Heat-Loving Microbes Offer Clues to Life's Origins

John Roach
for National Geographic News
May 26, 2004
Over the past 20 years scientists have warmed up to the idea that the majority of life on our planet lives not on Earth's surface but beneath its crust. The theory has spurred new ideas about life's origins on Earth and where to look for life on other planets.

Earth's crust gets warmer the closer it is to the molten iron-nickel believed to be at our planet's core. One question that scientists who study life beneath Earth's crust face is, at what temperature is it too hot for life to survive?

Since scientists believe Earth at one point was mostly molten, the answer to the question may shed light on how early life could have first evolved on our planet.

"If Earth had to cool to a certain temperature at which life was possible, maybe the high-temperature life could have existed that much sooner," said Derek Lovley, a microbiologist at the University of Massachusetts, Amherst.

Much of this life beneath the crust, which scientists refer to as biomass, are microbes that use hydrogen and minerals like iron to get energy from food sources in the same way that humans use oxygen to obtain energy from our food.

Lovley is at the forefront of research into such microbes. He has discovered dozens of different species, including Strain 121, a microbe that grows at 121° Celsius (250° Fahrenheit)—the highest temperature currently known for life.

The ability to grow at 121° Celsius is significant because for over a century it has been the temperature used to sterilize medical equipment. Scientists thought that such temperatures would kill all life-forms.

"It's kind of a benchmark," Lovley said. "This is like breaking the four-minute mile."

Strain 121, which goes dormant at temperatures below 80° Celsius (176° Fahrenheit), lives in environments known as hydrothermal vents on the ocean floor. The vents spew hydrogen- and mineral-rich hot water from deep in the Earth's crust to the surface.

For several years scientists have known that other microbes survive in and around hydrothermal vents at temperatures above 100° Celsius (212° Fahrenheit). Strain 121 just "opens that window where life can exist a little bit wider," Lovley said.

Jack Farmer, an astrobiologist at Arizona State University in Tempe, said that opening this window for life on Earth expands the potential for life to develop and persist elsewhere in the solar system and beyond.

"As the upper temperature limit for life has increased, new opportunities for habitable environments have opened up, and subsurface hydrothermal environments are among the most important," Farmer said.

"Poor Man's Drill"

John Delaney, a marine geologist at the University of Washington in Seattle, led the expedition that brought to the surface the chunk of hydrothermal vent from which Strain 121 was isolated.

Delaney said that examining such environments gives researchers a snapshot of what life is like deeper in the Earth's crust, where temperatures are higher. "Our way of doing it was a 'poor man's drilling program,'" he said.

The expedition team used a remotely operated submarine to cut out and bring to the surface a chunk of hydrothermal vent from the Juan de Fuca Ridge, which lies about 200 miles (322 kilometers) offshore from Washington's Puget Sound and nearly 1.5 miles (2.4 kilometers) deep in the Pacific Ocean.

The seafloor at the Juan de Fuca Ridge is cold, about 2° Celsius (36° Fahrenheit). But down beneath the seafloor the temperature warms gradually until, eventually, it is scalding hot.

"If you telescope those conditions by having hot water coming out along a fissure it will build a sulfide chimney," Delaney said. "And this sulfide chimney is very cold on the outside—two or three degrees—but on the inside it might be as much as 300° centigrade."

A chunk of one of these chimneys, or hydrothermal vents, is what Delaney and his team brought to the surface.

"We figured we would see different kinds of microbes in the wall as it got to hotter and hotter temperatures, and [that] pretty soon microbes wouldn't be there … [which would] indicate a limit to life under those conditions," he said.

Limits and Origins

Microbes like Strain 121 that live in environments lacking organic carbon are known as archaea, which literally means "ancient." Archaea are genetically different from seemingly similar bacteria, which need organic matter and photosynthesis to survive.

The discovery of Strain 121 bolsters the theory held by some scientists that Earth's first life-forms were archaea that could thrive at high temperature via chemical reactions with hydrogen and iron.

"They appear to be the branches closest to what is the last common ancestor of existing life," Lovley said. "All are hyperthermophiles that live at high temperatures."

Early in Earth's history, according to Delaney, volcanic eruptions occurred on the ocean floor as the planet's core separated from its crust. These eruptions could have allowed the mixing of hydrogen and minerals like iron and sulfur, upon which microbes could thrive.

"That may be one of the paths the origins of life takes," Delaney said. If that's the case, he added, then studying hydrothermal vents is a step in the process of understanding how the dynamics of such a system might work.

And understanding how such a system works on Earth may help in the search for life on other planets.

Farmer, the Arizona State University astrobiologist, said, "At the bottom line, hydrothermal systems were widespread in the early solar system and are thought to still be present in the subsurface of many other solar system objects, like Mars, Europa, and even the interiors of large asteroids."

So perhaps the question for scientists isn't is there life on other planets, but is there life inside them.

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