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| Extremophiles, Antarctica, and Extraterrestrial Life | |||||||||
| By Kate Dalke
July 25, 2003  |
Extremophiles are the ultimate adventurers. These organisms thrive where other microbes don’t dare venture: boiling water holes, freezing lakes, and toxic waste dumps.
Now, researchers have sequenced the genomes of two extremophiles that love life extremely cold. They live at the bottom of Ace Lake in Antarctica, where there is no oxygen and the average temperature is a brutal 33 degrees Fahrenheit. The two organisms, called Methanogenium frigidum and Methanococcoides burtonii, produce methane and are known as methanogens. Methanogens are unique among organisms in their ability to survive a wide range of temperatures, from the freezing point of water to 185 degrees Fahrenheit and everything in between. In a new study, scientists sequenced the genomes of M. frigidum and M. burtonii and compared their genomes with those of heat-loving methanogens to identify features that may help these microbes adapt to their cold surroundings. Some of these hardy organisms also live in oxygen-starved environments, without sunlight or carbon, and scientists believe that studying these microbes could reveal the boundaries of extreme environments that support life here on Earth and on other planets.
To answer questions about cold-loving methanogens, Australian biologists made several expeditions to a unique coastal ecosystem in Antarctica called the Vestfold Hills. The Vestfold Hills is one of the only places in Antarctica that is not covered with a permanent blanket of snow. There, a research outpost called Davis Station—built in 1957 by the Australian government—serves as a scientific haven for explorers. The largely ice-free oasis harbors many saltwater lakes. When glaciers retreated thousands of years ago, they trapped relic salt water in depressions that are now high-salinity lakes such as Ace Lake. The lake is covered with ice for several months of the year. Ace Lake is also full of methane, which led researchers to speculate that methanogens live at the bottom. Scientists drilled holes through the ice and isolated M. frigidum and M. burtonii from a depth of over 20 meters.
Through years of painstaking work in the laboratory, researchers kept these organisms alive outside their natural environment. Both survive without oxygen or sunlight, and M. frigidum is extremely slow growing. “During the three years I studied [M. frigidum], I thought I’d lost it a number of times,” says Peter D. Franzmann of CSIRO Land and Water in Australia, who isolated the microbes from Ace Lake. “I’d think, Bugger, I’ve lost it! And then there would be a nervous wait to see if it was growing,” he says. Now, over a decade after the organisms were isolated, scientists have unveiled the genome sequences of M. frigidum and M. burtonii—the first cold-adapted archaeans to be sequenced. Archaea is an ancient domain of life that is separate from bacteria, plants, and animals. The most significant finding is that both microbes have flexible proteins, which allow their cells to survive cold temperatures and carry out basic cell functions under extreme conditions. These proteins are more rigid and stable in bacteria that live at higher temperatures.
In M. frigidum, the researchers also identified cold-shock proteins that are not found in heat-loving archaeans. Cold-shock proteins are known to help other organisms, such as bacteria, adapt to cold environments. The researchers looked for broad trends in the composition and structure of proteins to find features that are involved in cold adaptation. “The focus of the project was not to do an exhaustive appraisal of genes specific to each organism,” says Ricardo Cavicchioli of the University of New South Wales in Sydney, Australia. Cavicchioli spearheaded the entire project. M. frigidum was sequenced by the Australian Genome Research Facility at the University of Queensland in Australia, and M. burtonii was sequenced by the U.S. Department of Energy’s Joint Genome Institute in Walnut Creek, California. M. burtonii’s ability to produce methane, a potential source of fuel, interests DOE, which funded the M. burtonii sequencing project. “In addition to our interest in methanogenesis, there is also a basic scientific interest in how [M. burtonii] works,” says Daniel Drell of DOE’s Office of Biological and Environmental Research in Germantown, Maryland. The remarkable flexibility of M. frigidum and M. burtonii allows these microbes to occupy some of the most extreme niches on this planet. M. frigidum has been found living in deep-sea trenches that are similar to the cold, stable ecosystem in Ace Lake. So what if Earth isn’t the only place these kinds of microbes live? Some scientists speculate that methanogens could provide clues to life on other planets, such as Mars, and Europa (Jupiter’s sixth moon).
Evidence suggests that beneath the icy surface of Europa, there may be subsurface oceans that could support extremophiles like M. frigidum. The Antarctic lakes of the Vestfold Hills and their hardy inhabitants may, in some way, resemble the environment on Europa. Other research suggests that some methanogens could survive life on Mars. Scientists at the University of Arkansas in Fayetteville have grown methanogens in Mars-like soil and under Mars-like conditions. After the Viking voyages to Mars in the 1970s turned up no trace of life, as we knew it, some scientists dismissed the idea of Martian life. Twenty years later, with the discovery of organisms that can survive without oxygen, carbon, or sunlight, researchers are rethinking the boundaries of what environments may support life.
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