Glaciers are vast, frozen rivers of snow, ice and rock, collected often over thousands, even hundreds of thousands, of years. As an area of scientific research, glaciers have long landed in the domain of physical geochemists. It’s easy to see why: these researchers study glaciers to investigate how water shapes the earth and track the increasingly observable signs of climate change. Many liken the annual growing and shrinking of glaciers to the deposits and withdrawals of a watery bank account, and warn that warming is driving the world dangerously close to being overdrawn.
But physical geochemists are not the only ones with a vital stake in predicting what the future will bring. Over the last 20 years or so, a growing group of microbiologists has also taken a critical interest in glaciers, due to the tiny passengers hitching a ride on these dense bodies of ice. Microorganisms can flourish on top of ice, beneath its surface (between the ice and the land) or even within—trapped, but not dead. Glaciers are ecosystems; they are not devoid of life. When they melt, their inhabitants change too.
"Most people think, 'It’s frozen, it’s dead, there’s nothing there.' But [glaciers are] not just frozen nothingness,” said microbiologist Dr. Christine Foreman, Ph.D., a professor at Montana State University in Bozeman, whose work looks at ways that microbes and climate change shape each other. “We now have this ability to think of glaciers as living systems. There’s a deep reservoir of living things there.” The deeper one goes, the older the ice, and presumably the more ancient the microorganisms. That means that to drill deep into the ice is to travel back in time. Studying glaciers can reveal both the biodiversity of life today and how it has shifted over centuries.
Glaciers hold roughly 75% of the freshwater on the planet, and as temperatures rise due to climate change, these “ice rivers” are quickly shrinking—in ways that will change how water cycles through the environment. Rates of retreat, and the impacts on life, vary by location. For example, according to research published in December 2021, glaciers in the Himalayan mountains, which run along the northern edge of the Indian subcontinent, are shrinking at an exceptionally fast rate. Given that glaciers feed rivers that supply water to more than 1 billion people, more melting may limit accessibility to drinking water and accelerate disasters like floods and landslides. In Antarctica, which has the most glaciers worldwide, the loss of large amounts of ice will likely boost sea levels. So will the melting of Greenland’s glaciers. A rising sea threatens coastal communities by eroding beaches and increasing the risk of devastating floods.
Melting glaciers won’t only impact human communities; they’ll also reshape microbial life. Viruses, bacteria and fungi that have been buried for hundreds of thousands of years could again see the sun. Those that live in, or on, the ice may be washed downstream by meltwater, changing the biological composition of the ecosystems they leave behind and the ecosystems where they land.
Microorganisms could also influence the changing climate in ways that aren’t well understood. For example, they might release methane, a potent greenhouse gas or, through biological-physical interactions, accelerate the rate at which snow and ice melt. “There’s essentially an amplifying process that we haven’t accounted for,” said microbiologist Dr. Arwyn Edwards, Ph.D., a researcher at Prifysgol Aberystwyth University in Wales, who has been studying ways that meltwater connects microbial populations in Greenland and Antarctica.“Glaciers respond to anthropogenic climate warming in ways that are entirely predictable. And then biology starts to drive feedbacks.”
The influence of minuscule microbes on big climate processes has long gone unappreciated—and likely underestimated. “We have yet to move past the tip of the iceberg in terms of understanding the diversity of microbes and microbial interactions in those environments,” said Edwards. But there’s a shift underway, as a growing number of studies investigate life on ice and how it’s changing. “These organisms can be searingly important,” said Edwards. “They are a missing piece in the jigsaw puzzle."
Microbial Life on Glaciers, but Not From Space
Today’s efforts to connect meltwater to microbes can, in a way, be traced back to a startling discovery made more than 150 years ago by Adolf Erik Nordenskiöld, a Swedish-Finnish aristocrat, scientist and explorer. Nordenskiöld was the first explorer to cross the Northeast Passage; throughout his career, he led 10 expeditions to the Arctic north. During one journey in 1870, he observed that the glacial ice surrounding his team wasn't a uniform field of white. Instead, it was pitted with small dimples, each 1-3 feet deep and containing a dark mixture. He called it a “clayey mud,” and worried that hidden holes (like potholes on an interstate) posed a danger to his expedition.
