Thawing microbial reservoirs – is another epidemic on the horizon?

Kate Walter

3 days ago

Introduction

The cryosphere, which comprises Earth’s glaciers and permafrost, has long been thought to be devoid of life. However, new evidence shows that it serves as a frozen archive, preserving both living and dead organisms from the past. The cryosphere has experienced significant consequences of global warming, and it is estimated that some regions have lost 39% of their glaciers and 25% of permafrost cover ¹. As these ice reserves thaw, ancient bacteria and fungi, previously dormant, are reanimated and threaten to contaminate the planet’s ecosystems, raising concern about potential outbreaks of disease. ². Furthermore, it is proposed that the increase in recent epidemiological events is closely linked to the destruction of natural barriers (such as permafrost) — for example, the exposure of human populations to new pathogens due to encroachment into areas previously uninhabitable.

Cryosphere Environments

The structure of glaciers allows microorganisms to persist within veins in the ice, as well as on the basal organic layer. Water-filled boundaries along the grains of ice contain nutrients, allowing psychrophilic (cold-loving) bacteria and viruses to persist. The glacier floor however, which moves over soil and grinds organic matter into the ice, contains carbon compounds and is typically warmer. This provides an environment suitable for a wide range of organisms, including aerobic chemoheterotrophs, anaerobic nitrate reducers, sulfate reducers, and methanogens which have been found to exist in this niche. ³ While some of these microbes are dead, others lie dormant and retain the ability to ‘reactivate’ when exposed to less hostile conditions, such as when the glacier melts.

Permafrost covers nearly 11% of Earth’s surface and is found in regions such as the Arctic, Tibet, North America, and Russia. Defined as ground or water that remains frozen for more than two years, it is highly vulnerable to global warming due to the release of carbon as it thaws. The water comprising some of these bodies can be dated back to the Middle to Late Pleistocene, 700,000 years ago, thus suggesting that microbes from this period are still present and at risk of release ⁴. Permafrost provides an environment with low temperatures and little oxygen, which is effective at preserving organisms by preventing natural decay. However, when these areas thaw, the threat of microbial reactivation is increased.

Emerging Bacteria And Viruses

The 2016 outbreak of anthrax in Siberia caused the death of one human and thousands of reindeer. This occurred on the Yamal Peninsula in Northern Siberia, resulting from a heatwave that melted permafrost in the region, releasing dormant Bacillus anthracis spores from a reindeer that had died decades earlier and been preserved in the frozen ground. ⁵. The soil of the peninsula is frozen to 1000 ft deep, thereby harbouring a vast range of potentially disease-causing microorganisms capable of reactivating once thawed. Scientists suggest that outbreaks such as these could occur every summer as more carcasses are uncovered, with the risk of exposure to older pathogens that the modern human immune system may struggle to combat increasing as more ice recedes. Plague and smallpox are examples of diseases expected to emerge as more permafrost melts, raising concerns about the threat to populations living in these areas and wildlife that could transmit infection over a wider geographic range.

Although viruses require a living host to replicate, some are able to persist in their frozen hosts or simply remain preserved in the ice. Because viruses are so ubiquitous, they are often found in many organisms — this means that cryogenically preserved individuals are likely to harbour viruses long after their time of death. A study in Alaska was successful in reviving H1N1 avian influenza using RNA collected from samples in permafrost, still armed with neuraminidase, the enzyme on the surface of influenza viruses responsible for infection of host cells, suggesting ability to cause infection ⁶.

This supports the theory that areas previously exposed to a disease can act as reservoirs which may release large amounts of microorganisms when melted. While not infectious to humans, Pithovirus sibericum was uncovered in Siberia and shown to infect and kill amoebas. This again demonstrates the ability of 30,000-year-old microbes to threaten life today and raises questions on the likelihood of other pathogens emerging which could infect humans.

Glaciers have been shown to contain a range of genes encoding antibiotic resistance (ARGs), which have allowed some reactivated bacteria to grow in cultures of high-dose antibiotics. This includes a strain of Staphylococcus hominis with genes identical to more than 96% of ARGs for antibiotics such as beta-lactams, aminoglycosides and phenicols. If these bacteria are released from glaciers in meltwater, they could devastate ecosystems while remaining invulnerable to current antibiotics and risking spread of resistance. Multiple studies performed in other sites have found ARGs encoding resistance to tetracycline, glycopeptides and macrolides, which enabled Escherichia coli to grow in antibiotic medium when cloned into the bacteria ⁷. It is possible these ARGs can be passed between bacteria within the host..

Moreover, the range of pathogenic bacteria surviving in glaciers was highlighted when a team investigated Yulong Mountain in China where they found 441 species isolated from snow, meltwater, and ice, many of which increased in abundance towards the end of summer. This included Mycobacterium tuberculosis, Clostridioides difficile, Vibrio cholerae, Staphylococcus aureus and Klebsiella pneumoniae, all of which are highly pathogenic and transmissible ⁸. The Baishui Glacier on Yulong Mountain flows into the Yangtze River, which is a major water supply for Southern China, therefore the possible release of these bacteria could impact up to 400 million people.

Threats To Populations

The thawing of glaciers and permafrost will see vast numbers of microorganisms drain into areas populated by humans and animals, which poses a risk for the transfer of ancient diseases. While these pathogens themselves can cause harm, it is also possible for their genes to transfer to existing microbes by the process of horizontal gene transfer. This may result in superbugs formed from today’s microbes inheriting genes for resistance and infectivity from ancient pathogens. This has been recorded in the Arctic, where the drainage of permafrost meltwater has allowed carbon-fixing genes from land-based actinobacteria to transfer to planktonic Chloroflexi bacteria ⁹.

Often, it is rural communities who live near melting permafrost and glaciers, where infected carcasses are more likely to be uncovered. This may slow transmission of outbreaks due to lower population density, however, as new areas are exposed to warming patterns, this could increase the number of people affected by these events. Disease outbreaks have been a feature of the Canadian Arctic for a long time, but concern about their frequency and severity is rising.

Glacier melt in mountain ranges such as the Alps is filtered and disinfected before being used as a water supply. Although many European facilities use multi-stage systems to ensure that water is safe to drink, there is a chance that they may not be able to cope with the number of microorganisms projected to be released from glaciers. While they are effective against known pathogens, there are concerns regarding their efficacy if this number increases. For populations without access to advanced filtration systems, the threat of compromised drinking water is even greater.

Conclusion

Although the threat to the wider population is not imminent, global warming continues to cause irreversible changes to the Earth which will present us with more challenges in the future. We have already seen evidence of fatal diseases emerging from the melting cryosphere, such as the Siberian anthrax outbreak, and we know that the range of other pathogens harboured in these environments is vast. The potential for ancient genetic material to be shared with existing microbes is also a cause for concern, potentially creating more resistant and infective bacteria. In order to combat this threat to public health, we must address the warming of our planet to prevent further thawing of these microbial reservoirs and invest in new methods of treatment.

Edited by Samiya Dash

Copy-edited by Cameron McKeddie

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Kate Walter

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