Researchers working together at the Photosynthetic Antenna Research Center imagine ways to adapt plants’ mechanisms, in hopes of producing renewable fuels and other beneficial outcomes.
by Tony Fitzpatrick
When it comes to using light, most of Earth’s plants are goldbrickers. From exotics to garden variety types, plants use only a fraction of potential light available to them. In fact, if plants had eyes, they would be able to “see” only as far as we advanced humans see, wasting the vast potential energy sources way out in the red and ultraviolet solar spectra.
In spring 2009, a new energy consortium, based at Washington University, formed to bring together the world’s best and brightest photosynthesis researchers in an effort to change plants and other photosynthetic organisms from goldbrickers to gold diggers. Named the Photosynthetic Antenna Research Center (PARC), it is one of the U.S. Department of Energy’s (DOE) 46 multimillion-dollar Energy Frontier Research Centers (EFRCs), formed to do research on novel energy initiatives. Washington University received a $20 million research award for PARC, the largest award ever received on the Danforth Campus.
Directed by Robert Blankenship, PhD, the Lucille P. Markey Distinguished Professor of Arts & Sciences, PARC hosts 17 principal investigators (PIs), including five at Washington University, 10 others at institutions and laboratories nationwide, and two in the United Kingdom. Three PIs are located at Los Alamos, Oak Ridge and Sandia national laboratories and boast some of the most sophisticated imaging and computational technical expertise in the world. And since its founding, more than a dozen other affiliated scientists — with locations ranging from Australia to the Czech Republic and Israel — have joined PARC forces.
The focus of PARC is not so much on the big picture of photosynthesis — which transforms light, carbon dioxide and water into chemical energy in plants and some bacteria — as it is on the front-end of the process. The front-end involves a light-gathering apparatus known as the antenna. There are roughly a dozen different types of photosynthetic antenna systems, with attendant variations within types. Blankenship likens the antenna to a satellite dish collecting signals and focusing them to a central receiving site, transducing them into stored energy and eventually a series of chemical reactions that produce useful products, such as the starch and sugars in a potato, for instance. The antenna is a place of such intrigue because it holds the potential for making plants and photosynthetic bacteria much more dynamic and beneficial than anyone ever envisioned.
Antennas are composed of pigments called chlorophylls in plants and bacteriochlorin in algae and cyanobacteria. In most systems they are nestled within scaffolding that holds the pigments at fixed distances and orientations from each other. Proteins, they absorb light like bodies in a tanning salon. Chlorophylls a and b, which differ by just one chemical group, are found in plants. Chlorophyll d, which taps into longer wavelengths, is only found in certain cyanobacteria.
The three-pronged approach “gives us the freedom to change things in major ways to explore areas that Mother Nature never did. We want to see if we can come up with something more efficient than found in any natural system.” —Robert Blankenship, PhD
“About half of the energy in sunlight is outside the visible range,” says Blankenship, who has published numerous landmark papers on photosynthesis and photosynthetic organisms. “We are trying to incorporate new kinds of chlorophylls that can expand a plant’s range and thus get additional energy, but right now no plant can take advantage of that.”
PARC is exploring basic science research aimed at understanding the principles of the harvesting of light and funneling of energy as applied to three antenna systems: natural photosynthetic, biohybrid and bio-inspired.
The three-pronged approach “gives us the freedom to change things in major ways to explore areas that Mother Nature never did,” Blankenship adds. “We want to see if we can come up with something more efficient than found in any natural system.”
Payoffs, which won’t come overnight, could be huge and include breakthroughs in agriculture, solar energy and green chemical manufacturing.
Himadri B. Pakrasi, PhD, the George William and Irene Koechig Freiberg Professor of biology in Arts & Sciences and professor of energy in the School of Engineering & Applied Science, leads a group of 10 PIs, all studying different natural antenna systems. Pakrasi, who heads WUSTL’s International Center for Advanced Renewable Energy and Sustainability (I-CARES), says antenna sizes differ based on the amount of light available to an organism.
“One of the key things our group does is determine how Nature has designed different antennas; we test what happens when an antenna is modified and an organism is grown under different light regimes.” —Himadri Pakrasi, PhD
“When we see a green plant being green, we are seeing its antenna,” he says. “A sun plant under bright light will have a paler green color, whereas canopy plants, under shade, will be much darker green — they have more antenna pigments to capture the same amount of light to allow growth.
“One of the key things our group does is determine how Nature has designed different antennas; we test what happens when an antenna is modified and an organism is grown under different light regimes.”
