And, well, it worked remarkably well. The plants carrying all the genes for the McG cycle weighed two to three times as much as control plants that only had some of the genes. They had more leaves, the leaves themselves were larger, and the plants produced more seeds. In a variety of growing conditions, the plants with an intact McG cycle incorporated more carbon, and they did so without increasing their water uptake.
Having a two-carbon output also worked as expected. By feeding the plants radioactive bicarbonate, they were able to trace the carbon showing up in the expected molecules. And imaging confirmed that the plants were making so many lipids that their cells formed internal pockets containing nothing but fatty materials. Triglyceride levels increased by factors of 100 or more.
So, by a variety of measures, the plants actually did better with an extra pathway for fixing carbon. There are a number of cautions, though. For starters, it’s not clear whether what we’re learning using a small weed will also apply to larger plants or crops, or really anything much beyond Arabidopsis at the moment. It could be that having excess globs of fat floating around the cell has consequences for something like a tree. Plants grown in a lab also tend to be provided with a nutrient-rich soil, and it’s not clear whether all of this would apply to a range of real-world conditions.
Finally, we can’t say whether all the excess carbon these plants are sucking in from the atmosphere would end up being sequestered in any useful sense. It could be that all the fat would just get oxidized as soon as the plant dies. That said, there are a lot of approaches to making biofuel that rely on modifying the fats found in plants or algae. It’s possible that this can eventually help make biofuels efficient so they actually have a net positive effect on the climate.
Regardless of practical impacts, however, it’s pretty amazing that we’ve now reached the point where we can fundamentally rewire a bit of metabolism that has been in operation for billions of years without completely messing up plants.
Plant cells are surrounded by an intricately structured protective coat called the cell wall. It’s built of cellulose microfibrils intertwined with polysaccharides like hemicellulose or pectin. We have known what plant cells look like without their walls, and we know what they look like when the walls are fully assembled, but we’ve never seen the wall-building process in action. “We knew the starting point and the finishing point, but had no idea what happens in between,” says Eric Lam, a plant biologist at Rutgers University. He’s a co-author of the study that caught wall-building plant cells in action for the first time. And once we saw how the cell wall building worked, it looked nothing like how we drew that in biology handbooks.
Camera-shy builders
Plant cells without walls, known as protoplasts, are very fragile, and it has been difficult to keep them alive under a microscope for the several hours needed for them to build walls. Plant cells are also very light-sensitive, and most microscopy techniques require pointing a strong light source at them to get good imagery.
Then there was the issue of tracking their progress. “Cellulose is not fluorescent, so you can’t see it with traditional microscopy,” says Shishir Chundawat, a biologist at Rutgers. “That was one of the biggest issues in the past.” The only way you can see it is if you attach a fluorescent marker to it. Unfortunately, the markers typically used to label cellulose were either bound to other compounds or were toxic to the plant cells. Given their fragility and light sensitivity, the cells simply couldn’t survive very long with toxic markers as well.
A few years back, the Internet was abuzz with the idea of vertical farms running down the sides of urban towers, with the idea that growing crops where they’re actually consumed could eliminate the carbon emissions involved with shipping plant products long distances. But lifecycle analysis of those systems, which require a lot of infrastructure and energy, suggest they’d have a hard time doing better than more traditional agriculture.
But those systems represent only a small fraction of urban agriculture as it’s practiced. Most urban farming is a mix of local cooperative gardens and small-scale farms located within cities. And a lot less is known about the carbon footprint of this sort of farming. Now, a large international collaboration has worked with a number of these farms to get a handle on their emissions in order to compare those to large-scale agriculture.
The results suggest it’s possible that urban farming can have a lower impact. But it requires choosing the right crops and a long-term commitment to sustainability.
Tracking crops
Figuring out the carbon footprint of urban farms is a challenge, because it involves tracking all the inputs, from infrastructure to fertilizers, as well as the productivity of the farm. A lot of the urban farms, however, are nonprofits, cooperatives, and/or staffed primarily by volunteers, so detailed reporting can be a challenge. To get around this, the researchers worked with a lot of individual farms in France, Germany, Poland, the UK, and US in order to get accurate accounts of materials and practices.
Data from large-scale agriculture for comparison is widely available, and it includes factors like transport of the products to consumers. The researchers used data from the same countries as the urban farms.
On average, the results aren’t good for urban agriculture. An average serving from an urban farm was associated with 0.42 kg of carbon dioxide equivalents. By contrast, traditional produce resulted in emissions of about 0.07 kg per serving—six times less.
But that average obscures a lot of nuance. Of the 73 urban farms studied, 17 outperformed traditional agriculture by this measure. And, if the single highest-emitting farm was excluded from the analysis, the median of the urban farms ended up right around that 0.7 kg per serving.
All of this suggests the details of urban farming practices make a big difference. One thing that matters is the crop. Tomatoes tend to be fairly resource-intensive to grow and need to be shipped quickly in order to be consumed while ripe. Here, urban farms came in at 0.17 kg of carbon per serving, while conventional farming emits 0.27 kg/serving.
Difference-makers
One clear thing was that the intentions of those running the farms didn’t matter much. Organizations that had a mission of reducing environmental impact, or had taken steps like installing solar panels, were no better off at keeping their emissions low.
The researchers note two practical reasons for the differences they saw. One is infrastructure, which is the single largest source of carbon emissions at small sites. These include things like buildings, raised beds, and compost handling. The best sites the researchers saw did a lot of upcycling of things like construction waste into structures like the surrounds for raised beds.
Infrastructure in urban sites is also a challenge because of the often intense pressure on land, which can mean gardens have to relocate. This can shorten the lifetime of infrastructure and increase its environmental impact.
Another major factor was the use of urban waste streams for the consumables involved with farming. Composting from urban waste essentially eliminated fertilizer use (it was only 5 percent of the rate of conventional farming). Here, practices matter a great deal, as some composting techniques allow the material to become oxygen-free, which results in the anaerobic production of methane. Rainwater use also made a difference; in one case, the carbon impact of water treatment and distribution accounted for over two-thirds of an urban farm’s emissions.
These suggest that careful planning could make urban farms effective at avoiding some of the carbon emissions of conventional agriculture. This would involve figuring out best practices for infrastructure and consumables, as well as targeting crops that can have high carbon emissions when grown on conventional farms.
But any negatives are softened by a couple of additional considerations. One is that even the worst-performing produce seen in this analysis is far better in terms of carbon emissions than eating meat. The researchers also point out that many of the cooperative gardens provide a lot of social functions—things like after-school programs or informal classes—that can be difficult to put an emissions price on. Maximizing these could definitely boost the societal value of the operations, even if it doesn’t have a clear impact on the environment.
