“Time-travelling” pathogens: science but no fiction | Polarjournal
Given the rapid warming in the Arctic, the days of permafrost soils such as here on Svalbard seem numbered. Dormant within them are countless microorganisms, including potential pathogens that can be released as the soils thaw. (Photo: Julia Hager)

Thawing permafrost soils pose an immense source of ancient pathogens. Researchers have now concluded that such “time-traveling” pathogens pose a potential threat to present-day biological communities, as well as to humans.

Releasing methane, another potent greenhouse gas, is only one of the problems associated with permafrost thawing. Another, non-negligible side effect may be the reappearance of pathogens that had been frozen for hundreds of thousands of years without contact with the surface world. A new international study has now concluded that these ancient pathogens escaping from permafrost may harm modern microbial communities and may even threaten human health. The study appeared in the journal PLOS Computational Biology.

While in science fiction novels and movies the ideas about the potentially catastrophic effects of newly emerged or escaped pathogens have been depicted in great detail, sometimes somewhat exaggerated, consequences to be expected in reality have been more difficult to assess.

The research team, led by Giovanni Strona, a scientist at the European Commission’s Joint Research Centre, therefore investigated the effects of digitally simulated virus-like pathogens from the past on communities of bacteria-like hosts in extensive artificial evolution experiments.

The digital communities under the influence of pathogens in a control group and an invasion group (upper part of the figure) at different times. The small coloured squares represent free-living organisms (digital hosts), pathogens are shown as coloured circles. Each large square around a community represents a moment in the experiment. The development of both groups, each starting from a single digital ancestor (a), initially proceeded in the same way. Only from the moment when a time-travelling pathogen (b) was injected into a future community (c) did the two groups evolve differently. In the lower part of the figure, the variations in species diversity are shown (green – control group, purple – invasion group). (Figure: Strona et al. 2023)

The computer simulations enabled the researchers to compare the impact of the invading pathogens on the diversity of host bacteria with the diversity in control communities with no invasion. This allowed them to quantify the ecological risks posed by the pathogens.

They concluded that the old invading pathogens were usually able to survive and establish themselves within the modern community, which can occasionally result in significant ecological changes. Although it is only a very small proportion of old pathogens (0.03%) whose invasion led to unpredictable results, in the worst case there was a significant reduction in host community diversity of about 30% compared to the control group. On the other hand, it also happened in the experiment that one pathogen would increase diversity by up to 12 %.

The team of authors describes their findings as worrisome, even though the risk of time-travelling pathogens re-emerging seems very low. However, it is these rare events that pose a real risk, with serious ecological consequences and a danger to human health, given the huge number of microbes that would be released.

The danger posed by these pathogens, which have been trapped for long periods of time, is significant for communities living today in part because of the lack of shared evolutionary history and thus shared adaptation between modern species and ancient pathogens. This would require extended overlapping in time and space.

Julia Hager, PolarJournal

Link to the study: Strona G, Bradshaw CJA, Cardoso P, Gotelli NJ, Guillaume F, Manca F, et al. (2023) Time-travelling pathogens and their risk to ecological communities. PLoS Comput Biol 19(7): e1011268. . https://doi.org/10.1371/journal.pcbi.1011268

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