The year 2019 was the second-hottest year on record, closing out the warmest decade. As the temperature rises, ice melts, and out come novel viruses. And there are many viruses frozen in ice, ready to spill out.

We know that viruses are frozen in ice: multiple giant viruses have been isolated from the 30,000 year old Siberian permafrost. The results prompted one of the authors, Jean-Michel Claverie, to write that the presence of such ancient viruses

…suggests that the thawing of permafrost either from global warming or industrial exploitation of circumpolar regions might not be exempt from future threats to human or animal health.

A new study of viruses in glacier cores drilled in 1992 from the plateau of the Guliya ice cap (pictured) reveals more new viruses. The ice samples, approximately 520 and 15,000 years old, were subjected to high-throughput DNA sequencing. The results revealed 33 different viral populations. Only four could be assigned to known taxa: three belong to genera within the Siphoviridae (tailed bacteriophages with isosahedral heads, like phage lambda) and one belongs to the Myoviridae (tailed bacteriophages with isosahedral heads, like phage T4). The two ice samples contained both shared and unique viruses.

It was possible to predict the potential bacterial hosts of 18 of the 33 different virus populations. Some of these were present in the ice cores: Methylobacterium, Sphingomonas, and Janthinobacterium. The implication of the finding is that these viruses were actively infecting their hosts in aqueous environments until frozen into a glacier many years ago.

The authors do not report identifying viruses that can infect eukaryotes, but I am sure they are present. Perhaps they are among the viral sequences that could not be classified.

There are only two previous reports of viruses in glacier ice. Nevertheless, more exploration is warranted. As the anthropogenic-enhanced warming of Earth continues, thawing of glaciers (and other ice structures) will release microbes that have been preserved for hundreds of thousands of years. As the authors point out, these specimens will no longer be available for studying past climates on Earth.

It is possible that the ice melt could release viruses that are pathogenic for humans or other animals. I think that it is unlikely that viruses released from ice will infect animals, unless these areas were inhabited with large numbers of hosts. Nevertheless, a study of these frozen viruses could provide insight into virus populations that were circulating tens to hundreds of thousands of years ago. Such information could supply a better understanding of the evolution of contemporary viruses.