Shedding light on photosynthesis at sea

Land plants mainly absorb red and blue light from the sun and use it for photosynthesis. However, only weak blue-green light reaches the ocean floor. Therefore, macroalgae growing in the ocean have developed a protein, a so-called photosynthetic antenna, that efficiently utilizes this blue-green light. The photosynthetic antenna of marine macroalgae is very similar to that of land plants but differs in the structure of the pigments bound to it. Land plants have two types of pigments bound to their photosynthetic antennae, namely carotenoids and chlorophylls. In the marine green macroalgaCodium fragile, the major carotenoids are substituted with siphonaxanthin while some chlorophyllamolecules are replaced by chlorophyllb分子。Siphonaxanthin因为rophyllbare known to contribute to increased absorption of green light and blue-green light, respectively, but the mechanism has not yet been fully understood.

Responding to this gap, a research team led by Associate Professor Ritsuko Fujii, from the Research Center for Artificial Photosynthesis (ReCAP) at Osaka Metropolitan University, and graduate student Soichiro Seki, from the Graduate School of Science at Osaka City University, used cryogenic electron microscopy to investigate the structures and binding environments of pigments bound to the photosynthetic antenna ofC. fragile. The results allow for the elucidation of the molecular mechanism by which blue-green light -- the only light available in deep seawater -- is efficiently utilized for photosynthesis. Their findings were published inBBA Advanceson November 11, 2022.

High-resolution analysis by cryogenic electron microscopy showed that siphonaxanthin inC. fragileis greatly distorted and forms hydrogen bonds with the surrounding protein at two locations. This structural feature is deemed a key factor in siphonaxanthin's ability to absorb green light. Additionally, the researchers successfully detected the difference between chlorophyllaand chlorophyllb, and they clarified several chlorophyll molecule substitution sites. When the substitution occurs, the adjacent region of chlorophyllbclusters becomes wider, enabling better absorption of blue-green light. In other words, the team was able to obtain information on the pigment coordinates, contributing to a better understanding of the mechanism of more efficient photosynthesis.

"I think increasing the utilization of photosynthesis simply by changing the pigment structure would be a cost-effective strategy," explained Professor Fujii. "Learning such survival strategies of organisms would lead to improved use of sunlight and the development of renewable energy sources for human beings."