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.
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 H2from 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.