FIFTY MILLION YEARS AGO, palm trees grew in the Arctic. Alligators lazed on land devoid of ice; rain fell heavily in the warm, thick air; swollen rivers flooded the ocean with freshwater. And a fern called Azolla covered the surface of the Arctic Ocean in a massive blanket the size of Europe. Over the course of about 800,000 years — a blip in the history of the planet — the fern likely pulled trillions of tons of carbon dioxide out of the air and sank to the bottom of the ocean. Eventually the freshwater tropical fern’s glory days in the Arctic came to an end, and Azolla largely disappeared beneath the sea floor.

In July 2004, a group of scientists and oil drillers set out to study the ocean floor on the world’s first (and only) Arctic scientific drilling expedition. The multinational undertaking was run by the Integrated Ocean Drilling Program (now called the International Ocean Discovery Program). Two ships from Russia and Sweden led the way, cracking the thick ice, while the Vidar Viking of Norway brought up the rear, a 1.5-kilometer-long hollow steel pipe — the drill — dangling delicately beneath its hull.

The ships arrived at the Lomonosov Ridge in the North Pole, where the team drilled and collected 428 meters of core from the ocean floor. Nearly all the cores were broken and incomplete. The expedition could easily have been a failure. But by a wild stroke of luck, expedition scientist Henk Brinkhuis says, “exactly the most critical parts of the history of Earth happened to be in these bits and pieces” — specifically, pieces from 49 million years ago, a time when global temperatures were in dramatic decline.

After traveling by helicopter from the Vidar Viking to the Swedish ship Oden, Brinkhuis prepared a small sample of the core for analysis under a microscope. Looking through the lens, he saw hundreds of thousands of remains that looked like “slimy balls with spaghetti strings sticking out”: fossilized Azolla fern.

Again, another stroke of luck. If Brinkhuis hadn’t been the one looking through the microscope, there may have been no discovery. He was the only scientist on the ship who recognized the specimens because Azolla had shown up frequently during his former oil drilling ventures. Jonathan Bujak, an English paleontologist who has worked in the Arctic since 1973, had seen their microscopic fossils in cores for years too. “But I hadn’t attached any significance to it,” he says.

The proliferation of Azolla 50 million years ago is now referred to as the Arctic Azolla event.

The proliferation of Azolla 50 million years ago is now referred to as the Arctic Azolla event. Scientists still aren’t certain what led to the fern’s decline, but by the end of Azolla’s 800,000-year reign, the world had started cooling to the relatively temperate climate we have today. Because high levels of atmospheric carbon dioxide typically lead to global increases in temperature, scientists believe that Azolla’s immense appetite for carbon dioxide contributed to Earth’s cooling. The fern may have cut atmospheric carbon levels by as much as half.

While this part of its fascinating story only recently came to light, the Azolla fern has been entangled with human history for thousands of years. Today, some see it as a pest — a fast-growing invasive weed that blooms like algae. But many see its vast potential to clean up chemical runoff, act as a sustainable fertilizer, or provide a high-protein food source. Some even believe that the fern could contribute to curbing global warming — a natural solution to an unnatural crisis. I’m determined to find out: Does Azolla live up to the claims?

AZOLLA DOESN’T LOOK MUCH like a fern. With leaves only slightly bigger than a grain of sand, a clump of Azolla could easily fit on your thumbnail. It floats on the surfaces of freshwater ponds and lakes, with its roots hanging limply like wispy hairs in a corn husk. Its color ranges from neon green to rusty red. While the quintessential fern has long green fronds and fiddleheads — single leaves coiled up like a roly-poly bug — a patch of Azolla fern looks vaguely like a miniature leaf pile in the middle of a New England autumn.

But as Azolla makes clear, one should never underestimate something for being small: It’s one of the fastest growing plants on Earth, doubling in size every three to ten days in optimal conditions. If humans exhibited the same tendency, an infant would grow to be four times the height of basketball player Shaquille O’Neal in two months.

How does Azolla grow so fast? The fern allied with a cyanobacterium called Anabaena (Nostoc azollae) that resides in small pockets in Azolla’s leaves. The two species help each other out: Anabaena provides Azolla with nitrogen, and Azolla provides Anabaena with sugars. Truly a friend for life, Anabaena has stuck by Azolla’s side for the past 80 million years. The bacterium has even shed genes that it no longer needs since it cohabits with Azolla. Now, according to Kathleen Pryer, a professor of biology at Duke University, “They can’t live without each other.”

