For decades, doctors and dentists have assumed that the 121°C water used to sterilize their tools was hot enough to kill any bacteria. But now scientists have discovered an amazing new microbe that thrives and multiplies at this temperature and can survive in water as hot as 130°C. Fortunately for patients, the microbe is a harmless inhabitant of hydrothermal vents.

For 2 decades, the temperature limit for known organisms has hovered around 110°C, despite concerted efforts by scientists to find a record-breaking bug whose molecular machinery could stand up to more heat. Because the previous record holder used sulfur rather than oxygen for its respiration, most researchers focused their search on sulfur-consuming bugs. But microbiologists Derek Lovley and Kazem Kashefi of the University of Massachusetts, Amherst, noticed that most high-temperature bugs were using iron for respiration at least part of the time. So they went looking for iron-metabolizing microbes in the superheated water flowing from a deep-sea vent in the Pacific Ocean. They put the vent water into culture tubes along with iron oxide and found a bug that was reducing the iron oxide and multiplying at 100°C. When they began heating the water to find the new microbe's temperature limit, they were stunned to find that the organism kept growing in water as hot as 121°C and survived in 130°C water. "It was pretty exciting," Lovley says. The team is now trying to figure out just what makes the bacteria so heat resistant.

The discovery of this heat-loving iron reducer opens a new realm of high-temperature microbes to search for. It's also possible that the first living organisms may have been iron reducers similar to this new microbe and could have arisen earlier than previously thought, as Earth slowly cooled after its formation. Lovley suspects that the temperature ceiling may be even higher, and he plans to keep hunting. The researchers report their find in the 15 August issue of Science.

"This is the first really significant jump in maximum temperature in a very long time," says microbial geochemist Jan Amend of Washington University in St. Louis, Missouri. "By increasing the temperature at which life can exist, they've expanded the habitable zone to greater depths," he says. "The biosphere is significantly greater than we once thought." The new finding also expands the habitable zone for other planets, making the possibility of extraterrestrial life even more tantalizing, Amend says.

Derek Lovley's research

Jan Amend's research

PHOTO (BLACK & WHITE): Heat lover. A newly discovered bacterium (round object, left) thrives at temperatures that would kill any other living thing.

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By Betsy Mason