There will be many ways to make ammonia in the future and, regardless of breakthroughs in chemical catalysts and engineering design, genetically modified organisms will play an increasingly important role.
At this week’s American Chemical Society meeting, Daniel Nocera from Harvard University introduced his new ammonia synthesis technology. It builds on his “artificial leaf” that produces and stores hydrogen using power from sunlight. Nocera’s latest innovation is to couple this system with a microbe that naturally contains nitrogenase, the enzyme that fixes atmospheric nitrogen into ammonia.
The end result – a robust population of nitrogen fertilizer-emitting microbes – can be delivered to the soil simply by watering the plants.
Nitrogenase is the enzyme that has, for millennia, created most of the available nitrogen on this planet by ‘fixing’ it as ammonia. The enzyme is found in microbes in the soil that live in a symbiotic relationship with plants: in return for food, the microbes supply the plant’s roots with nitrogen. But not all plants are able to have this relationship: only legumes, like lentils, peas, soy, clover.
(This is why crop rotation is one of the most important and overlooked ammonia production technologies available: legumes naturally replenish the nitrogen that other crops, like corn or rice or wheat, suck out of the soil. Monoculture and the homogeneity of the modern diet preclude this system of nutrient regeneration.)
While I’ve written about a lot of industrial synthesis technologies in development, GMO technologies are making significant progress as well. We can now, for example, increase ammonia production by doubling nitrogen fixation in soybeans, or decrease consumption by improving nitrogen use efficiency in rice varieties.
Nocera’s GMO is not the crop but the nitrogenase-containing microbe Xanthobacter autotrophicus, which is engineered to be able to produce ammonia without needing the symbiotic relationship with the plant. Each microbe has its own biomass fuel store, generated from artificial photosynthesis, so it needs no food from the plant: its only purpose is ammonia production.
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 … 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.”
American Chemical Society press release, A ‘bionic leaf’ could help feed the world, 04/03/2017
Monday’s press conference at the ACS meeting quickly generate broad coverage. Science Magazine reported that “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.”
And in Forbes Magazine:
He envisions this technology being used in developing nations that can’t or don’t want to invest in chemical plants and distribution systems for ammonia … Nocera says the system is self-sustaining. Even the catalysts on his artificial leaf will heal themselves as they get damaged over time. And each bacterium can run through the nitrogen to ammonia cycle about 8 trillion times before it dies.
Forbes, Bionic Leaf Makes Fertilizer From Sunlight And Air, 04/05/2017
Nocera’s full abstract for the ACS presentation follows and, beneath it, the video of his press conference which includes an interesting question and answer session.
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.
ACS abstract, Sustainable solar-to-fuels and solar-to-fertilizer production, 04/03/2017