Azolla is the only plant in the world with such a loyal companion. When Azolla reproduces, the fern spores are bathed in a solution containing the bacteria. So when the spore relocates and begins to grow, the bacteria accompany the spore, and their relationship continues across generations.

Widely regarded as the world expert on Azolla, Francisco Carrapiço describes the symbiosis as a “new level of biological organization.” Carrapiço, an emeritus cell biology professor at the University of Lisbon, has studied the symbiosis for over three decades. “It’s a question of curiosity,” he tells me over Skype. His interest in Azolla is eclectic: He has just returned from a trip to the Muséum National d’Histoire Naturelle in Paris to examine the first known collected Azolla specimens in the Western hemisphere. A few years ago, he wrote a children’s book about a medicine man using Azolla in Africa.

Carrapiço and other scientists continue to study the symbiosis in hopes of better understanding it. So far, the answers that have come with research have only led to more questions. But Jonathan Bujak believes the fern could be a valuable teacher as we work towards living more sustainably.

“Azolla and Anabaena,” he says simply. “They developed this process over 80 million years. How can our technology compete with that?”

Indeed, the Azolla-Anabaena coevolution has turned the plant into a little growing machine. Its stealthy ability to overtake large bodies of water has earned the Azolla genus a bad reputation in many parts of the world. Of the seven different species, Azolla pinnata occupies a spot on the “Federal Noxious Weed List” in the US, and Azolla filiculoides has been classified as a weed in Iran since its introduction in 1986.

Excess nutrients in the water can cause Azolla populations to “bloom,” or accumulate rapidly, sometimes to the point of harming the ecosystem. Like algal blooms, this can block sunlight from reaching the organisms below and rob the water of oxygen. In 1993, the fern grew out of control in Portugal’s Guadiana River due to low water flow and chemical contamination. A mat of Azolla blanketed several kilometers of the river so densely that photos of the river in 1993 show only the fern cover, with the water nowhere to be seen. More recently, an Azolla bloom in Sonoma County, California, got some press when it covered a small lake, likely fertilized by fire retardant runoff.

But perhaps Azolla isn’t the bad guy: “People pass laws banning Azolla as an invasive weed when actually it’s mopping up harmful chemicals and runoff from the nitrogen-based compounds in chemical fertilizers used on crops,” says Bujak. “Azolla is not a nuisance at all. In fact, it’s an ally.”

WEEDY REPUTATION ASIDE, most people don’t know much of anything about Azolla. The fern was widely unknown in North America until 2014 when Pryer, a self-described “fern lady,” crowdfunded more than $20,000 to sequence Azolla’s genome, making Azolla the first fern to ever garner $20,000 of interest.

Pryer has loved ferns since her first college botany course in the 1970s. Her Twitter account features a photo of her holding a fern above her upper lip like a mustache.

Pryer, a self-described “fern lady,” crowdfunded more than $20,000 to sequence Azolla’s genome.

In 2010, she named a genus with 19 fern species after Lady Gaga, who then sent a team of biologists to Costa Rica to collect a sample for her. “I received a photo of Lady Gaga with the fern in a Versace vase,” Pryer says as we chat over the phone.

In 2000, researchers sequenced the first plant genome — a wild species in the mustard family — and since then the focus has been on sequencing genomes for the plants we eat. But Pryer believed that understanding Azolla’s genome could provide valuable information about plant evolution and potential future applications for agriculture.

Her enthusiasm fell on deaf ears. Grantmakers dismissed Pryer’s funding requests as “too unconventional” or “too risky,” she says. In 2014, she sent one last entreaty to the National Science Foundation, a major funder of scientific research. She received a rejection email within 15 minutes telling her not to waste their time.

Frustrated, Pryer had an idea: What if she could raise the money herself? She turned to her charismatic graduate student Fay-Wei Li and asked him what was the lowest amount of money they would need to sequence the genome. “We stared at each other with wide eyes,” she says. “And we decided on $15,000.” Immediately, they set up a crowdfunding page to raise the money needed to sequence two genomes, Azolla and a fern called Salvinia. Before long, Pryer received $5 from a high schooler in Oklahoma. And then $30 from a science-fiction writer who finished his book after being inspired by Azolla. Then $1,000 from an intrigued psychiatrist. Within a few months, 123 people had raised $22,160. The publicity turned heads, and when the Beijing Genomics Institute in China heard about the crowdfunding, it offered to back the entire project. With their financial needs covered, Pryer decided to use the money raised via crowdfunding to help fund the later phases of Azolla’s genome assembly — a long, multifaceted process that’s still underway today.

