RIVERSIDE, CALIFORNIA—For the first astronauts to visit Mars, what to eat on their 3-year mission will be one of the most critical questions. It’s not just a matter of taste. According to one recent estimate, a crew of six would require an estimated 10,000 kilograms of food for the trip. NASA—which plans to send people to Mars within 2 decades—could stuff a spacecraft with prepackaged meals and launch additional supplies to the Red Planet in advance for the voyage home. But even that wouldn’t completely solve the problem.
Micronutrients, including many vitamins, break down over months and will need to be synthesized en route. And food isn’t just a source of calories, says Jennifer Fogarty, chief science officer at the Translational Research Institute for Space Health at Baylor College of Medicine. Taste, texture, freshness, and other factors all play major roles in maintaining our well-being. Simple survival “is not the goal,” Fogarty says. Today’s preserved food system, she concludes, “is completely inadequate for a Mars mission.”
Robert Jinkerson, a chemical engineer at the University of California (UC), Riverside, thinks the answer is for astronauts to grow their own on-board garden—in the dark, with plant growth fueled by artificial nutrients rather than sunlight. It won’t be easy; after all, plants evolved for hundreds of millions of years to extract energy from sunlight. But Jinkerson believes it can be done by reawakening metabolic pathways plants already possess—the same ones that power the germination of seeds buried in the ground and then shut off once a seedling’s leaves start to reach for the Sun. In his vision of the future, electricity from solar panels could transform water and carbon dioxide (CO2) exhaled by a spacecraft’s crew into simple, energy-rich hydrocarbons that genetically modified plants could use to grow—even in the darkness of space or the dim light on Mars, which receives less than half as much sunlight as Earth.
His team has already shown that modified plants can survive, if not yet thrive, on a light-free regimen. If the UC Riverside researchers can get them to flourish, Jinkerson believes they’ll be on a path to not only feeding astronauts, but also growing a wide variety of crops on Earth without land and abundant sunshine. “It could be implemented in places like the South Pole, in places where agriculture is not possible,” says Jinkerson, whose team has already won two rounds of a NASA competition.
Others have tried to wean plants from light, and there’s a reason why decades of attempts haven’t worked. “They need light for everything,” says Sue Rhee, a plant biologist at Carnegie Science. Light plays a role not just in photosynthesis, but also in germination, growth, flowering, and the ripening of fruit. But she thinks Jinkerson’s vision is “bold” and worth trying, because it involves a combination of genetic tweaks and added nutrients that haven’t been tried before. Patrick Shih, a plant molecular biologist at UC Berkeley, agrees. “They are trying to rethink how we do agriculture,” Shih says. “It’s a really out-of-the-box way of looking at it, which is refreshing.”
JINKERSON’S ORIGINAL DREAM was to redesign fuel production, not food. As a postdoctoral researcher from 2014–17 he helped rewrite genes in photosynthetic algae to boost the production of oils that could be captured and converted into biofuels. At the time, chemists around the world had similar ambitions, backed by hundreds of millions of dollars from the likes of Shell and Exxon Mobil. Ultimately, the high cost of algal biofuels compared with petroleum, among other issues, doomed Jinkerson’s project along with most of the others. But along the way he learned that some types of algae could grow in the dark when fed a simple liquid hydrocarbon called acetate.
In 2017, a few months after joining UC Riverside, Jinkerson found himself sitting in on a seminar by Feng Jiao, a chemist at the University of Delaware. Jiao was describing his team’s experiments with an electrolyzer, a paperback book–size device that zaps CO2 and water with electricity to create acetate and ethylene, a building block for plastics. Because of the massive market for ethylene, $176 billion annually at last count, most of the buzz around Jiao’s electrolyzer centered on this compound. But it occurred to Jinkerson that if Jiao could make acetate from CO2, his own team could feed it to algae, and one day perhaps food crops. “I thought it was a perfect match,” Jinkerson says. “I pitched him in the parking lot and again 2 months later at a meeting.”
The two teamed up. Jiao and his students revamped their electrolyzer to turn down ethylene production and turn up acetate. Last year, they reported in Nature Catalysis that they had shifted the output from 30% acetate to 99%, a purity high enough to feed directly to plants. The Delaware team has since carried out a technoeconomic analysis of its process, showing that producing acetate in an electrolyzer with regular grid power can be cheaper than the conventional process for producing acetate for use in textiles, among other things.
The milky-white pearl oyster mushrooms growing inside a stainless-steel refrigerator at UC Riverside’s Plant Transformation Research Center illustrate what else can be done with that acetate. Each mushroom grows out of an oversize test tube packed with sand-like vermiculite bathed in liquid growth media, a mixture of acetate and inorganic nutrients including phosphorus and nitrogen. In work published last year in Nature Food, Jinkerson, Jiao, and colleagues fed acetate generated in their electrolyzer to mushrooms, yeast, and algae grown in the dark. Those organisms converted chemical energy into new biomass up to 18 times more efficiently than if they had been fed plants grown by photosynthesis.
