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World’s Hottest Microbe: Loving Life in Hell
By Nancy Touchette

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Move over, Pyrolobus fumarii. A new entry for the record books has just been discovered. The hottest organism known to man has been isolated from a thermal vent deep in the Pacific Ocean.

The Finn thermal vent lies within the Faulty Towers complex along the Juan de Fuca Ridge in the Pacific Ocean.

The previous record-holder, P. fumarii, could live at temperatures as high as 113 °C (235 °F), well above the boiling point of water. But the new microbe, for now called “Strain 121,” thrives at 121 °C and can even survive for two hours at 130 °C.

The new organism is also unusual because it relies on iron to digest food and produce energy. Such organisms show promise in generating electricity from waste products and in removing radioactive metals from the environment.

“No one had ever seen a bug like this before,” says Derek R. Lovley of the University of Massachusetts in Amherst, who along with colleague Kazem Kashefi reported their discovery in Science. Researchers believe that many high-temperature microbes rely on iron to grow, but none had ever been isolated or cultured until now.

“The trick was to grow it in the presence of iron,” says Lovley. Many underwater structures are rich in metals and the microbes that live there are likely to use things like iron in their metabolism, he says.

Strain 121 is a type of extremophile, a microorganism that thrives in the harshest environments of the planet—from hot springs, volcanoes, and thermal vents at the bottom of the ocean floor to the frozen lakes of Antarctica.

Scientists are interested in extremophiles because they grow in environments that are thought to resemble those of other planets or a primordial earth and may give clues to how life began.

In addition to their possible use in cleaning up toxic wastes and in generating energy, the microbes may also yield useful substances and pharmaceuticals with commercial and technological applications, such as heat-resistant enzymes that can be used in detergents.

Strain 121 was isolated from an underwater chimney recovered by John A. Baross of the University of Washington in Seattle. A portion of the chimney, a “black smoker” called Finn, was removed from a vent on the Juan de Fuca Ridge, which lies two miles below the ocean’s surface about 250 miles off the coast of Washington.

To remove the sample, Baross and his crew ventured out into the Pacific and sent an unmanned submarine to the ocean floor. With a remotely controlled chainsaw, the researchers cut about a meter from the top of Finn’s chimney.

A thin section through strain 121 reveals its single-layer envelope (S) and cytoplasmic membrane (CM).

Working quickly, the researchers transferred the chunk of rock into an oxygen-free container. From that sample, Lovley was able to culture strain 121.

Baross has examined a cross section of the chimney sample and looking from the outside surface to the interior, he sees a diverse populations of microbes.

“It’s likely we will find microbes that can grow at even higher temperatures,” says Baross. “I can’t venture a guess as to how high that might be, but 121 degrees will probably not be the highest. We see all kinds of microbes growing in the hot interior.”

Populations on the outside of the chimney are a mixture of bacteria and a primitive life form called archaea. Within the interior of the chimney, the hottest region, archaea are the sole survivors. Archaea, which gave rise to bacteria and multi-celled organisms, are the most ancient branch of organisms in the tree of life.

Lovley is most interested in how strain 121 uses iron to metabolize food. Most organisms break down organic matter and transfer electrons to a network of molecules inside the cell, which in the end produces carbon dioxide.

Strain 121, in contrast, appears to deliver electrons to iron molecules on the exterior of the cell. To Lovley, this presents some exciting possibilities.

“This type of microbe transfers electrons from the inside to the outside of the cell,” says Lovley. “This creates an electron flow. In principle you could use these bugs to generate electricity from organic matter.”

Lovley says the bugs can also be used to transfer electrons to radioactive waste metals such as uranium. When this occurs, the uranium drops out of the water supply and can be recovered from the soil. Such a process could be useful in cleaning up contaminants from nuclear reactors.

Clusters of microbes found within a hot interior section of the Finn black smoker.

Scientists are also intrigued as to how strain 121 and other extremophiles can survive such harsh conditions. Most biological compounds such as DNA, proteins, and the lipids that form the cell’s outer membranes, are degraded at much lower temperatures. Lovley is studying the lipids in strain 121 to figure out how its membrane survives temperatures above the boiling point of water.

Alexei Slesarev, who studies high-temperature extremophiles at Fidelity Systems in Rockville, Maryland, says the key to their survival may be the unusually high concentrations of salt and ions inside the cells.

“Some of these high-temperature organisms have salt concentrations 10 to 20 times higher than normal cells,” says Slesarev. “That means there is very little free water present in these bugs. This could stabilize DNA and many of the other molecules in the cells and may be a key to their survival.”

While others work out the details of how these extraordinary microbes survive in such extreme environments, Baross revels in the thrill of discovery.

“We are going after the most incredible organisms that exist,” he says. “This gives us the chance to look at the most ancient, primordial events and understand how life began and to think about the possibility of extraterrestrial life. It’s really exciting science.”

Baross’ interest in ocean life grew out of the tales of high-sea adventures he heard from his grandfather, who left Italy and worked as a cabin boy at sea for ten years before arriving in the United States.

“When I was a kid, I sat on my grandfather’s lap and he told me unbelievable stories about giant squid and sharks and whales,” says Baross. “It turned out that most of his stories weren’t true, but it got me hooked on life in the sea.”

To view a movie of the thermal vents in the Faulty Towers complex, click here.

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K. Kashefi and D.R. Lovley. Extending the upper temperature limit for life. Science 301, 934 (August 15, 2003).
M.O. Schrenk et al. Incidence and Diversity of microorganisms within the walls of an active deep-sea sulfide chimney. App. Env. Microbiol. 69, 3580-3592 (June, 2003).

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