In 2015, the global effort began: Forty-two scientists from Belgium to Nova Scotia, Taiwan to Utah, all worked on sequencing Azolla’s genome. In July 2018, with the initial genome complete, Azolla made the cover of Nature Plants as the first fern ever sequenced.

With the genetic sequence in hand, Pryer thinks that scientists could someday engineer crops to fix their own nitrogen, “an achievement that could truly revolutionize modern agriculture,” according to her 2014 op-ed in one of Canada’s most widely-read newspapers, The Globe and Mail. Sequencing Azolla’s genome also provided insight into the evolutionary history of a protein that makes Azolla and other ferns highly resistant to insects. In 2016, scientists successfully transferred this protein, called Tma12, to a cotton crop that subsequently showed resistance to insects.

“We have new tools now,” says Henriette Schluepmann of Utrecht University, who helped Pryer pioneer the genome project. “The sequencing of these plants gives novel insights to all kinds of applications and fundamental biological questions, which now we can start tackling.”

AS THE GLOBAL COMMUNITY struggles to address the climate crisis, the trillion-dollar question is this: Could Azolla help cool down our planet again as it did 50 million years ago? Azolla grows fast, doesn’t require much space, can be removed easily when it dies, and consumes so much carbon dioxide that the fern is a potential carbon-sequestration miracle plant. The equation seems simple: more Azolla = less carbon dioxide = cooler planet.

Could Azolla help cool down our planet again as it did 50 million years ago?

When I ask Henk Brinkhuis, now the director of the Royal Netherlands Institute for Sea Research, if he thinks we could reenact the Arctic Azolla event, he says, “Why not?” Instead of the Arctic, which has become too cold and salty for Azolla’s taste, he suggests the Baltic Sea. But though he dreams big, he acknowledges it’s an unlikely possibility. There are too many massive hurdles: international negotiations, large financial investments, unknown environmental impacts, and a much shorter time frame than a million years.

So the conversation among geoscientists has turned to other ways to grow Azolla in bulk. Brinkhuis proposes massive warehouses, an easy place to maintain optimal conditions for the fern’s growth. Another option is man-made marshes. Salt marshes are incredible carbon sinks, superior even to tropical forests. With the help of the tides, their muddy sediments bury organic remains which would otherwise release carbon into the atmosphere. The Netherlands, along with many other delta regions around the world, have drained salt marshes to build cities and farms. Scientists at Wageningen University are researching how to construct new marshes in hopes of improving water quality and protecting the Netherlands’ coasts from rising seas.

“We call this a ‘building with nature’ program, where we try to make use of the existing natural systems in this kind of way,” explains Brinkhuis.

If Azolla could be added to these marshes, the quantity of carbon pulled from the air could be enormous. But there’s a wrinkle here too: Azolla doesn’t usually tolerate salty water. Azolla nilotica, native to the brackish Nile delta, could be a possible candidate for wetland propagation, but introducing a nonnative, fast-growing fern to wetlands across the world raises its own set of questions.

In order to someday grow Azolla in warehouses or salt marshes, we need to know how to breed the fern. At Utrecht University in the Netherlands, Schluepmann is working on this first step. Her research involves optimizing Azolla yield for livestock feed and perhaps, someday, human use as well.

“We have never domesticated ferns before!” says Schluepmann. “Let alone a fern that is in symbiosis with a cyanobacterium.”

The symbiosis makes it more challenging to use common breeding techniques, such as adding sugars, which would affect Anabaena. Currently Schluepmann’s team can grow Azolla in a plant nursery and transfer it to flooded fields, but ideally Azolla could be planted and grown directly in the fields. “We don’t know how to do that yet,” she tells me. “It’s not like a seed plant, where you collect the seed and just throw it into your field. Because of their spores and their tendency to split rather than sexually reproduce, this represents one of many challenges for domestication.”

In addition to the promise of carbon sequestration, researchers are also eyeing Azolla’s potential to rebuild land that’s being lost to subsidence in delta regions. In places like the Netherlands — where soil is sinking at an average rate of eight millimeters per year, causing highways to buckle and buildings to sink — this prospect is particularly appealing.

‘There is a trend to think, Hey, can we make land? We looked at Azolla and thought, Hm.’

“If we keep going like this, we will have to invest in a tremendous amount of infrastructure. So there is a trend to think, ‘Hey, can we make land?’” says Schluepmann. “We looked at Azolla and thought, ‘Hm.’”