He named the mud “kryokonite” (the word is now spelled cryoconite) and speculated about its origin. Because the dimples were evenly distributed, he ruled out the possibility that the material had washed down from surrounding mountains or been deposited by rivulets of flowing water. He rightly concluded that the substrate had been carried by the wind; he also wrongly concluded that the presence of iron meant the goo included cosmic dust from space. When Sven Berggren, a Swedish botanist who was along on the voyage, examined the cryoconite, he observed a “peculiar ice-flora, consisting of a quantity of microscopical plants (algae).” Snow algae wasn’t new; observations of snow-fields colored pink, red or green by various types of algae—sometimes called “watermelon snow”—date back to Aristotle. Clearly, the cryoconite could hold life.
However, Nordenskiöld saw something else in the small holes scattered across the ice. He surmised that the dark color of the cryoconites absorbs more heat from the sun than the bluish ice and, as a result, would accelerate the melting of the ice sheet. “Undoubtedly we have, in no small degree, to thank these organisms for the melting away of the layer of ice which once covered the Scandinavian peninsula,” he wrote in an 1883 publication.
Nordenskiöld’s ideas were among the first to connect the glacial microbes to larger climate processes. Now, scientists have a better handle on the finer details. Gusts of wind deposit dust from local and distant sources, from industrial emissions and natural landscapes. The dust coats the snow and becomes trapped by filamentous cyanobacteria with super-sticky polysaccharides on their surfaces. “Cryoconite microbes do a good job of organizing that dust,” Edwards said.
Recent studies have helped researchers gauge the contribution of snow algae to glacial melting. One gauge of that contribution is how algae changes the albedo, which measures the degree to which a material can reflect radiation from the sun. Snow, which has a high albedo, can reflect up to 90% of incoming sunlight. Cryoconites, which have a low albedo, absorb much of that energy, heat up as a result and, necessarily, heat the surrounding snow as well (a newish term for their contribution is “bioalbedo”).
An international, interdisciplinary team of microbiologists, glaciologists and others recently used satellite data, runoff modeling and other tools to try to estimate the microbial contribution to melting. Their study, published in January 2020, found that algal growth in 2017 boosted annual runoff from bare ice by around 5 gigatons, more than 10% of the total. In patches dense with biomass, the algae accelerated melting by more than 25%.
"The dark color of many species of snow algae," said Edwards, "acts as protective sunscreen for the microbes, shielding them from harmful radiation." But it’s also clearly contributing to glacial warming. “They produce meltwater; they reduce the albedo,” he explained, adding that climate models don’t always account for this contribution, but they should.
Waking Up Microbes Frozen Within Glaciers and Ice Sheets
Microbes can also be trapped within glaciers and ice sheets, but just a few decades ago it would have been difficult to convince anyone of their existence. “Until 15-20 years ago, no one really thought there was life in ice,” said Dr. John Priscu, Ph.D., Regents Professor Emeritus at Montana State University in Bozeman and senior research scientist, Polar Oceans Research Group. Priscu has been studying microbes in icy environments for more than 40 years.
Priscu helped bring attention to those frozen critters. In the 1990s, Priscu and his team studied an ice core collected by Russian drillers from more than 3,500 meters beneath the ice’s surface—and about 150 meters above Lake Vostok, a subglacial lake discovered in the 1970s. The core represented lake water that had frozen, or accreted, on the bottom of the ice sheet. Antarctica has about 400 known subglacial lakes, but Vostok is believed to be the deepest (at about 1,000 meters) and oldest. It may have formed more than 15 million years ago, even before the continent was covered in ice. When Priscu’s group thawed the core and studied the meltwater, they discovered genetic material from bacteria and showed that the organisms were capable of active metabolism. The microbes could have lived in the lake and been trapped in water that had accreted to the bottom of the glacier, providing a glimpse of life in an icy world that has not seen sunlight for many millions of years, or they may have blown across the ice when the glacier was forming.
The Lake Vostok ice cores ignited interest in deep, frozen life, and since then, researchers have not only sequenced the genomes of the collected microorganisms to gauge biodiversity, but also analyzed the abundance and metabolic potential of living things in ice cores taken from within glaciers and ice sheets around the world.
In ice cores, “we indeed have a time machine—2 miles [deep] of time machine,” Priscu said. “When a microbe gets blown onto the surface of the ice, it eventually becomes part of the climate record as new layers form at the surface. It’s like cosmic fly paper.” Later, when scientists thaw that ice, sometimes the microbes wake up. “We can take ice that’s 300,000 years old and melt it, and in 5 minutes they’re metabolizing."
