A hybrid method for producing ammonia using solar or wind power could be the long-sought tool for maintaining global food supplies, without disrupting the climate.
Along with sunlight and water, nitrogen and phosphorus are the essential ingredients for plant growth. The atmosphere has plenty of nitrogen, but it is in a form plants can't use directly. For millions of years, some plants got their nitrogen from symbiotic relationships with bacteria or archaea that are capable of transforming nitrogen into useable forms. The rest of the terrestrial biosphere got along by re-using the nitrogen from these symbioses until it was turned back into molecular nitrogen.
For an industrial society, natural production was insufficient, so we invented the Haber-Bosch process, which turns molecular nitrogen (N2) into ammonia (NH3) for fertilizer and industrial uses. The quadrupling of agricultural productivity that this has made possible has been essential to keeping the world fed. Unfortunately, the Haber-Bosch process relies on natural gas and emits a lot of carbon dioxide, more than 1 percent of the world's total greenhouse gas emissions.
Nocera and Silver split water electrically to produce hydrogen and oxygen, a well-established process. They then fed the hydrogen to Xanthobacter autotrophicus, a nitrogen-fixing bacteria originally found in sludge from a German pool, which combined it with atmospheric nitrogen to make ammonia. By adding a chemical that inhibits the formation of certain molecules, the authors reported in the Proceedings of the National Academy of Sciences that they were able to prevent the ammonia getting incorporated into biomass. Instead, it was left free to be collected and used where needed.
The electricity to split the water could come from any source, but if powered by solar or wind, the result is a sustainable source of ammonia with few negative consequences for the environment.
X. autotophicus can also be a potent fertilizer when put directly in contact with the roots of plants. The authors cultured the bacteria by feeding it nitrogen, carbon dioxide, and electrically generated hydrogen and then applied it to the roots of radishes grown in a greenhouse. They achieved a 130 percent increase in the size of the edible radishes compared to unfertilized controls, and a 100 percent increase in total plant size.
Nocera and Silver have teamed up before to supercharge the chemical reactions that power the planet. In 2015, they announced a “bionic leaf” that can convert sunlight and air into isopropanol, a solvent and transportation fuel. As in this case, that involved combining inorganic chemical reactions with the bacteria, although the species used was different.