In coordination with ecologists Leon Lamers and Fons Smolder at Radboud University in Nijmegen, Schluepmann’s team grew Azolla in experimental plots for the past year, during which the fern produced two centimeters of organic soil. Their research, which has yet to be published, suggests that Azolla could be used to build rich soil at a rate faster than the land is subsiding. And while creating soil, the Azolla would also do some of that sequestration work climate scientists have dreamed about, by burying carbon in that same soil.

“I think this service will be the first product that people will want to pay for. People won’t want to pay for CO₂ drawdown on a massive scale with a plant we don’t know how to handle yet,” says Schluepmann. “But people might be more interested to first develop it on a smaller scale on fields where they need to lift the land. I think that’s much more realistic.”

WHILE WAITING on the science and the market, the little fern has already had some big impacts in other ways. In East Asia, rice farmers have used it as a fertilizer for thousands of years. Rather than disrupt the rice crops in flooded paddies, Azolla actually encourages faster growth by suppressing weeds and harmful bacteria — and, most importantly, increasing the amount of nitrogen available to the rice plants. Due to these agricultural applications, the Chinese and Japanese have spearheaded a great deal of basic research on Azolla. The Fujian Academy of Agricultural Science in China, for instance, has a national Azolla nursery, and in 1985, the academy held its first international conference on Azolla use and produced a book on its proceedings.

Other countries have started to pick up on Azolla’s benefits as well. In Iran, the fern is used to filter radioactive wastewater from nuclear plants. In Sweden, it has its own cookbook. It’s been used to reduce mosquito populations in Kenya by choking larvae under its dense cover. And in 1980, it traveled to outer space with astronaut Phạm Tuân to be studied in closed-loop life support systems.

In particular, the fern’s use as a cheap, sustainable, and healthy fertilizer alternative has become popular in developing countries around the world, from Guinea-Bissau to the Philippines. In Ecuador, Dr. Mariano Montaño has converted rice fields into what he calls “green fertilizer factories.” He found that Azolla fertilizer increased rice yield by nearly a ton per hectare. Water and soil quality, as well as the health of the field workers, all improved with its use.

Azolla also has a vibrant grassroots following, including Lexy Bujak, who co-founded the Azolla Foundation with her father Jonathan Bujak in 2012. I give her a call one morning. She talks briefly about her dissertation, in which she quantified Azolla’s carbon sequestration capabilities, but I quickly ascertain that Azolla represents a different sort of impact for her.

She spends her weekends and evenings after her day job as a technician Skyping with thousands of Azolla Foundation members, many of whom live in the Philippines. She tells me about a man who uses Azolla as a supplement feed for his livestock to save money on commercial brands. He also sells the fern online to help pay his rent, she explains. Part of her mission is to work with all her Skype contacts to figure out how Azolla can best help them financially.

I ask her a question that I’ve asked all the Azolla acolytes: “What’s your favorite aspect of Azolla?” Most of the scientists describe their fascination with its symbiosis. But Lexy Bujak has a different answer.

“Just how many people it’s helped,” she says after a moment. “It sounds daft, but it means more to me than anything. Life can be quite dreary, sometimes. Just listening to people, and hearing how Azolla has helped them pay the rent, or put their children through school…” She trails off. But I understand.

ON A COLD DAY IN OCTOBER, I go to see Azolla in person for the first time. I’m meeting up with Jacob Suissa, who studies ferns and lycophytes as a PhD student at Harvard. Having recently acquired Azolla for the Arnold Arboretum collection, he agreed to introduce me to the fern I had heard so much about.

I meet Suissa in the lobby of the Harvard Herbaria. A friendly fellow who small-talks about ferns instead of the weather, he says hello quickly before launching into an explanation of why the correct term is “herbaria” rather than “herbarium.” I try to take notes as he leads me through a maze of hallways and staircases. By the time we emerge in a tropical greenhouse, there is no way I could find my way back outside without a guide.

Eventually, we reach two glass tanks filled with water. I peer at the floating plants on the surface. Something like a chestnut with leaves and thick hair floats in one tank. In the other, something that looks like green pasta shells lined with soft teeth. Suissa is busy pointing out the different parts of all the ferns, but he knows I came here for one thing.

“Here it is,” he says suddenly, pointing to a little leaf hiding behind the pasta shells. “A beautiful and curious plant.”

He transitions into talking about sporangia as I gaze at Azolla. I think about Carrapiço and Lexy Bujak, who have dedicated so much of their lives to this plant, which looks like nothing more than a delicate clump of leaves. Like them, I’m enchanted by it.

“Can I take some home with me?” I ask.

When we part ways, I am carrying a couple dozen leaves in a small tub of water, and a hope for the little fern that could, and that someday, perhaps, will again.