Above 20,000 feet in the Himalayas, yaks are one of your only transportation options. It takes dozens of the furry beasts to help you move an entire glacier core back down to the freezer truck, each carrying about 40 feet-worth of segments of long-frozen ice on its back. “We have to have a whole herd of yaks,” says Lonnie Thompson, a paleoclimatologist and glaciologist at Ohio State University who has gone on several such expeditions to drill and retrieve ice cores. “Yaks are kind of like cats, they have their own idea of what should be done,” he tells Popular Science. But if you can wrangle them (with the help of a Tibetan Whistler), then you’ve retrieved a slice of ancient ecological history.
Inside you might uncover something never documented before–for instance, the preserved genomes of 1,705 virus species, going back as far as 41,000 years. A study published August 26 in the journal Nature Geoscience presents and analyzes exactly that. From an ice core collected from Guliya Glacier on the Tibetan Plateau, Thompson and a team of interdisciplinary co-researchers have discovered and cataloged about 50 times more viral information than we’ve ever previously had from glaciers before. The findings are a window into Earth’s climate past, and could help us better understand the microbial future we’re headed towards.
Viruses shape the world
Oftentimes when you see headlines about frozen viruses, it’s about the human risk of pathogens revived from permafrost–as in 2016, when it’s thought that soil thaw spurred an anthrax outbreak in Siberia. Thankfully, the 1,705 genomes retrieved from the Guliya core are all from viruses of prokaryotes; they infect bacteria and archaea, not humans, animals, or even plants.
Yet still the ancient microbes once played a critical role in their local environment, dictating diversity and evolution from the bottom up, through processes like selection pressure and viral-mediated gene transfer, says Matthew Sullivan, a co-senior study author along with Thompson, and a microbiologist at OSU. Viruses can even shape the ecological metabolism of an ecosystem (i.e. what compounds are taken up from the environment and which are produced as byproducts, and at what rate), he adds. “Among these glacier archives are viruses that potentially played key ecological roles in the past before freezing,” write the researchers in the study.
“These kinds of data are just so foundational for asking any questions about what Earth looked like previously,” says Sullivan. “Collecting these 1,700 genomes now empowers scientists doing glacial work elsewhere to unlock the stories that are in these other ice freezers, so to speak.” In the present, the long-preserved viral material shows that climate shifts dictate and direct microbial communities.
Climate shapes viruses
The scientists extracted all the DNA they could from nine sections of the 310 meter (~1,016 foot) long ice core, each representing a different time horizon and climactic period. The youngest sample was from as recently as 160 years ago, and the oldest from more than 41,000 years ago. Taking all the DNA from each sample in bulk, they used a process called metagenomic analysis to pick out as many individual virus strains from the resulting genetic soup as they could. They found that the viral community looked significantly different depending on the climatic conditions captured at each depth and time in the ice core. “We saw clear shifts in the viruses that existed under colder climates versus warmer climates,” Sullivan explains.
During cold periods in our planet’s past, the Guliya Glacier viral community tended to revert to a similar–though not identical–makeup. In contrast, each warm period had a completely distinct assemblage of virus species. The most diverse and idiosyncratic mixture of microbes came from about 11,500 years ago, during the major transition from the Last Glacial Stage to the stable and temperate Holocene. That slice of time “shared the least amount of viruses with all the other samples,” says Sullivan. “It’s interesting because this was a big moment in this 41,000-year time period,” says Sullivan. It was “the most dramatic change in climate,” from that timespan, adds Thompson–and so to see it reflected in the viral community indicates just how closely climate and microbes are linked, and how fundamentally climate can sway the course of an ecosystem–from the microscopic viruses infecting microscopic bacteria and upwards to everything else.
A melting museum
Additional analyses revealed the likely hosts of many of the viruses and how the viruses may have been changing those hosts’ functions and metabolic patterns. Finally, the scientists compared their samples with the known viral record and assessed the biogeographic origins of the microbes. They found that most of the identified viruses collected from the ice core–upwards of 70 percent–are unique to Guliya Glacier and haven’t been found anywhere else on Earth. Only 12 percent of the total viruses are also known from outside of Asia, and less than one percent have been documented in non-glacier environments.
The new research is by far the most comprehensive of its kind that we’ve had to date–despite the fact that it’s just one ice core from one glacier. We’re in the very early days of this kind of microbial analysis. But down the line, with more research and more ice cores, a better understanding of ancient frozen viruses could help us better forecast our own future, says Sullivan. “We’re doing a grand experiment in the era of the Anthropocene,” he explains. “These ice signatures give us a little flavor for what [microbial] communities looked like tens of thousands of years back, under different climate scenarios.”
Soon, climatologists may be able to include that information in their assessments and predictions. Seemingly simple questions that have proven difficult to answer like, ‘is a wetland or forest a sink or source of carbon dioxide?’ could be clarified by understanding the link between viruses, bacterial metabolism, and climate, he adds. “Having a genomic resource like this is key to informing our modeling capabilities.” That is, if we manage to collect the data before it’s gone.
Glaciers are uniquely suited to storing and preserving genetic data and biotic material. They are records of everything in the air and environment at the moment they froze. As they melt, that invaluable bank of climate history disappears. “We’re losing viral diversity as we lose ice,” says Sullivan, and we don’t yet know enough to understand what that means. Luckily, Thompson is already headed out on his next collection mission along with dozens of international human colleagues and a herd of yaks, ready to ferry another wave of discovery down the mountain.
