image: The radishes on the right were grown with the help of a bionic leaf that produces fertilizer with bacteria, sunlight, water and air. view more
Credit: Nocera lab, Harvard University
SAN FRANCISCO, April 3, 2017 -- In the second half of the 20th century, the mass use of fertilizer was part of an agricultural boom called the "green revolution" that was largely credited with averting a global food crisis. Now, the challenge of feeding the world looms again as the population continues to balloon. To help spur the next agricultural revolution, researchers have invented a "bionic" leaf that uses bacteria, sunlight, water and air to make fertilizer in the very soil where crops are grown.
The team will present the work today at the 253rd National Meeting & Exposition of the American Chemical Society (ACS). ACS, the world's largest scientific society, is holding the meeting here through Thursday. It features more than 14,000 presentations on a wide range of science topics.
"When you have a large centralized process and a massive infrastructure, you can easily make and deliver fertilizer," Daniel Nocera, Ph.D., says. "But if I said that now you've got to do it in a village in India onsite with dirty water -- forget it. Poorer countries in the emerging world don't always have the resources to do this. We should be thinking of a distributed system because that's where it's really needed."
The first "green revolution" in the 1960s saw the increased use of fertilizer on new varieties of rice and wheat, which helped double agricultural production. Although the transformation resulted in some serious environmental damage, it potentially saved millions of lives, particularly in Asia, according to the United Nations (U.N.) Food and Agriculture Organization. But the world's population continues to grow and is expected to swell by more than 2 billion people by 2050, with much of this growth occurring in some of the poorest countries, according to the U.N. Providing food for everyone will require a multi-pronged approach, but experts generally agree that one of the tactics will have to involve boosting crop yields to avoid clearing even more land for farming.
To contribute to the next green revolution, Nocera, who is at Harvard University, is building on the work he's most famous for -- the artificial leaf -- to make fertilizer. The artificial leaf is a device that, when exposed to sunlight, mimics a natural leaf by splitting water into hydrogen and oxygen. This led to the development of a bionic leaf that pairs the water-splitting catalyst with the bacteria Ralstonia eutropha, which consumes hydrogen and takes carbon dioxide out of the air to make liquid fuel. Last June, Nocera's team reported switching the device's nickel-molybdenum-zinc catalyst, which was poisonous to the microbes, with a bacteria-friendly alloy of cobalt and phosphorus. The new system provided biomass and liquid fuel yields that greatly exceeded that from natural photosynthesis.
"The fuels were just the first step," Nocera says. "Getting to that point showed that you can have a renewable chemical synthesis platform. Now we are demonstrating the generality of it by having another type of bacteria take nitrogen out of the atmosphere to make fertilizer."
For this application, Nocera's team has designed a system in which Xanthobacter bacteria fix hydrogen from the artificial leaf and carbon dioxide from the atmosphere to make a bioplastic that the bacteria store inside themselves as fuel.
"I can then put the bug in the soil because it has already used the sunlight to make the bioplastic," Nocera says. "Then the bug pulls nitrogen from the air and uses the bioplastic, which is basically stored hydrogen, to drive the fixation cycle to make ammonia for fertilizing crops."
Nocera's lab has analyzed the amount of ammonia the system produces. But the real proof is in the radishes. The researchers have used their approach to grow five crop cycles. The vegetables receiving the bionic-leaf-derived fertilizer weigh 150 percent more than the control crops. The next step, Nocera says, is to boost throughput so that one day, farmers in India or sub-Saharan Africa can produce their own fertilizer.
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A press conference on this topic will be held Monday, April 3, at 10:30 a.m. Pacific time in the Moscone Center. Reporters may check-in at the press center, South Building, Foyer, or watch live on YouTube http://bit.ly/ACSLive_SanFrancisco. To ask questions online, sign in with a Google account.
Nocera acknowledges support from private funders.
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Title
Sustainable solar-to-fuels and solar-to-fertilizer production
Abstract
Hybrid inorganic | biological constructs have been created to use sunlight, air and water to accomplish carbon fixation and nitrogen fixation thus enabling distributed and renewable fuels and fertilizer generation. The carbon fixation cycle is achieved by interfacing the oxygen evolving and hydrogen evolving catalysts of the artificial leaf with an engineered bioorganism. Using the tools of synthetic biology, a bio-engineered bacterium has been developed to convert carbon dioxide, along with the hydrogen produced from the artificial leaf, into biomass and liquid fuels, thus closing an entire artificial photosynthetic cycle. This hybrid microbial | artificial leaf system scrubs 180 grams of CO2 from air, equivalent to 230,000 liters of air per 1 kWh of electricity. This hybrid device, called the bionic leaf, operates at unprecedented solar-to-biomass (10.7%) and solar-to-liquid fuels (6.2%) yields, greatly exceeding the 1% yield of natural photosynthesis. Extending our approach, we have discovered a renewable and distributed synthesis of ammonia at ambient conditions by coupling solar-based water splitting to a nitrogen fixing bioorganism in a single reactor. Nitrogen is fixed to ammonia by using the hydrogen from the artificial leaf to power a nitrogenase installed in the bioorganism. The ammonia produced by the nitrogenase can be diverted from biomass formation to an extracellular product with the addition of an inhibitor. The nitrogen reduction reaction proceeds at a low \ driving force (~ 0.16 V) with a turnover number (TON) of 8 × 109 per cell and operates at 15 to 23% of the theoretical yield without the use of any sacrificial chemical reagents and carbon feedstock (which is provided by CO2 from air). This approach can be powered by distributed renewable electricity, enabling the sustainable production of nitrogen fertilizer.