The mushroom cultivation strategy was part of the team’s entry in the Deep Space Food Challenge, a competition launched by NASA and the Canadian Space Agency in 2021 to come up with innovative ways to feed astronauts on extended deep space missions. The UC Riverside and Delaware collaborators call themselves Nolux (Latin for “no light”), and in October 2021 they were chosen out of more than 300 applicants as one of 18 U.S.-based finalists. (Other finalists include an outfit that feeds dried insect cells to a bioreactor to “emulate traditional meat products” and a project to create “meat” from pluripotent stem cells.)
In the next phase of the contest, teams put together prototype food production systems. Based on its results, Nolux calculates that acetate-fed mushrooms grown in a 2-cubic-meter reactor could provide about 8.5 kilograms of food per day, more than one-third of what a six-person crew on a deep space mission would need. Last month it was named one of five U.S. winners awarded a $150,000 prize.
Even a mushroom lover might balk at eating 1 kilogram or more of the fungi every day, however. And most plants will be harder to grow in the dark. Mushrooms, yeast, and some algae are heterotrophs, organisms that consume other plants or animals for their nutrition and are naturally able to grow in the dark. But nearly all plants are autotrophs, producing their own food from sunlight.
In the same Nature Food study, Jinkerson, Jiao, and their colleagues fed acetate to nine crop plants raised in the dark, including lettuce, tomatoes, and peppers. Although the plants survived, they didn’t grow larger. Additional tests using acetate labeled with carbon-13, a heavy isotope of carbon, showed the plants could incorporate acetate into their tissues—an encouraging sign—but apparently not enough of it to support the growth of new tissue.
Further investigation revealed why. The acetate, it turns out, was processed through a metabolic cycle known as the Krebs cycle. Plants normally use this pathway to break down sugar molecules built and stored during photosynthesis. They do so bit by bit, extracting energy in each step that can be used to synthesize proteins and other essential cellular building blocks. Leftover sugars can then be used for building plant tissue. But unlike sugar molecules, which have either five or six carbon atoms, acetate molecules only have two. And with each full turn of the Krebs cycle, plants release two molecules of CO2, leaving no carbon atoms for the plant to build leaves, roots, or stems. It stays healthy but its growth stalls.
Growing plants in the dark
Plants normally rely on photosynthesis to make sugars they use as fuel and to construct new tissue. Tweaking plant metabolism to allow them to grow in the dark could help feed astronauts on deep space missions and grow crops year-round on Earth.
To create plants that can actually grow on acetate, and ultimately produce edible leaves and fruit, Jinkerson and his colleagues are now using CRISPR and other gene-editing tools to restart genes that are briefly active when seeds germinate. A metabolic pathway called the glyoxylate cycle allows seedlings to feed on the starches, proteins, and oils stored in seeds. The glyoxylate cycle bypasses steps in the Krebs cycle where carbon is lost as CO2, so it should conserve carbon that could be used for growth. “They already have the pathway and already use it,” Jinkerson says. But it shuts down as plants grow.
He notes that humans have a similar genetic switch that allows babies to process lactose, the sugar in milk, but then turns off in many adults, causing them to become lactose intolerant. Over the course of human evolution, that switch got turned back on for many, allowing them to drink milk. “Now, we’re trying to do the same thing in plants,” he says.
The initial results are promising. First, Jinkerson and his colleagues engineered Arabidopsis, a mustard plant commonly used for genetic studies to turn up a gene for an enzyme called acetyl-CoA synthetase (ACS) that chemically alters the acetate they consume. The change enabled the plants to grow normally when fed acetate at a high concentration, which would otherwise stunt growth. They are also now ramping up activity of two other genes—for isocitrate lyase (ICL) and malate synthase (MLS)—that enable germinating plants to bypass the Krebs cycle and metabolize acetate via the glyoxylate pathway. If that works, the UC Riverside team will test whether the engineered plants can flourish in the dark.
Rhee says the general strategy is a good one and notes that the modified Arabidopsis’s increased tolerance for acetate is a hopeful sign. But she notes that plants rely on light to trigger a host of other processes, including initiating signaling cascades that trigger flowering and ripening That could make it extremely difficult to coax them to grow completely in the dark. J. Clark Lagarias, a plant biochemist at UC Davis, agrees and points to additional challenges. These include a risk that fungi and algae might invade the growth medium and gobble up all the acetate before plants could consume it. That concern alone could be too much for NASA to stomach, says Adam Arkin, a bioengineer at UC Berkeley who also works on feeding astronauts on deep space missions. “We are very, very risk averse in space,” Arkin says.
