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article imageMaking fuel out of sunlight and a ‘bucket of bacteria’ Special

By Les Horvitz     Jul 15, 2015 in Science
Boston - Can scientists make a bionic leaf and why would they want to? As it turns out, green leaves aren’t terribly efficient at converting sunlight to energy.
That’s why Pam Silver and her colleagues Daniel Nocera and Joseph Torella of Harvard Medical School were convinced they could improve on nature. “We think we can do better than plants,” Silver told attendees at the Synthetic Biology: Engineering, Evolution & Design (SEED) conference in Boston, MA last month.
Plants convert sunlight into chemical energy by photosynthesis (from the Greek for light and synthesis). The energy is stored in carbohydrate molecules like sugars, which are further broken down into carbon dioxide and water; oxygen is released as a waste product. Even though photosynthesis works perfectly well for green plants and flowers, scientists believed that they could engineer the process so it could produce much more energy.
Why should photosynthesis be so inefficient? Blame evolution.
Photosynthesis developed at a time when carbon dioxide levels in the atmosphere were far higher than they are today. Five hundred million years ago during the Cambrian Period, CO2 levels hovered around 7,000 parts per million or ppm. Photosynthesis functions just as it did eons ago even as atmospheric conditions have changed dramatically: CO2 levels currently range from 280 to 400 ppm, still higher than they’ve been in centuries, but nothing comparable. (As a greenhouse gas, CO2 is considered one of the major culprits in climate change.) It was Nocera, an inorganic chemist, who came up with the idea of a bionic leaf. He wanted to “one up nature” by creating a leaf that could harvest and store energy in a novel way that mimics photosynthesis but with far greater efficiency than plants or bacteria. His bionic leaf is, in effect, a genetically engineered “interface between living organisms and chemistry” — a pairing of machine and microbe, but with “the best features of both.”
Optimally, the artificial ‘leaf’ is meant to function rather like a micro-factory which could turn sunlight into useful molecules like food, fuel, even pharmaceuticals. But the question the scientists were most interested in learning the answer to was whether sunlight could be converted directly into a liquid fuel, one that was abundant, cheap and reliable. The Harvard team began by seeing whether they could use their process to make hydrogen, which as a fuel, produces zero pollution and no greenhouse gases. They proved that it could be done, but hydrogen just hasn’t gained much traction as an alternative fuel. For one thing, hydrogen cars are expensive (although costs are coming down); for another, it’s expensive to transport and there only a few stations, mainly in California, where it’s possible to find any. So the scientists turned their attention to producing another type of alternative fuel — isopropanol, an alcohol molecule, which is the principal component of rubbing alcohol. In this case, they made use of a genetically modified soil bacteria (Ralstonia eutropha) that feeds on hydrogen and carbon dioxide. In February, the team succeeded in demonstrating that their process could in fact yield a burnable biofuel. Their findings were published in Proceedings of the National Academy of Sciences.
Isopropanol is already one of the world’s most widely used solvents, found in coatings and industrial products as well as in household and personal care products. Although it’s unlikely that an isopropanol-based biogasoline will soon replace petrochemicals and other fossil-based fuels, it does have the potential of becoming a source of what Silver calls “personalized energy” because it wouldn’t require access to the power grid. “Instead of having to buy and store fuel, you can have your bucket of bacteria in your backyard."
Such a bucket would be especially useful for people in developing countries where electricity and gasoline are in short supply.
The U.S. Government is taking the research seriously: Silver’s lab received funding from the Air Force Office of Scientific Research and the Office of Naval Research and the National Science Foundation.
Silver, Nocera and Torella have already proven that they can surpass the energy efficiency of photosynthesis, which is a mere one percent, aiming for five percent — what Silver calls “the magic number.” Nocera says that that’s not bad considering that they’ve only been working on this for a year and a half and nature has had a head start of 2.6 billion years.
More about bionic leaf, Synthetic biology, renewable fuel, Photosynthesis, Pamela Silver