AgBioResearch

Blue-green algae may look unappealing in nature, settling on the surface of swamps and other bodies of water. But it could be one of the catalysts for developing the bioeconomy of the 21st century, including renewable energy sources that fuel the future. The algae, also known as cyanobacteria, use photosynthesis to convert light energy from the sun into the chemical energy
needed for growth and function. Scientists at Michigan State University (MSU) are learning more about the photosynthetic process from these abundant organisms — with an eye on efficiency.

Cheryl Kerfeld, the John A. Hannah Distinguished Professor of Structural Bioengineering and an AgBioResearch scientist, and her team of researchers have been studying a process by which cyanobacteria protect themselves from too much light.

“Without a molecular mechanism to deal with excess light energy, the cyanobacteria would, essentially, fry,” Kerfeld said. “They have to have a way to take in only the energy they need without overloading their systems.”

Cyanobacteria have antennae that are used to capture light energy. A protein in the cyanobacteria, known as the orange carotenoid protein (OCP), changes from orange to a protective reddish color when a carotenoid pigment attached to the protein detects too much light. In this activated state, the protein plugs into the cyanobacteria’s antennae, helping the organism dissipate excess light energy as heat. The molecular structure of the activated state and the cause of the protein’s color change were previously unknown.

A paper published in Science, written by Kerfeld’s group with lead author and research assistant Ryan Leverenz, details the structure of the activated form of the OCP and reveals an unexpected movement of the carotenoid. Kerfeld’s team — which includes Leverenz and co-lead author Markus Sutter, among other MSU researchers, the Kirilovsky Lab in France and scientists from Berkeley National Laboratory — is the first to see this behavior. Previously, carotenoids were thought to be static and fixed to a protein scaffold.

“It’s known that carotenoids have photoprotective functions, but the OCP is unique in that the carotenoid is also used as part of a switch that’s turned on and off by light,” Leverenz said. “Now that we can see the switched ‘on’ form of the protein structurally, which we were able to see in the lab, we’re learning more about how it binds to the antennae of the cyanobacteria and how it helps dissipate energy after it binds. Once we fully learn how this process is performed in nature, we hope to apply the principles to design new artificial photosynthetic systems.”

Researchers have noticed that, though cyanobacteria possess this unique mechanism for dissipating excess light energy, they don’t always perform the task in the most efficient way. Harnessing the energy lost as heat will be important in the development of artificial photosynthetic systems as a reliable energy source.

“Our group spends some of our time in California and some in Michigan. With the drought in California, people are very mindful of dripping faucets and their overall water consumption,” Kerfeld said. “In Michigan, there’s plenty of water, so people don’t think about it much at all. It’s kind of like that with cyanobacteria. They’re so used to having so much sun that they don’t bother to be careful about their photoprotective process — they turn it on and forget to turn it off. We want to help cyanobacteria be smarter about photoprotection and not waste so much of that energy as heat. This is important for modifying cyanobacteria to be microbial cell factories.”

Cyanobacteria are also being tested for viability as a chemical precursor for plastics, in addition to fuels. Nearly all precursors in the chemical industry are currently petroleum-based, so sustainability and environmental impacts are concerns. The U.S. Department of Energy has set a goal of generating 25 percent of industrial chemicals from biological processes by 2025. Improving the efficiency of photosynthesis in cyanobacteria can increase yields and address questions surrounding its use as a practical solution to energy challenges.

“There is great potential in using cyanobacteria as a way to understand the complex process of photosynthesis,” Kerfeld said. “We have to better understand how photosynthesis is carried out in nature and how that translates to realworld applications, from the perspectives of biology, physics and chemistry. So we need to continue to collaborate as multidisciplinary teams to push this research forward. Our team is really excited about what we’ve seen so far, but we know there’s a lot of work still to be done.”

Funding for the project has been provided by the endowment of Kerfeld’s position, the MSU-DOE Plant Research Lab and MSU AgBioResearch, with additional funding from the Kirilovsky Lab in France and the Berkeley National Laboratory.