A Yale-led research team has discovered a molecular “device” found in nature that harvests a particular sliver of the sunlight spectrum in order to convert it into chemical energy.
In a study led by Yale’s Gary Brudvig and Christopher Gisriel, and Donald Bryant of Pennsylvania State University, the researchers describe a helix-shaped nanotube structure that forms within photosynthetic organisms called cyanobacteria.
The discovery of this structure offers new insights into how nature collects and stores light energy in challenging conditions – something researchers seek to mimic for new solar technology and more resilient crops.
The study appears in the journal Science Advances. Other authors include researchers at Yale, Pennsylvania State University, City University of New York, and Freie University in Amsterdam.
According to their findings, the helical nanotubes harvest light photons from the far-red part of the light spectrum and deliver the photons for conversion into chemical energy during photosynthesis. These tiny, nanotube “devices” are deployed in low-light environments, where cyanobacteria compete with other cyanobacteria and diverse photosynthetic bacteria for every photon of sunlight they can find.
“Cyanobacteria are microscopic photosynthetic organisms that are masters of adaptation,” said Gisriel, a postdoctoral associate in Brudvig’s lab and the study’s first author. “They’re found in every habitat imaginable, from desert crusts and hyper-saline lakes to hot springs and dark caves.”
Researchers have found that in low-light conditions, certain species of cyanobacteria activate a gene cluster that launches the production of proteins known as far-red light phycobiliproteins. These phycobiliproteins assemble themselves into helical nanotubes, distinct from previously discovered, similar proteins that produce cylindrical structures. The cylinder-shaped structures collect photons from the visible light colors in the solar spectrum, such as yellow and orange, whereas the helical nanotubes collect photons from the invisible, far-red portion of the solar spectrum, the researchers say.
Although the component proteins are closely related evolutionarily, their components are structurally incompatible, which allows these cyanobacteria to assemble two complementary types of light harvesting devices at the same time – a clear advantage when light is the limiting resource for growth.
“Photosynthetic organisms such as plants, algae, and cyanobacteria are integral components of life on Earth,” said Brudvig, the Benjamin Silliman Professor of Chemistry in Yale’s Faculty of Arts and Sciences and director of the Energy Sciences Institute at Yale’s West Campus. “They are the entry point for nearly all the energy that supports life in our biosphere, and they exhibit a diversity of molecular machines used to convert light to chemical energy. The design principles used by nature provide blueprints to develop artificial processes for solar energy utilization.”
For the study, the researchers used two techniques to determine the far-red light-absorbing structure and function: cryo-electron microscopy and time-resolved absorption spectroscopy.
Cryo-electron microscopy, which flash-freezes biomolecular samples and pelts them with electrons to produce images of molecules, allowed the researchers to see the helical shape of the nanotubes and how they were assembled. Time-resolved absorption spectroscopy, which looks at the way a material’s absorbance changes after it is exposed to light, allowed the researchers to track how quickly energy is transferred through the nanotubes – and the route the energy takes.
“Seeing in detail the helical structure of these unique phycobiliproteins was completely unexpected,” Gisriel said.
Bryant is the study’s senior author. Co-authors of the study were David Flesher of the Department of Molecular Biophysics and Biochemistry at Yale; Roberta Croce and Eduard Elias of Freie University, Amsterdam; Gaozhon Shen and Nathan Soulier of Pennsylvania State University; and Marilyn R. Gunner of City College of New York.
Gisriel, Brudvig, Croce, and Bryant are co-corresponding authors of the study.
The U.S. Department of Energy’s Office of Basic Energy Sciences, the National Science Foundation, the National Institutes of Health, and the Netherlands Organization of Scientific Research funded the research.