Antarctic ice loss reflected in octopus DNA

Various subpopulations of an octopus species (right, © Dave Barnes, BAS) occur in the coastal waters around Antarctica (left, © Rainer Lesniewski/iStock).

Populations that are now separated by the ice were apparently once connected: Researchers have discovered evidence of a collapse of the West Antarctic ice sheet during a warm phase around 120,000 years ago in the genome of an Antarctic octopus species. What is worrying is that it is assumed that average global temperatures at that time were only about one degree Celsius above pre-industrial values. The results confirm previous fears that the loss of West Antarctic ice and the resulting rise in sea level could be triggered by even moderate increases in temperature, say the researchers.

It is probably the clearest sign of global warming: the ice masses on our planet are visibly shrinking and this is becoming a danger to humanity: due to the influx of gigantic amounts of meltwater, global sea levels are rising and this threatens flooding in some densely populated coastal regions. In addition to the ice masses in the north, the worried gaze is also directed at the icy south. The West Antarctic ice sheet is considered particularly vulnerable to rising temperatures. It is a kilometer-thick mass of ice that lies partly on the mainland of West Antarctica and also extends into the adjacent sea.

Did the ice sheet once disappear?

According to studies, it is already becoming apparent that climate change is eating away at the West Antarctic ice sheet. But how this could develop further in the wake of climate change is unclear. However, it seems clear that there is great potential for danger if the gigantic ice masses continue to melt: If the West Antarctic ice sheet were to be completely lost, sea levels would rise by around three to five meters. Some studies also show that this could happen. The tipping point that leads to a complete loss of ice masses may even lie within the current global climate targets of 1.5 to 2 degrees Celsius of warming.

As evidence of this, there is geological evidence that the West Antarctic ice sheet may have melted during the last interglacial period, 129,000 to 116,000 years ago. According to estimates, the average global temperature at that time was only about one degree Celsius above pre-industrial values. Whether the ice sheet had actually disappeared is still controversial, as there were also study results that contradicted this scenario.

Eight-armed witnesses to the loss of ice?

In order to provide new clues to this geological-climatic question, researchers led by Sally Lau from Australia's James Cook University in Townsville have now pursued an approach that may initially seem surprising: They looked for traces of possible ice loss in the genetic history of an octopus -Species that occurs in the marginal seas around Antarctica. The crux of the matter is that the populations of Pareledone turqueti in the Ross Sea and the Weddell Sea are now geographically isolated from each other by the ice masses of the West Antarctic Ice Sheet and its foothills. Does the species' genome perhaps reflect that this was not the case in the past?

To answer this question, the researchers sequenced the genomes of a total of 96 specimens of the Pareledone octopus that were caught in various marginal seas in Antarctica. They examined the animals' genetic makeup for so-called single nucleotide polymorphisms (SNPs) - characteristic changes in just one DNA base that can be used to distinguish population groups. First of all, it was shown that the subpopulations in the different areas of the ocean actually differ genetically in a characteristic way. But between the now separated octopuses of the Ross Sea and the Weddell Sea, the researchers found striking signs of mixing in the genome.

The team then conducted bio-geographic modeling to shed more light on what might have been the most plausible explanation for this gene flow in the species' history. Mixing through an exchange of individuals via the circum-Antarctic current does not fit the mixing patterns found in both octopus populations. Instead, the data suggests that there was once direct contact between the octopuses of both ocean regions. Specifically, the researchers conclude: The octopuses mixed through a direct marine connection between the Ross Sea and the Weddell Sea, which apparently formed in the last interglacial period. In other words, the genetic finding is clear evidence that the West Antarctic ice sheet melted during the last interglacial period.

Is the same thing happening now?

This now raises a critical question: Can the process that once led to the loss of the ice still be averted? The researchers write: “Our results provide evidence that the tipping point for the loss of the West Antarctic ice sheet could be reached even under strict climate protection scenarios.” If global warming is plus 1.5 to 2 degrees Celsius above pre-industrial levels, meltwater from the West Antarctic ice sheet could contribute several meters to sea level rise in the coming centuries, scientists say.

In order to shed light on possible developments, further research is now necessary, write Andrea Dutton from the University of Wisconsin–Madison and Rob DeConto from the University of Massachusetts in Amherst in a comment on the study. According to them, it should be clarified when exactly and under what conditions the melting occurred. For example, it is also conceivable that changes in ocean currents rather than air temperature led to the process.

Source: Science, doi: 10.1126/science.ade0664

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