial lineages via horizontal gene transfer (HGT).

While the diversification of chlorophyll-based systems is widely recognized as a key driver of cyanobacterial adaptation, this study highlights the importance of rhodopsins as an alternative light-harvesting strategy that has supported the expansion of cyanobacteria into diverse environments.

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