Viruses from the past trapped in glacier ice

It has been known since the beginning of the last century, that glaciers store in their depths microorganisms from thousands of years ago.

The study of ice samples collected using core drills has allowed determining the environmental conditions of different ages, based on the characteristics of the bacteria they contain. In some cases, bacteria found in ice samples were still alive, and it was possible to reactivate their metabolism to understand which nutrients they used, which molecules they produced, and in which optimal conditions they lived.

While several studies have been done about bacteria trapped in the glaciers, information about viruses is still scarce. This is due both to the very low concentration of viral particles in this kind of sample (about 1000 fold lower than in seawater) and to the fact that is technically more difficult to identify viral nucleic acids than bacterial DNA.

Recently, a group of researchers from Ohio State University, University of Nebraska, and Lawrence Berkeley National Laboratory has isolated bacteria and viruses from two ice samples collected at two different depths in the Guliya ice cap (Tibet, China). Using modern sequencing techniques, the researchers were able to determine 33 different viral sequences trapped in the ice 500 and 15000 years ago; comparison with the sequences of 2304 known virus, has allowed a partial classification.

The genetic material found in the samples belongs to different populations of bacteriophages, viruses that infect bacteria, and the bacterial target populations of some of them were predicted through bioinformatics analyses. The frequency of the different types of bacteriophages found in the two samples from different ages correlates with the distribution of the bacterial species: in each of the two samples, the most abundant bacteria are indeed the target populations predicted for the dominant bacteriophages.

Even if it was not possible to determine the target bacteria for half of the isolated viruses, the results of this study confirm that viruses, as well as other environmental conditions such as temperature, radiations, and chemical concentrations, play an important role in shaping the ecosystems.