From “The MIT Technology Review”
6.8.23
Boyce Upholt
A Silicon Valley startup wants to supercharge trees to soak up more carbon and cool the climate. Is this the great climate solution or a whole lot of hype?
Credit: ANA MIMINOSHVILI.
Fifty-three million years ago, the Earth was much warmer than it is today. Even the Arctic Ocean was a balmy 50 °F—an almost-tropical environment that looked something like Florida, complete with swaying palm trees and roving crocodiles.
Then the world seemed to pivot. The amount of carbon in the atmosphere plummeted, and things began to cool toward today’s “icehouse” conditions, meaning that glaciers can persist well beyond the poles.
Icehouse Earth
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What caused the change was, for decades, unclear. Eventually, scientists drilling into Arctic mud discovered a potential clue: a layer of fossilized freshwater ferns up to 20 meters thick. The site suggested that the Arctic Ocean may have been covered for a time in vast mats of small-leaved aquatic Azolla ferns. Azollas are among the fastest-growing plants on the planet, and the scientists theorized that if such ferns coated the ocean, they could have consumed huge quantities of carbon, helping scrub the atmosphere of greenhouse gasses and thereby cooling the planet.
Patrick Mellor, paleobiologist and chief technology officer of the biotech startup Living Carbon, sees a lesson in the story about these diminutive ferns: photosynthesis can save the world. Certain fluke conditions seem to have helped the Azollas along, though. The arrangement of continental plates at the time meant the Arctic Ocean was mostly enclosed, like a massive lake, which allowed a thin layer of fresh river water to collect atop it, creating the kind of conditions the ferns needed. And crucially, when each generation of ferns died, they settled into saltier water that helped inhibit decay, keeping microbes from releasing the ferns’ stored carbon back into the atmosphere.
Mellor says we can’t wait millions of years for the right conditions to return. If we want plants to save the climate again, we have to prod them along. “How do we engineer an anthropogenic Azolla event?” he says. “That’s what I wanted to do.”
At Living Carbon, Mellor is trying to design trees that grow faster and grab more carbon than their natural peers, as well as trees that resist rot, keeping that carbon out of the atmosphere. In February, less than four years after he co-founded it, the company made headlines by planting its first “photosynthesis-enhanced” poplar trees in a strip of bottomland forests in Georgia.
This is a breakthrough, clearly: it’s the first forest in the United States that contains genetically engineered trees. But there’s still much we don’t know. How will these trees affect the rest of the forest? How far will their genes spread? And how good are they, really, at pulling more carbon from the atmosphere?
Living Carbon has already sold carbon credits for its new forest to individual consumers interested in paying to offset some of their own greenhouse gas emissions. They’re working with larger companies, to which they plan to deliver credits in the coming years. But academics who study forest health and tree photosynthesis question whether the trees will be able to absorb as much carbon as advertised.
Even Steve Strauss, a prominent tree geneticist at Oregon State University who briefly served on Living Carbon’s scientific advisory board and is conducting field trials for the company, told me in the days before the first planting that the trees might not grow as well as natural poplars. “I’m kind of a little conflicted,” he said, “that they’re going ahead with this—all the public relations and the financing—on something that we don’t know if it works.”
Roots of an idea
In photosynthesis, plants pull carbon dioxide out of the atmosphere and use the energy from sunlight to turn it into sugars. They burn some sugars for energy and use some to build more plant matter—a store of carbon.
A research group based at the University of Illinois-Urbana-Champaign supercharged this process, publishing their results in early 2019. They solved a problem presented by RuBisCO, an enzyme many plants use to grab atmospheric carbon. Sometimes the enzyme accidentally bonds with oxygen, a mistake that yields something akin to a toxin. As the plant processes this material, it must burn some of its sugars, thereby releasing carbon back to the sky. A quarter or more of the carbon absorbed by plants can be wasted through this process, known as photorespiration.
The researchers inserted genes into tobacco plants that helped them turn the toxin-like material into more sugar [Science (below)]. These genetically tweaked plants grew 25% larger than controls.
The breakthrough offered good news for the world’s natural landscapes: if this genetic pathway yields more productive crops, we’ll need less farmland, sparing forests and grasslands that otherwise would have to be cleared. As for the plants’ ability to remove atmospheric carbon over the long term, the new trick doesn’t help much. Each year, much of the carbon in a crop plant’s biomass gets returned to the atmosphere after it’s consumed, whether by microbes or fungi or human beings.
