A rotifer and the Alazeya River. Image: Michael Plewka (left), Tatiana A. Visnivetskaya (right)
Some 24,000 years ago, a group of microscopic animals called bdelloid rotifers became frozen into a layer of Siberian permafrost and entered a state of suspended animation. Now, in a microbial version of Encino Man, scientists have discovered that the ancient rotifers could not only be revived after their eon-spanning nap, they could also successfully produce offspring. The mind-boggling discovery “constitutes the longest reported case of rotifer survival in a frozen state” and “is of great interest not only for evolutionary biology but also for practical purposes of cryobiology and biotechnology,” according to a study published on Monday in Current Biology.
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Rotifers are tiny freshwater organisms that can be found all around the world. Their capacity to survive in a frozen state for about a decade was already known to scientists, but researchers at the Soil Cryology Laboratory at the Institute of Physicochemical and Biological Problems in Soil Science in Russia set out to test the limits of their endurance by collecting ice cores from a site near the Alazeya River in northeastern Siberia. “The cores were extracted from a site around 50 meters from the river bank,” said Stas Malavin, a researcher at the Soil Cryology Laboratory who co-authored the new study, in an email. “The depth at which the core used for the isolation was extracted is well above the river water level, as those relic permafrost sediments, called 'yedoma', actually form permanently frozen hummocks that the river cuts through.”In a previous study, Malavin and his colleagues had demonstrated that nematodes, a type of hardy roundworm, could be revived after at least 30,000 years in a frozen state, so they were already well-aware that extremophiles could slumber in permafrost for tens of thousands of years. “Bdelloid rotifers are known for their ability to enter cryptobiosis in response to different adverse events like drying or freezing of the environment (and also starvation and low oxygen content),” said Malavin. “In fact, together with tardigrades, the ‘water bears,’ they are among the toughest animals on the planet known to date. Thus, considering also the previous finding of nematodes, we were expecting to once find a bdelloid rotifer in our samples.”
The team used radiocarbon dating, along with the depth of the core layer, to estimate that this permafrost froze some 24,000 years ago, during an era when mammoths still wandered the Siberian wilderness. Microbes are not able to move vertically through ice or ice-cemented ground, according to the study, which means that these ancient organisms are as old as the permafrost they were found in.Dozens of rotifers were found in the samples; an analysis of their gene sequences revealed that these elders belong to the genus Adineta, which still has extant lineages. As the tiny aniamls were thawed back in the laboratory, many of them became active again, and astonishingly, some were able to asexual reproduce in a process called parthogenesis. A new generation of clone offspring were thereby brought into the world by parents 24,000 years older than them. Clearly, rotifers have evolved special adaptations that enable them to withstand extreme conditions over tens of thousands of years. But these biological superpowers are not well-understood, which is why Malavin and his colleagues plan to follow-up on their new discovery with further analysis of the rotifers, in addition to studies of other extremophiles.“We're going to study some of the mechanisms involved in the cryptobiosis of our strain,” Malavin said. “And also we're now eager to isolate from the permafrost an alive tardigrade—or show it's not possible—and to develop a technique allowing us to find single dormant organisms directly in the sample and film their recovery.”Unraveling the mysteries of cryobiosis in these tough creatures will shed light on their amazing abilities, but this research also has implications beyond the world of microbes. In science fiction, cryopreservation offers an opportunity for characters to dramatically extend their lifespans so that they can voyage across the universe and effectively travel through time. While these visions are a long way from reality, Malavin said that his team’s work with frozen microbes presents an exceptional opportunity to investigate the basic foundations of long-term cryopreservation in a human context.“The next step in research would be to investigate the physiological/biochemical mechanisms that allow these organisms to survive during cryptobiosis, especially such an extremely long one,” he said. “Those mechanisms may then be applied to cryopreservation of cells, tissues, and organs of human importance, and also the organisms during interplanetary voyages.”“This is an actively developing research field with much already known and much remaining to be investigated,” Malavin concluded, noting that “organisms ‘stored’ in permafrost represent a natural experiment that we can't replicate due to its extreme duration, and thus organisms isolated alive from permafrost potentially represent the best models for cryobiology research.”