Cryogenics, gene popsicles and the oldest life on Earth

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While the notion of “cryogenic freezing”, or putting a person into a state of frozen suspension, has been a common theme in science fiction for decades (think the Alien movies, Star Wars, Sleeper, Vanilla Sky) bacteria have probably been doing their own version of cryogenic sleep for billions of years.

Researchers studying ice cores from the Dry Valleys of Antarctica have found viable, frozen bacteria that are thousands to millions of years old. The ice in this region of the Dry Valleys ranges from modern to about ten million years old, making it some of the oldest known ice on earth. By analyzing the ice crystal structure and isotopic data, these researchers determined their ice samples had likely been permanently frozen (i.e., no thawing/refreezing), implying that the bacteria encased within the ice have been trapped since its formation.

Resuscitation

The scientists incubated meltwater from ice core samples at temperatures just above freezing for up to 300 days, adding supplemental nutrients to encourage bacterial growth. The samples they incubated represented a broad range of timescales, with ages ranging from 10,000 years to 8 million years. Astoundingly, bacterial growth was observed in all samples, though growth rates declined with sample age: bacteria that had been encased in ice for shorter periods of time grew much more rapidly than bacteria frozen for millions of years.

Caveats to cryogenic

The study concluded that even bacteria cannot maintain cryogenic preservation forever. In addition to slower growth rates for older bacteria, the study found an exponential decline in the size of the community DNA pool over time, suggesting the DNA is slowly degrading, even in a deep freeze. Very slowly. The estimated half-life for the reduction in DNA pool size (i.e., the amount of time it takes to reduce the amount of DNA in a sample by 50%) was 1.1 million years. (I think I just heard the microbial ecologists breathe a collective sigh of relief.) So, it may be perfectly reasonable to find frozen bacteria that are hundreds of thousands of years, even a couple million years old, that can still be resuscitated.

{An aside: why does DNA degrade, even in a deep freeze? The jury is still out, though one suspect in the present study is cosmic radiation (high-energy particles that bombard the Earth from space). Antarctica receives the highest levels of incoming cosmic radiation on the planet.}

Gene popsicles in a melting world

Bacteria encased in ice for thousands to millions of years are literally a gene bank. Collectively, the community DNA frozen in ice can be thought of as a “gene popsicle” that provides a snapshot into the past and another clue scientists can use to piece together ancient Earth environments. Moreover, it is well known that bacteria are capable of transferring genes amongst each other in a process known as lateral gene transfer. Could the periodic melting of ice sheets, due to shifts from glacial to interglacial periods, result in an influx of ancient genes into modern bacterial communities? Could genetic information perhaps be preserved for hundreds of millions, or even billions of years, through freezing, melting, and re-uptake of ancient genes by living bacteria?

And finally, the million dollar question: what are the implications for of melting gene popsicles on present-day Earth? As glaciers and ice sheets across the world continue to melt due to climate change, will hordes of ancient bacteria start to “wake up”? Could they plague the world with ancient diseases that no modern humans have resistance to? (Hmm…sounds like a good idea for a science fiction story 🙂 The answer to the former question is, probably yes, the latter, probably not. But time, and a lot more research on the microbial ecology of melting ice sheets is needed to answer these questions.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0702196104)

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