ACETATE ISN’T the only potential food source for dark-raised plants, however. Sugars are another attractive option, if they could somehow be supplied in space. Plants already make sugars through photosynthesis and circulate them throughout their tissues to be metabolized in the Krebs cycle. Adapting plants to use sugars supplied artificially might not require the extensive rerouting of plant metabolism needed for acetate.
Vertical farming is used today for some specialty crops, but the lighting required makes it an energy-intensive process.AEROFARMS
Soon after Jinkerson joined UC Riverside, he came across a paper by an international team of researchers showing cotton plants grew in the dark when fed sugar water and wondered whether the same could be done with food crops. He teamed up with Martha Orozco-Cárdenas, head of UC Riverside’s Plant Transformation Research Center, to investigate. They started with tissue cultures from lettuce and showed these plants could grow in the dark when fed sucrose. Initial results showed only modest growth, but the taste was fine. “It tasted just like lettuce and was crispy,” Jinkerson says. The team has also added genes for making vitamin B12 to the lettuce and shown that the dark-reared plants produce the vitamin—an advantage in space, where supplying this typically animal-derived vitamin will be a challenge.
The team is also trying to raise cherry tomatoes on sugar water, experimenting with dwarf plants they genetically modified to have almost no leaves or stems so that they channel the majority their energy into making fruit. Dubbed Small Plants for Space Exploration, or SPACE tomatoes, they’re now scheduled for testing aboard the International Space Station to see how microgravity affects their growth and whether they can produce seeds that can be replanted. If so, they might offer astronauts an efficient way to produce fresh tomatoes, while minimizing inedible plant waste.
Lagarias thinks the sugar-fed dwarf tomatoes might thrive in the dark, particularly if the plants are given doses of light at select times. Previous work from his group and others, he says, shows that “plants can go through a complete life cycle in darkness with sugar.” But they don’t grow well. “To make this really effective you need light to turn on some aspects in development.”
Jinkerson’s group is working to tease apart the amount and timing of light needed to trigger signaling and other key processes. “It may only be important to give them light at certain times,” Jinkerson says. He notes that only tiny amounts of light—1000 times less than is normally needed for plant growth—may be enough to trigger key developmental steps.
Even those dim flickers may not be essential. In the 27 April 2020 issue of Plant Direct, Lagarias and his colleagues reported that a particular mutation they discovered in rice plants alters a light-sensitive protein called phytochrome B, enabling it to trigger growth and development in the absence of light. “The plants develop in the dark as if in the light,” he says.
Hauling tons of sugar into space as plant food may not make much sense, but recent advances in synthesizing sugars in electrolyzers may make that unnecessary. Compared with acetate, the molecular structures of sugars are more complex, making them more difficult to synthesize. But in the 19 October 2022 issue of Joule, researchers led by Peidong Yang, a chemist at UC Berkeley, reported using an electrolyzer to convert CO2 and water to compounds called glycolaldehyde and formaldehyde. A metal catalyst then caused the molecules to react and form sugars, including glucose. And in the May 2022 issue of Nature Catalysis, researchers led by Tingting Zheng at the University of Electronic Science and Technology of China reported that they engineered yeast to convert electrolyzer-produced acetate to glucose at high yield. Though neither of these approaches yet matches the yield of Jiao’s acetate electrolyzer, they raise hopes that future astronauts may be able to recycle their breath into plant food—and ultimately into fresh vegetables.
ON EARTH, light is free and plentiful, so there’s no need for Jinkerson’s metabolic wizardry in most agriculture. But for some high-priced vegetables, it might make sense. Dark rearing would allow growers to convert sugar, which currently costs just 45 cents per kilogram on the commodities market, into a product with far higher value. It also could enable those crops to be grown year-round anywhere in the world without worry about drought, storms, deep freezes, and long-distance transportation.
“It allows us to reimagine what agriculture can be,” Jinkerson says. It might also save costs for vertical farming, a method already used to produce expensive fruits and vegetables (think artisanal greens) for restaurants and other high-end consumers. In today’s vertical farms, plants are grown indoors on racks stacked floor to ceiling and illuminated with banks of high-efficiency LED lights. But the electricity needed to power those lights, among other energy demands, means vertical farms use on average seven times the energy of regular greenhouses. The added costs limit vertical farming’s reach. “Our approach would eliminate the lights, and have stacks of plants growing in the dark,” Jinkerson says.
“It’s early days,” says Lucas van der Zee, a plant biologist at Wageningen University who has also been trying to grow plants in the dark, but so far without success. “Since the beginning of agriculture, food production has been dependent on the Sun,” he says. “But if you can find a way to separate growth from light that would be huge, and change food production around the world.” Perhaps beyond our world as well.
Author: Robert F. Service
Bob Service is a news reporter for Science in Portland, Oregon, covering chemistry, materials science, and energy stories.