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Thursday, Feb. 21, 2002
Living under pressure
By ROWAN HOOPER
Life, as we knew it only a few decades ago, needed sunlight and warmth. No one imagined that anything could survive in extreme environments -- in intolerable places such as high-pressure, high-temperature deep-sea vents or under Antarctic ice sheets.
That was before the discovery, in 1977, of extremophiles, a special class of organism that lives in extreme conditions. Scientists thought that being an extremophile was like being an astronaut: To qualify, you've got to have the right stuff. But now scientists have shown that even common, ordinary bacteria, such as Escherichia coli (which is present in the human stomach), can survive under extreme conditions. In this study, that meant immense gigapascal pressures, 16 thousand times that of atmospheric pressure at sea level.
That life can exist under extreme conditions encourages those looking for extraterrestrial life, and might help us recognize the value of life on our own planet.
Anurag Sharma and James Scott, both at the Geophysical Laboratory of the Carnegie Institution of Washington, led a team that adapted the tools of high-energy physics to microbiology. In lab experiments they subjected two "ordinary" (i.e., not extremophile) bacteria species -- E. coli and Shewanella oneidensis -- to pressures equivalent to those about 50 km beneath the Earth's crust, or a hypothetical ocean depth of 160 km. Their paper is published in today's issue of Science.
"This is a very high-pressure condition for biology," said Sharma, who is an experimental geochemist. "Since [under this pressure] liquid water turns into a solid, high-pressure ice even at room temperature, these conditions are typically considered inhospitable."
But the bacteria survived and were found to be viable when the pressure was returned to normal.
"Our results," said Scott, who is a microbiologist, "raise important questions about the evolution of life."
The work also illustrates the value of collaboration. "This is what happens when two scientists with very diverse backgrounds get together and test the validity of past assumptions," said Scott.
The work was supported by the NASA Astrobiology Institute, which investigates the conditions necessary for extraterrestrial life. Sharma and Scott's study suggests that pressure is not as much of a hindrance to life as was thought. Deep water or ice structures thought to exist on Jupiter's moons Europa, Callisto and Ganymede might also be environments that could contain life.
"Soon the only thing that should limit our investigation of the survivability of life on Earth and beyond is our imagination," said Scott.
However, according to Norman Pace, University of Colorado at Boulder professor of molecular, cellular and developmental biology, we might need extensive use of our imagination if we are going to find extraterrestrial life in this solar system.
Pace gave a lecture titled "Molecular Perspectives of Extreme Life," at the 2002 American Association for the Advancement of Science meeting in Boston, which ended Tuesday. An expert on extremophiles, Pace said that the chances of finding primitive life on Mars or Europa are not high.
"If you look at what is required for life, it really is a narrow window," Pace said. "Our solar system outside Earth doesn't seem too promising to sustain life, but we don't know what sort of extreme conditions conducive to life may be found elsewhere in the universe."
Elsewhere in the universe, more and more planetary star systems are being discovered all the time. Astrobiology is advancing, driven by our increased understanding of the diversity of habitats on Earth. The key to abundant and diverse life on the surface of Earth, said Pace, is photosynthesis.
"Life has changed the surface of Earth dramatically," he said. "If life is really going to succeed and flourish for an extended period, I think it has to take over and modify a planet on the surface, like it has on Earth." A primitive microorganism deep below the surface of a frozen moon is unlikely to do that.
So where does that leave us? Complex life is rare, and, according to Pace, primitive life might not even be found in our solar system.
But Scott and Sharma, in e-mail interviews, are more hopeful. "Our study shows that we have to seriously re-evaluate the limitation put on the viability of life," said Sharma. "Even microorganisms that we do not consider extremophiles are capable of functioning beyond previously defined habitable limits."
His opinion? Sharma says he wouldn't be surprised if microbial life is discovered on another planet.
"It will surely make us feel very small and insignificant. Maybe that will help us preserve and understand the fragility of life on our planet."
Scott agrees. "The biggest threat to our survival as a species is intolerance. At the root of all fanaticism (racism, fascism etc.), is fear and ignorance and a desire to shield yourself from some great 'unknown.' "
Another paper published this week, in the Proceedings of the National Academy of Sciences, highlights why an understanding of this fragility is so urgently needed. Wes Sechrest and colleagues at the University of Virginia have found that the greatest concentration of all primate and carnivore evolutionary history exists in only 25 biodiversity "hotspots." The hotspots cover just 1.4 percent of Earth's land but contain more than 60 percent of total terrestrial biodiversity. These hotspot species -- whose combined evolutionary ages total 2.6 billion years -- represent genetic lineages vital to future biodiversity and evolution.
"We are facing double jeopardy," said Gustavo Fonseca, co-author of the paper. "Not only are we in danger of losing species, but we are facing the loss of their legacy."
Given that our first sight of extraterrestrial life, if it ever happens, is more likely to be down the barrel of a microscope than down the barrel of a laser gun, our attention might better be turned to sorting out the problems facing life on Earth.
E-mail Rowan Hooper at firstname.lastname@example.org