Studies of ice-dwelling microbes also reveal chemical clues to past and present climate patterns. Foreman, a former postdoctoral student of Priscu’s, recently launched a project to study englacial ice, designed to learn what exactly those microbes are doing. “What if they are actively respiring in there?” she asked.
Though fears of ancient zombie pathogens have largely gone unfounded, more of these microbes will again see the light of day and be ferried away by meltwater. “As ice sheets are melting and ice shelves are breaking up, the melted water goes into the ocean,” Priscu said. “It’s taking these ancient genomes and pumping them back into the sea.” But that mixing raises an important question: what happens when ancient microorganisms find themselves in a new ecosystem, commingling with newer ones? Can they live in that habitat?
This process is an example of what scientists call “genomic recycling,” and it’s not limited to the Southern Ocean. It happens globally, even at high altitudes. "In the Himalayas, we’re getting a lot of mixing too,” Priscu said. But in most cases, it’s still not clear where the microbes end up, or what happens next. “What we do know is that ice-bound microorganisms can tell us what the climate was like when they were deposited at the surface.
Tracing the Throughline
One place where scientists are trying to answer questions about melting and mixing is in and around the fjords of the Svalbard archipelago in Norway. These stunning, watery and remarkably deep channels wind through craggy mountains and pass by enormous glaciers. When the glaciers melt, the water drains into the fjord. Microbiologist Dr. Karen Lloyd, Ph.D., an associate professor at the University of Tennessee at Knoxville, wants to know how shrinking glaciers will shape microbial communities living on the floor of the fjords.
“We don’t totally know what’s happening with the retreat,” she said. In the Svalbard fjords, glaciers play a critical role in feeding the microbial communities living at the bottom. “They sort of bulldoze iron into the seafloor,” she said. But that influx of iron doesn’t cause an algal bloom on the surface; instead, the iron settles in the deep, where it is processed by microorganisms. “The question is: What’s going to happen when the bulldozers stop bulldozing?” said Lloyd.
"If glaciers stop dumping iron in the water, more of the mineral might be available to the microbes on land," she said. They may flourish, but right now it’s not clear how the shifting availability will affect climate change. If these land-dwelling microbes became more active and produced more greenhouse gases, for example, they could amplify warming.
As part of her work, Lloyd is also collaborating with scientists who are looking for connections between land microbes and those that live at the bottom of the fjords. In March 2021 and 2022, she and her collaborators traveled by snowmobile to drill through frozen permafrost. The soil in Svalbard isn’t as carbon-rich as that in places like Siberia, so scientists don’t know as much about how the exposure and thawing of permafrost will affect the atmosphere.
Permafrost microbes may produce carbon dioxide, methane or possibly both. Yet how much of these compounds they produce—and at what rates—remains unclear. By studying carbon-degrading genes in the microbes, researchers may be able to answer a slew of lingering questions around which genes control the activity of microbes and attenuate in their CO2 production. Predictions of carbon emissions from thawing permafrost are often based on gross carbon levels, but those predictions could be more accurate if they take into consideration the constraints posed by microbes.
For almost every ice-related process known to scientists, a better understanding of microbes and meltwater will improve models of how life in ice will affect and respond to global warming. Researchers in the U.K. are now quantifying the bioalbedo and incorporating the results into models of the Greenland Ice Sheet to determine the magnitude of ice melt caused by microbes. Right now, its contribution remains unclear. “You might have 2 or 3 data points but can’t extrapolate to the entire system,” Foreman explained. For a more accurate idea, researchers simply need more data.
And to better understand the influence of microbes within and beneath melting glaciers, she said, “we need more ice, more material.” Priscu agreed, adding that a typical sample of meltwater from an ice core contains only about 1,000-10,000 organisms per milliliter. In order to collect enough data (e.g., genomic sequences, enzymatic analyses), researchers will need access to more cores removed from some of the planet’s most remote places. "Including microbial research in studies of those cores," Priscu said, "is critical to understanding how microorganisms participate in global systems."
“Arctic and glacier environments are changing rapidly,” said Edwards, the microbiologist from Wales. “The big question now is, how do these microorganisms matter, in a century where we will see a significant increase in climate change, for every human on the planet? Our overriding priority is on understanding this impact.”