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As humans change the Earth’s climate and ecosystems, scientists are looking to Earth’s history to try to predict what will happen as climate changes. To this end, huge ice structures like glaciers act as natural freezers, storing detailed records of past climates and ecosystems, including viruses.
We are a team of microbiologists and paleoclimatologists studying ancient microbes, including viruses, preserved in glacial ice. Together with my colleagues Ronnie Thompson, Virginia Rich, and other researchers from the Ice Core Paleoclimatology Group at Ohio State University, we are investigating interactions between viruses and their environment recorded in an ice core from the Griya Glacier on the Tibetan Plateau.
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By linking the genomes of ancient viral populations to specific climatic conditions preserved in glacial ice, our newly published study provides insight into how these viruses have adapted to Earth’s changing climate over the past 41,000 years.
Reading the history of viral genes
We primarily used metagenomics (a collection of genomes that captures the total gene content of all microbes present in an environmental sample) to reconstruct viral genomes from nine different time intervals within the Gulya ice core. These time ranges span three major cold-warm cycles, providing a unique opportunity to observe how viral communities changed in response to different climatic conditions.
Through analysis, genomes representing 1,705 viruses were recovered, expanding the pool of known ancient viruses preserved in the glacier by more than 50-fold.
Ice lakes and snow-capped mountains in southeastern Tibet. People living downstream from where the ice core was taken have been drinking meltwater from these glaciers for thousands of years.
Only around a quarter of the viral species we discovered shared species-level similarity with viruses identified in around 1,000 metagenomes captured in global datasets to date. Most of these overlapping species also originated from the Tibetan Plateau. This suggests that at least some of the viruses preserved in the Griya Glacier originated in this region, but also indicates the relative paucity of glacial viruses in the available databases.
We set out to use these new reference genomes to “read” their stories.
One of the key findings is that the viral communities are very different between cold and warm periods. The most distinctive viral species communities on the glaciers appeared around 11,500 years ago, coinciding with the major transition from the last glacial period to the Holocene. This indicates that the unique climatic conditions of the cold and warm periods significantly influenced the composition of the viral communities. We believe that these effects are likely due to viruses that were blown in from elsewhere by changing wind patterns and under selection pressure due to temperature changes on the glaciers.
“We dug even deeper to look at how the virus interacts with its host. To do this, we used computer models to compare the virus’ genome with the genomes of other microbes present in this environment. We found that the virus was consistently infectious. FlavobacteriumBacterial lineages commonly found in glacial environments.
They also found that viruses from Gulya Glacier need to “steal” genes to manipulate their host’s metabolism. The viral genome encoded 50 auxiliary metabolic genes related to metabolism, including the synthesis and breakdown of vitamins, amino acids and carbohydrates. Some of these genes were abundant in all nine time intervals studied, suggesting that they help the microbial host cope with the harsh conditions of the glacier surface, thereby improving viral fitness.
Thus, the virus not only infects and kills cells, but also appears to alter the host’s fitness during infection, affecting its ability to survive the extreme conditions of the glacial environment.
Climate change over time
Our findings provide a new perspective on how life in the form of viruses has responded to climate change over tens of thousands of years.
Understanding these ancient interactions represents a unique opportunity for future research in both virology and climate science: by studying how ancient viruses responded to past climate changes, researchers can gain valuable insights into how viruses adapt to ongoing global climate change.
We believe that glacial ice will continue to be a vital resource for understanding the history of Earth’s climate and the life it has supported by providing a long-term record of microbial life and ecosystems in each layer, especially as glacial ice reserves are rapidly declining.
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