Industrial fertilizers help feed billions of people every year, but they remain beyond the reach of many of the world’s poorest farmers. Now, researchers have engineered microbes that, when added to soil, make fertilizer on demand, producing plants that grow 1.5 times larger than crops not exposed to the bugs or other synthetic fertilizers. The advance, reported here this week at a meeting of the American Chemical Society, could help farmers in the poorest parts of the world increase their crop yields and combat chronic malnutrition.
A key component of fertilizer is nitrogen, an element essential for building everything from DNA to proteins. Nitrogen is all around us, comprising 80% of the air we breathe. But that nitrogen is inert, bound up in molecules that plants and people can’t access. Some microbes have evolved proteins called nitrogenases that can split apart nitrogen molecules in the air and weld that nitrogen to hydrogen to make ammonia and other compounds that plants can absorb to get their nitrogen.
The industrial process for making fertilizer, invented more than a century ago by a pair of German chemists—Fritz Haber and Carl Bosch—carries out that same molecular knitting. But the Haber-Bosch process, as it’s now known, necessitates high pressures and temperatures to work. It also requires a source of molecular hydrogen (H2)—typically methane—which is the chief component of natural gas. Methane itself isn’t terribly expensive. But the need to build massive chemical plants to convert methane and nitrogen into ammonia, as well as the massive infrastructure needed to distribute it, prevents many poor countries from easy access to fertilizer.
A few years ago, researchers led by Harvard University chemist Daniel Nocera devised what they call an artificial leaf that uses a semiconductor combined with two different catalysts to capture sunlight and use that harvested energy to split water molecules (H2O) into H2 and oxygen (O2). At the time, Nocera’s group focused on using the captured hydrogen as a chemical fuel, which can either be burned directly or run through a device called a fuel cell to produce electricity. But last year, Nocera reported that his team had engineered bacteria called Ralstonia eutropha to feed on the H2 and carbon dioxide (CO2) from the air and combine them to make hydrocarbon fuels. The next step, says Nocera, was to broaden the scope of their work by engineering another type of bacterium to take nitrogen out of the air to make fertilizer.
Nocera and his colleagues turned to a microbe called Xanthobacter autotrophicus, which naturally harbors a nitrogenase enzyme. But they still needed a way to provide the bugs with a source of H2 to make ammonia. So they genetically engineered Xanthobacter, giving them an enzyme called a hydrogenase, which allows them to feed on H2 to make a form of cellular energy called ATP. They then use that ATP, additional H2, and CO2 from the air to synthesize a type of bioplastic called polyhydroxybutyrate, or PHB, which they can store in their bodies.
This is where the microbes’ nitrogenase enzyme kicks in. The bacteria harvest H2 from their PHB store and use their nitrogenase to combine it with nitrogen from the air to make ammonia, the starting material for fertilizer. It doesn’t just work in the lab: Nocera reported yesterday at the meeting that when he and his colleagues put their engineered Xanthobacter in solution and used that solution to water radish crops, the vegetables grew 150% larger than controls not given either the bugs or other fertilizers.
Leif Hammarström, a chemist at Uppsala University in Sweden who also works on making fuels from solar energy, says he was impressed with the work. Making ammonia without using an industrial process “is a very challenging chemistry,” he says. “This is a good approach.” It may even be one that could help many of the world’s poor. Nocera says Harvard has licensed the intellectual property for the new technology to the Institute of Chemical Technology in Mumbai, India, which is working to scale up the technology for commercial use around the globe.