For more than 20 years, Pakrasi has studied cyanobacteria, also known as blue-green algae. He has made landmark contributions to the understanding of how these organisms — whose antennas capture additional light than plants do — produce lipids, or fats, that are easily converted to biodiesel fuel, as well as hydrogen, a clean energy source. Cyanobacteria are producing these fuels daily in WUSTL laboratories, but perhaps more exciting is the potential of these organisms, with the proper genetic modifications, to churn out useful chemicals and drugs.
Deciphering the natural photosynthetic antenna systems is Pakrasi’s PARC goal, and the heart of his overriding goal of “taking cyanobacteria and making them photosynthetic cell factories that can produce fuel and useful chemicals,” he says.
Moreover, Pakrasi states that PARC’s strength is “the contributions of the many different groups to enable many energy possibilities.”
For example, two WUSTL researchers in Pakrasi’s group, working with Oak Ridge scientists, recently got valuable time on the Oak Ridge Laboratory’s neutron-scattering instrument, the largest equipment of its kind in the world. They used the instrument to make measurements and detail changes in cell membranes around the antenna after modifications to the natural antenna system. “This kind of collaboration is necessary for big science to make big changes,” Pakrasi says.
On the biohybrid front, Blankenship and PARC PI Pratim Biswas, PhD, the Lucy and Stanley Lopata Professor and chair of the Department of Energy, Environmental & Chemical Engineering, recently produced a solar cell. The cell — part natural, part artificial — combines antennas from well-understood photosynthetic bacteria Blankenship provided with a titanium dioxide substrate, or base, that Biswas created, using flame ionization. The two showed that light shone on the device is absorbed by the antenna complexes and makes its way to the titanium oxide, creating an electric current flow.
“The biohybrid complex is not something you’ll probably put on your roof, it’s not stable enough,” Blankenship says. “But the idea would be at some point to come up with a completely artificial system to replace the natural antenna component.”
“We don’t want to miss anything that happens in the natural system. That’s actually the purpose of the bio-inspired and biohybrid approaches. Even though the natural antennas are beautiful, maybe we’re not going to need some of the complexities of the natural system in an artificial one.” —Dewey Holten, PhD
Chemistry Professor Dewey Holten, PhD, PARC PI and associate director of the center, calls such molecular maneuvering “decorating.” He leads the PARC team building bio-inspired, or completely artificial, antenna systems. And, as in the instances of Blankenship and Pakrasi, many of his collaborators are scientists with whom he has worked for decades. One such collaborator, PARC PI Jonathan Lindsey, PhD, of North Carolina State University, can synthesize analogs of all known native chlorophylls that absorb light across the visible and near-infrared regions of the solar spectrum. These artificial chlorophylls, called chromophores, are attached to synthetic proteins called peptides by PARC PIs P. Leslie Dutton, PhD, and Christopher Moser, PhD, of the University of Pennsylvania. The results are fully artificial, bio-inspired antennas, also referred to as maquettes. A change of tactics allows production of a type of biohybrid antenna in which synthetic chlorophylls made by Lindsey are attached to the amino acid cysteine incorporated in mutants of the natural antenna provided by fellow PARC PI Neil Hunter, PhD, of Sheffield University, UK. The attachment chemistry and components assembly to make biohybrid antennas are accomplished by PARC collaborators Paul Loach, PhD, and Pamela Parkes-Loach, PhD, of Northwestern University. Holten and PARC PI David Bocian, PhD, of the University of California, Riverside, characterized the properties of all these new antenna systems, in Holten’s case through sophisticated laser spectroscopy that he and collaborator-spouse Christine Kirmaier, PhD, run in their Louderman Laboratory.
“We’re looking to understand their properties so that we can design molecules to provide whatever characteristics we want,” Holten says. “One idea is to make compact artificial antenna systems, decorate them with molecules absorbing different wavelengths of light, and deliver the energy efficiently to a cellular or surface site for energy processing.
“We don’t want to miss anything that happens in the natural system,” he continues. “That’s actually the purpose of the bio-inspired and biohybrid approaches. Even though the natural antennas are beautiful, maybe we’re not going to need some of the complexities of the natural system in an artificial one.”
Publications have been coming out in all three areas of PARC’s research thrust, reflecting well on the large-scale collaborative vision of the EFRCs program.
“One of the things these centers do is enable work that couldn’t be done in an individual lab or even a small research group,” Holten says. “We’re doing things now we never even dreamed of doing before.”
Tony Fitzpatrick is a freelance writer based in St. Louis.
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