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Heating Up Science with the World’s Oldest Ice

CAS Prof’s Antarctic ice samples reveal ancient microbes, new answers

When most people open their freezers, they may grab a few fresh ice cubes for a drink. When David Marchant opens his freezers, he’s probably reaching for an eight-million-year old block of Antarctic ice — a research tool that has already provided new insight into ancient forms of life and could offer more answers about interplanetary exploration.

Marchant, a College of Arts and Sciences associate professor of earth sciences, has been traveling to Antarctica since the 1980s and has more than 400 pounds of the world’s oldest ice stored in secure freezers in his department. Now, several of the glacial ice samples that he and his students took from the Dry Valleys of the Transantarctic Mountains are revealing that the DNA of ancient microorganisms, long frozen in glaciers, may return to life as the ice melts — findings published earlier this year in the Proceedings of the National Academy of Sciences.

The genesis of the study came when Marchant lectured at Rutgers on the origin and chronology of buried ice in Antarctica in spring 2004. Afterwards, Kay Bidle, a Rutgers assistant professor of marine microbiology, microbial ecology, and biogeochemistry, and the study’s lead author, and Paul Falkowski, a Rutgers professor of biochemistry and biophysics, became interested in studying the biology of the ice. “Because the ice is so old and because of the conditions in that particular valley, you can start to ask some fundamental questions about whether there are any time limits on how long cells maintain viability,” says Bidle. “What sort of physical factors are in play to reduce or constrain viability over long geologic time periods?”

Marchant packed his ice samples — collected in 1998 on a trip to Antarctica with BU students, and ranging in age from 100,000 to 8 million years old — into crates, kept them chilled with blue ice packs, and sent them via a regular courier service to New Jersey. He spoke with BU Today about ancient ice and its research application.

BU Today: How do you determine how old ice is?
Marchant:
It is very difficult to date ice directly. The approach is to date other materials that rest on top of the ice, or are included in it, and then, through numerical modeling, determine if the ages calculated make geologic sense. For this study, we employed four different dating techniques: first, cosmogenic-nuclide analyses of boulders on top of the ice. Very small amounts of cosmogenic nuclides, or isotopes affected by cosmic rays, build up in the outer surfaces of rocks that are exposed to the atmosphere. If the production rates of these nuclides are known and none are lost during erosion, then an exposure-age can be calculated. Using this technique, we determined that the exposure time for the oldest boulders resting on top of the ice were as much as 7.6 million years, suggesting that the underlying ice was even older. This was our first bit of quantitative data that called for very old ice.

We then numerically modeled the flow of alpine ice and found that the results matched the cosmogenic data with great precision. Then, during the course of our fieldwork, we mapped several deposits of volcanic ash on top of the glacier and dated them — the oldest deposit came in around eight million years old. The final method involved satellite measurements of modern ice-flow velocity. If the distal regions of the glacier were really eight million years old and the system is near equilibrium today, then the ice should be nearly stagnant, with horizontal velocities of less than one millimeter per year. All of these techniques yielded consistent results, with the age of the ice ranging from essentially modern at the valley headwall to as much as eight million years old at its distal end. The end result has been that this glacier system is one of the best dated in Antarctica.

We were curious about the thickness of the ice remaining in this alpine system as well. Our multichannel seismic surveys demonstrated that the ice is around 45 to 100 meters thick in Antarctica’s Mullins Valley and up to 150 meters thick in upper Beacon Valley.

How do you melt ancient ice, extract the microbes and the DNA, and then grow the microbes in the lab?
To monitor bacterial viability, we added a variety of standard microbiology nutrient formulations to decontaminated meltwater and incubated them at four degrees Celsius. We followed their growth over a one-year period by measuring optical density and by direct-staining cells with the DNA-binding stain SYBR Gold, a gel stain that identifies nucleic acid. Both are standard methods to detect microbial growth.

To check for the state of encased DNA, we collected encased microbes via filtration and centrifugation. We then extracted the community DNA using standard molecular biology techniques. We were able to evaluate how degraded the DNA was by measuring its size via agarose electrophoresis, which separates molecules by size by moving them through an electric field of agar, a gel substance derived from algae. Then we measure base pairs, which means the genetic code letters strung together in a line. The young, 300,000-year-old ice had sizes of around 18,000 base pairs, which is very different from the 8-million-year old ice, with sizes of around 210 base pairs. For comparison, a typical bacterium has a genome of three million base pairs.

What do you know about the microbes? They are bacteria, but what type?
We examined the identity of the microbes encased in the ice by sequencing their 16S rDNA genes — this gene has been used for the past 10 years to identify microorganisms in the environment, even if you cannot grow them. The 16S rDNA molecule is an integral component of the ribosomal machinery involved in the making of proteins. It is used to identify all organisms and their relationships to one another. The microbes that we found in both ices were most closely related to common environmental microbes that have been found in various ice and soil environments.

How did the microbes stay alive for so long?
Given that the Antarctic receives the highest cosmic radiation flux on the planet, it is clear that any organisms that did survive had to possess sophisticated DNA repair capabilities. If your DNA gets chopped into small bits, you are in trouble. And given that the average DNA size in the old ice was 200 base pairs, most of the organisms’ DNA was degraded beyond repair, However, a subpopulation was able to maintain sufficiently large DNA sizes to grow. The ice is a time capsule for bacteria, one in which we can plot the pace of bacterial evolution and DNA degradation.

Are there lessons from them that we can apply to other planets?
The climate implications for this ice, and its relevance to Mars and the Phoenix Mission to find water on the planet, are part of other funded studies we’re working on at BU. In addition to housing ancient bacteria, the ice provides a tremendous amount of paleoclimate information. First, its great longevity indicates that surface-air temperatures in the Antarctic Dry Valleys have not been significantly greater during the past eight  million years. Second, gas bubbles trapped within the ice can be measured to shed light on past concentrations of natural “greenhouse” gases. As a point of comparison, the oldest ice within the active ice sheet on Antarctica today is only about 900,000 years old, making the stagnant ice in the Dry Valleys a unique and considerably older paleoclimate archive.

The similarities of this region to certain Martian terrains thought to contain buried ice makes it one of the best proxies that we have here on Earth. Mars also receives very high cosmic radiation, since it doesn’t have a magnetic field. This study has given us a better idea of the physical factors encountered by microbes over long geological time periods and how they impact their potential viability and genetic integrity. It also provides insight into the time frame over which viability is possible.

Some of the articles in the press suggested that as the climate changes, causing the glaciers to melt, the microbes will return to life. Is that true? And if so, what would be the impact on modern organisms?
At the moment we have no idea how resuscitated microbes will impact existing microbial communities. They will surely mix with existing microbial populations and perhaps enhance microbial diversity. We did not find any harmful bacteria in the ice samples, so the possibilities of impacting human health are extremely slim. Some colleagues have talked about the possibility of resuscitating potentially harmful bacteria and viruses — not necessarily harmful to humans, but perhaps to marine organisms — but to our knowledge there is not data to support this.

The other, equally important. issue is the release of ancient DNA and the effect this could have on microbial communities and on the tempo of microbial evolution. Environmental microbes evolve through lateral gene transfer, or the taking up and assimilation of foreign DNA. This genetic information can be assimilated and either be beneficial or deleterious, to be decided by natural selection. Microbes gain new traits and can serve to increase microbial diversity and the rate at which they evolve. They are the only organisms that have such a capability, and we know they are quite good at it.

We emphasize that these processes have undoubtedly happened many times in Earth’s history as glaciers have formed and melted. It is a part of Earth’s natural process.