Still, the result caught the attention of Maddie Hall, a veteran of several Silicon Valley startups who was interested in launching her own carbon-capture venture. Hall reached out to Donald Ort, the biologist who’d led the project, and learned that the same tweaks might work in trees—which stay in the ground long enough to serve as a potential climate solution.
Late in 2019, Hall settled on the name for her startup: Living Carbon. Not long afterward, she met Mellor at a climate conference. Mellor was then serving as a fellow with the Foresight Institute, a think tank focused on ambitious future technologies, and had become interested in plants like Pycnandra acuminata. This tree, native to the South Pacific islands of New Caledonia, pulls huge quantities of nickel out of the soil. That’s likely a defense against insects, but as nickel has natural antifungal properties, the resulting wood is less prone to decay. Mellor figured if he could transfer the correct gene into more species, he could engineer his Azolla event.
When Mellor and Hall met, they realized their projects were complementary: put the genes together and you’d get a truly super tree, faster-growing and capable of more permanent carbon storage. Hall tapped various contacts in Silicon Valley to collect $15 million in seed money, and a company was born.
In some ways, Living Carbon’s goal was simple, at least when it came to photosynthesis: take known genetic pathways and place them in new species, a process that’s been conducted with plants for nearly 40 years. “There’s a lot of mystification of this stuff, but really it’s just a set of laboratory techniques,” Mellor says.
Since neither Mellor nor Hall had substantial experience with genetic transformation, they enlisted outside scientists to do some of the early work. The company focused on replicating Ort’s enhanced-photosynthesis pathway in trees, targeting two species: poplars, which are popular with researchers because of their well-studied genome, and loblolly pines, a common timber species. By 2020, the tweaked trees had been planted in a grow room, a converted recording studio in San Francisco. The enhanced poplars quickly showed results even more promising than Ort’s tobacco plants. In early 2022, Living Carbon’s team posted a paper [Forests (below)] claiming that the best-performing tree showed 53% more above-ground biomass than controls after five months.
Through the loophole
Plant genetics research can be a long scientific slog. What works in a greenhouse, where conditions can be carefully controlled, may not work as well in outdoor settings, where the amounts of light and nutrients a plant receives vary. The standard next step after a successful greenhouse result is a field trial, which allows scientists to observe how genetically engineered (GE) plants might fare outside without actually setting them fully loose.
US Department of Agriculture (USDA) regulations for GE field trials aim to minimize “gene drift,” in which the novel genes might spread into the wild. Permits require that biotech trees be planted far from species with which they could potentially reproduce, and in some cases the rules dictate that any flowers be removed. Researchers must check the field site after the study to ensure no trace of the GE plants remain.
Before planting trees in Georgia, Living Carbon launched its own field trials. The company hired Oregon State’s Strauss, who had given Living Carbon the poplar clone it had used in its gene transfer experiments. In the summer of 2021, Strauss planted the redesigned trees in a section of the university’s property in Oregon.
Strauss has been conducting such field trials for decades, often for commercial companies trying to create better timber technologies. It’s a process that requires patience, he says: most companies want to wait until a “half rotation,” or midway to harvest age, before determining whether a field trial’s results are promising enough to move forward with a commercial planting. Living Carbon’s trees may never be harvested, which makes setting a cutoff date difficult. But when we spoke in February, less than two years into the field trial and just before Living Carbon’s initial planting, Strauss said it was too early to determine whether the company’s trees would perform as they had in the greenhouse. “There could be a negative,” he said. “We don’t know.”
Strauss has been critical of the US regulatory requirements for field trials, which he sees as costly, a barrier that scares off many academics. The framework behind its rules emerged in the 1980s when, rather than wait on the slow grind of the legislative process, the Reagan administration adapted existing laws to fit new genetic technologies. For the USDA, the chosen tool was its broad authority over “plant pests,” a term meant to describe anything that might injure a plant—whether an overly hungry animal, a parasitic bacterium, or a weed that might outcompete a crop.
