Eight million years on ice?

The oldest, coldest bacteria

Posted: August 07, 2007

Chunks of glacier dug up from Antarctica have revealed a startling cargo, Rutgers University scientists announced yesterday: bacteria that had apparently lain dormant in the ice for up to eight million years.

Despite their badly degraded DNA, some of the ancient microbes were able to reproduce after being warmed up in a New Brunswick laboratory, and their genetic code has offered a snapshot of the distant past.

The team said its findings, published in the online edition of Proceedings of the National Academy of Sciences, could help tackle a wide range of scientific questions, from the threat of antibiotic-resistant bacteria to the mysteries of life on Mars.

Also on the horizon is the question of what would happen if glaciers, continuing to melt as a result of global warming, were to release DNA from long ago into the oceans.

Eight million years is by far the oldest estimate for microbial life frozen in ice, and yesterday's report is being met with some skepticism. Yet the research draws on a broad range of evidence from geology and microbiology, winning admirers such as John C. Priscu, a microbial ecologist at Montana State University.

"They are good scientists," said Priscu, who was not involved with the study. "They stick their necks out. But if you're a person working in this field, it fits other new findings."

Barely a decade ago, this line of research was nonexistent. Ice was thought to be virtually devoid of life, and thus Antarctica - with 70 percent of the world's fresh water locked in its frozen grip - was assumed to be a barren landscape. No more.

Previous studies on Antarctic ice cores have found bacteria several hundred thousand years old. Some of the microorganisms in the Rutgers study were from that era, too; the scientists analyzed a range of samples from different periods to calculate how fast DNA degrades - the result of many centuries of exposure to cosmic radiation.

The ice chunks were dug up with ice axes and other tools in the late 1990s by David Marchant, a Boston University earth sciences professor who was using the samples to study geology. When the Rutgers team asked him if it could look for bacteria in his ice, he agreed to collaborate.

The samples were shipped to New Jersey packed in insulated boxes with dry ice. To guard against contamination, scientists dipped the chunks in a bath of ethanol and dry ice, then melted away the outermost inch or two of ice by rinsing it in sterilized water.

They then added nutrients to the melted water and incubated it in the lab, thereby growing an ancient version of Arthrobacter - a type of bacterium common in modern dirt.

The experiments were done by Kay Bidle, a Rutgers assistant professor of marine and coastal sciences, along with SangHoon Lee, a visiting scientist who has since returned to his native South Korea. Bidle codirected the project along with Rutgers professor Paul Falkowski.

Some scientists have speculated that, as glaciers continue to melt, the release of ancient microbes could be harmful to modern organisms.

But Bidle called that a remote possibility, as the recovered bacteria were not the kind that cause disease. Nor were numerous fragments of DNA from a variety of other microbes in the samples.

Rather, Bidle said, some of the genes the team recovered from the fragments might someday prove useful in fighting disease.

Among the genes were some that enabled the ancient bacteria to resist certain natural antibiotics that were present in the environment at the time.

That genetic material might allow scientists to work backward and develop new antibiotics for bugs that have developed resistance to today's drugs, Falkowski said.

Ordinarily, bacteria are constantly evolving in response to changing conditions in their environment - a process that plays out quickly because they can reproduce in as little as a half-hour. But the Rutgers samples had remained unchanged.

"This type of very rapid evolution has been frozen for eight million years, literally and figuratively, in these ice samples," Falkowski said.

Still unclear is how the bacteria stayed alive for so long. One possibility is that they remained dormant. Another is that they sustained themselves with a low level of metabolism, enabled by the presence of thin films of water that remain unfrozen even within a glacier, said Brent Christner, a Louisiana State University biologist who studies Antarctic bacteria.

Another possibility would be that the bacteria are not eight million years old after all - either because the age of the ice was miscalculated or newer microbes contaminated the ice samples.

The scientists took rigorous steps to guard against these problems. In addition to the careful handling in the Rutgers lab, Boston University's Marchant determined that there was no intrusion of modern bacteria at the site in Antarctica - an environment believed to resemble, in some ways, the surface of Mars.

Estimates for the age of the ice were calculated using several methods, including one that involved dating nearby rock samples.

Certain isotopes of helium and neon are formed when cosmic rays interact with minerals in rock. The more of these isotopes that is present in a sample, the older the rock and thus the nearby ice, Marchant said.

Whatever its age, the continued discovery of icebound life echoes a similar shift decades ago. Scientists used to think the deep sea was also devoid of life, Montana State's Priscu said. No longer.

"We're seeing history repeat itself here," he said, "in one of our last unexplored frontiers."

Contact staff writer Tom Avril

at 215-854-2430 or tavril@phillynews.com.

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