Bionic leaf breakthrough could revolutionise fertiliser production, but environmental questions remain

Harvard chemistry professor Daniel Nocera has already wowed the scientific world with breakthroughs in artificial photosynthesis – including the “artificial leaf”, a metallic wafer that produces hydrogen and oxygen from sunlight and water, and the “bionic leaf”, which uses a bio-engineered bacterium that consumes hydrogen and carbon-dioxide to produce liquid fuels.

This week he revealed another potential game changer in human replication and super-charging of natural chemistry: a different kind of “bionic leaf” that uses another bio-engineered bacteria to make fertiliser in the very ground where crops are grown.

Illustrating the success of his research team’s efforts with super-sized radishes grown using the bacteria (150% the size of control radishes), Nocera told an audience at the national meeting of the American Chemical Society in San Francisco last Monday that the most important aspect of his work was achieving at a “backyard” level what is currently done through large-scale industrial processes and costly distribution networks. “That’s what I’ve always been focused on,” he said. “The poor.”

Working in tandem with the artificial leaf, the modified Xanthobacter bacteria feeds on hydrogen and carbon dioxide to produce a hydrogen-rich bioplastic, which it then stores and consumes for energy while sucking in atmospheric nitrogen (N₂) to produce ammonia (NH₃) – the mainstay chemical fertiliser of the industrial farming revolution that has dramatically increased crop yields over the past century. The final stage of the fertilisation process, by which the ammonia is converted into a soluble form of nitrogen plants can then suck up, is performed by a naturally occurring “partner microbe” that the research team has yet to identify.

Ammonia’s capacity as a nitrogen-delivery mechanism to spur crop growth has made it one of the most commonly manufactured chemicals in the world, with global production exceeding 145 million metric tonnes in 2015, according to the US Geological Survey. About 80% of that production is used for fertiliser.

Commercial production of ammonia is based on reacting nitrogen and hydrogen under high temperatures and pressures. The main industrial procedure used is called the Haber-Bosch process – named after the German chemists Fritz Haber and Carl Bosch who developed the technique a century ago. Ammonia production now accounts for nearly 20% of energy consumed by the chemical and petrochemical sector and 1–2% of total global energy consumption.

“Sometimes when you’re doing science you just want to get to the top of mountain to prove it can be done.”

Nocera’s breakthrough in producing ammonia at the soil level therefore has obvious potential to reduce agriculture’s carbon footprint while maintaining, or even improving, the crop yields needed to feed a global population projected to grow from about 7.5 billion now to 9.7 billion in 2050 and 11.2 billion in 2100.

“I don’t think we’re doing anything different than a commercial fertiliser,” he said during his presentation. “Except that if you’re a poor country in Africa, you don’t have to convince somebody to build a Haber-Bosch plant and you don’t need all the money to build a distribution system. I don’t think sometimes my colleagues understand that we’re really up against an infrastructure issue.”

That said, there are reasons to be circumspect about heralding Nocera’s new “bionic leaf” as the precursor to a second “green revolution” that will boost crop yields in developing countries and end world hunger. For the all the success of the first green revolution in boosting crop yields, it has had a significant environmental downside, and any scientific fix that reinforces its “nitrogen-fixing” methodology may not be sustainable.

Nocera’s achievement, if it can be affordably scaled up, could be a much better way of fixing nitrogen from the atmosphere than the current industrial processes, agrees Trevor Garnett, director of technology development at the Plant Accelerator, part of the Australian Plant Phenomics Facility at the University of Adelaide. “However, it still fixes nitrogen from the atmosphere which is a big worry.”

Garnett is particularly interested in nitrogen use. While it is the fertiliser plants most need, and nitrogen-fixing has helped sustain a much greater global population than would otherwise be possible, Garnett has written of the environmental consequences of overstepping planetary boundaries. He notes that about half of all nitrogen in applied fertiliser is not taken up by plants but ends up polluting waterways and oceans, causing algal blooms and aquatic dead zones. Unused nitrogen can also, through microbial activity, return to the atmosphere as nitrous oxide (N₂O), an extremely potent greenhouse gas.

Garnett points to the 2009 paper “Planetary Boundaries: Exploring the Safe Operating Space for Humanity” (published in Ecology & Society) in which Johan Rockström and international colleagues argue that interference with the nitrogen cycle – removing nitrogen from the atmosphere and converting it to reactive nitrogen for human use – is one of three earth-system processes already compromised to the point of eroding system sustainability. The other two processes are climate change and biodiversity loss.

If Nocera’s breakthrough ultimately ends up perpetuating the problem of putting reactive nitrogen into the biosphere, Garnett says, “that scares me”.

Nocera, for his part, did admit that his fertilising bionic leaf was still a long way from being more than a scientific curiosity. For one thing, all the concerns around genetically modified organisms certainly apply to his “weird” bio-engineered bacteria. “There’s a bunch of societal issues and everything else we’ll have to deal with,” he said, “but sometimes when you’re doing science you just want to get to the top of mountain to prove it can be done.”