At the time, gene transfer in plants was almost entirely accomplished with the help of Agrobacterium tumefaciens. This microbe attacks plants by inserting its own genes, much like a virus. But scientists found they could convince the bacterium to deliver whatever snippets of code they desired. Since Agrobacterium itself is considered a plant pest, the USDA decided it had the authority to regulate the interstate movement and environmental release of any plant that had had its genes transformed by the microbe. This meant nearly comprehensive regulation of GE plants.
In 1987, just one year after the USDA established its policy, a team of Cornell researchers announced the successful use of what’s become known as a “gene gun”—or, less colorfully, “biolistics”—in which bits of DNA are literally blasted into a plant cell, carried by high-velocity particles. No plant pest was involved. This created a loophole in the system, a way to produce GE plants that the current laws did not cover.
Since then, more than 100 GE plants, mostly modified crop plants, have thus escaped the USDA’s regulatory scrutiny.
Agrobacterium remains a common method of gene transfer, and it’s how Living Carbon produced the trees discussed in its paper. But Mellor knew going to market with trees considered potential plant pests “would be a long and depressing path,” he says, one with tests and studies and pauses to collect public comment. “It would take years, and we just wouldn’t survive.”
Once Living Carbon saw that its trees had promise, it dove through the loophole, creating new versions of its enhanced trees via biolistics. In formal letters to the USDA the company explained what it was doing; the agency replied that, because the resulting trees had not been exposed to and did not contain genes from a plant pest, they were not subject to regulations.
Other federal agencies also have authority over biotechnology. The Environmental Protection Agency regulates biotech plants that produce their own pesticides, and the Food and Drug Administration examines anything humans might consume. Living Carbon’s trees do not fit into either of these categories, so they could be planted without any further formal studies.
A year after Living Carbon announced its greenhouse results—before the data from the field trial had any meaning, according to Strauss—the company sent a team to Georgia to plant the first batch of seedlings outside strictly controlled fields. Mellor indicated that this would double as one more study site, where the trees would be measured to estimate the rate of biomass accumulation. The company could make an effort to start soaking up carbon even as it was verifying the efficacy of its trees.
Out in the wild
Experiments with genetically modified trees have historically evoked a strong response from anti-GE activists. In 2001, around 800 specimens growing in Strauss’s test plots at Oregon State University were chopped down or otherwise mutilated.
In 2015, in response to the news that the biotech firm ArborGen had created a loblolly pine with “increased wood density,” protesters descended on the company’s South Carolina headquarters. (The company had taken advantage of the same loophole as Living Carbon; ArborGen has said the pine was never commercially planted.) But after the New York Times wrote about Living Carbon’s first planting in February, there were no notable protests.
One reason could be that the risk is far from clear-cut. Several forest ecologists I spoke to indicated that trees that grow substantially faster than other species could outcompete rivals, potentially making Living Carbon’s “super tree” a weed. None of these scientists, though, seemed particularly worried about that happening.
Fig. 1 Alternative photorespiratory pathways.
(A) Model of three alternative photorespiration pathway designs. AP1 (red) converts glycolate to glycerate using five genes from the E. coli glycolate pathway encoding the enzymes glycolate dehydrogenase, glyoxylate carboligase, and tartronic semialdehyde reductase. AP2 (dark blue) requires three introduced genes encoding glycolate oxidase, malate synthase, and catalase (to remove hydrogen peroxide generated by glycolate oxidase). AP3 (blue) relies on two introduced genes: Chlamydomonas reinhardtii glycolate dehydrogenase and Cucurbita maxima malate synthase. (B) qRT-PCR analysis of the two transgenes in AP3 and the target gene PLGG1 of the RNAi construct. Results for three independent transformation events are shown with (1, 5, and 8) and without (8, 9, and 10) PLGG1 RNAi. Error bars indicate SEM. * indicates statistical difference at P < 0.05 compared to WT based on one-way ANOVA. Actual P values are shown in supplementary data set 15. (C) Immunoblot analysis from whole leaves and isolated chloroplasts, including the insoluble membrane fraction, using custom antibodies raised against the indicated target genes, cytosolic marker actin, and chloroplast-specific marker platoglobulin 35 (PGL35). Five micrograms of protein was loaded per lane. Arrows indicate detected protein based on molecular weight. The kinetic properties of CrGDH, as well as numerous malate synthase enzymes, have been previously characterized (table S3) (17*).
*References
See the science paper for further instructive material with images.
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