ARLINGTON, Va.—It may be small, its habitat harsh, but a newly discovered single-celled microbe leads the hottest existence known to science.
Indeed, its discoverers have preliminarily named the roughly micron-wide speck "Strain 121" for the top temperature at which it survives: 121 degrees Celsius, or about 250 degrees Fahrenheit. On the standard stovetop, water boils at 100 C, or 212 degrees F.
Strain 121, however, comes from water at the ocean bottom, from a surreal deep-sea realm of hydrothermal vents. There, heated to extremes by the earth's magma, water spouts forth through leaks in the ocean floor. The pressure of the immense depths prevents such hot water from turning to steam—even as it sometimes emerges at temperatures near 400 C (750 F).
Previously, the upper known temperature limit for life had been 113 C (235 F), a record held by another hyperthermophilic—or extreme-heat-liking—microbe called Pyrolobus fumarii.
Supported by the National Science Foundation's Life in Extreme Environments program, Derek Lovley and Kazem Kashefi of the University of Massachusetts, Amherst, conducted the research. Their NSF project may also yield clues to the formation of important ore deposits, the remediation of toxic contaminants, and more efficient recovery from petroleum reserves.
Announcing the Strain 121's record-breaking ability to beat the heat in the August 15 issue of the journal Science, they write, "The upper temperature limit for life is a key parameter for delimiting when and where life might have evolved on a hot, early Earth; the depth to which life exists in the Earth's subsurface; and the potential for life in hot, extraterrestrial environments."
As scientists ponder the kinship of hyperthermophiles to the origins of life on this planet and to the possibility of life on others, Strain 121 carries on in a habitat that seems a collaboration of Dr. Seuss, The Outer Limits and Jules Verne.
The sample cultured by Lovley and Kashefi was collected about 200 miles offshore from Puget Sound and nearly a mile and a half deep in the Pacific Ocean by a University of Washington team led by biological oceanographer John Baross.
Baross's crew, also supported by NSF, used a remotely operated submarine to retrieve it from the Mothra hydrothermal vent field of the Pacific's Juan de Fuca Ridge, a lightless seascape where vents called "black smokers" rise up like three- and four-story chimneys and continuously spew a blackening brew laced with iron and sulfur compounds. The neighborhood is called the Faulty Towers complex.
(For more on this collection effort and the vent field, see the accompanying sidebar.)
While suffocating, crushing, scalding, toxic and downright abysmal by most living standards, the arrangement is not so bad for Strain 121 and its ilk. They are archaea, single-celled microbes similar to, but not quite, bacteria. They often live amid extreme heat, cold, pressure, salinity, alkalinity, and/or acidity.
Archaea literally means "ancient," and Lovley and other biologists tend to call them "deep branchers" because these microbes were among the first branches on the "tree of life."
According to Lovley, Strain 121—it will be given a species name after his lab finalizes the microbe's description—uses iron the way aerobic animals use oxygen.
"It's a novel form of respiration," Lovley says, explaining how Strain 121 uses iron to accept electrons. (Many archaea also use sulfur.) As oxygen does in humans, the iron allows the microbe to burn its food for energy. Chemically, the respiration process reduces ferric iron to ferrous iron and forms the mineral magnetite.
The back end of this equation—magnetite—is what first attracted Lovley to the project. The presence of vast deposits of magnetite deep in the ocean, its presence as a respiratory byproduct of some archaea, and the abundance of iron on Earth before life began all led Lovley and Kashefi to write that "electron transport to Fe (III) may have been the first form of microbial respiration as life evolved on a hot, early Earth."
In their cool, modern lab, the researchers tested the process with Strain 121 cultures kept at 100 C in oxygen-free test tubes.
"It really isn't technically difficult. You just need some ovens to get it hot enough—and remember not to pick it up with your bare hands," he says, speaking from experience.
They fed the microbes formic acid (the same simple acid released by some ants), which provided the electrons for their iron to accept. Using a microscope that focused ultraviolet light on the culture and gathered fluorescent light back from it, they watched its cell count increase. With X-ray diffraction analysis, they confirmed the formation of magnetite, which, they also noted, did not form in the organism's absence. (Simply placing a magnet alongside the test tube also revealed magnetite's formation.)
They sampled Strain 121's DNA sequences and found it indeed to be a member of the archaea, with a greater than 95 percent match to two species of Pyrodictium, a genus known to survive to 112 C and known to develop centimeter-wide colonies of individual cells connected by a matrix of tiny tubes.
Most startling, they discovered that Strain 121 grew at temperatures from 85-121 C (185-250 F). (Meanwhile, Pyrolobus fumarii, the former top-temperature record-holder, wilted. After an hour at 121 C, only 1 percent of its cells were intact and none appeared viable.)
"Growth at 121 C is remarkable," report Lovley and Kashefi in Science, "because sterilization at 121 C, typically in pressurized autoclaves to maintain water in a liquid state, is a standard procedure, shown to kill all previously described microorganisms and heat-resistant spores."
Not only did Strain 121 survive such autoclaving, its population doubled in 24 hours at such heat and pressure. While they could not detect growth at higher temperatures, the researchers found that cultures that spent two hours at 130 C (266 F) still grew when transferred to a fresh medium at 103 C (217 F), with each new single-celled member appearing like a tiny tennis ball filled with cytoplasm and covered with about a dozen whip-like flagella.
What gives this organism its unparalleled ability to stand the heat?
"That's the big question," says Lovley. Perhaps to cool the air of distinction, he adds, "It is restricted. It can only grow on iron." And it perishes when temperatures dip below 80 C (176 F).
While the big question lingers on one burner, his lab is cooking on something else.
"We're trying to grow something that's even hotter," he says, not yet ready to discuss the ingredients, recipe or proverbial Petri dish.
Additional expert: John Baross; professor, Department of Oceanography; (206) 543-0833, firstname.lastname@example.org
Background resources, related news available on the web:
NSF's Life in Extreme Environments Program: The LExEn research program will explore the relationships between organisms and the environments within which they exist, with a strong emphasis upon those life-supporting environments that exist near the extremes of planetary conditions (includes program announcement, workshop report and lists of research awards). http://www.nsf.gov/home/crssprgm/lexen/start.htm
Related previous NSF news releases:
"Hydrothermal Vent Systems Could Have Persisted for Millions of Years, Incubated Early Life" - July 24, 2003:
The staying power of sea-floor hydrothermal vent systems like the bizarre Lost City vent field is one reason they also may have been incubators of Earth's earliest life, scientists report in a paper published in the July 25 issue of Science....
"Researchers Uncover Extreme Lake -- and 3000-Year-Old Microbes -- in Mars-Like Antarctic Environment" - December 16, 2002:
"Scientists Find Underground Environment on Earth That Supports Ancient Life Forms" - January 22, 2002 (news tip):
"NSF-Funded Researchers Discover Evidence of Microscopic Life at the South Pole" - July 6, 2000:
"Limits of Life on Earth: Are They the Key to Life on Other Planets? New NSF Grants to Foster Answers" - October 10, 1997:
"Introduction to the Archaea - Life's extremists" (University of California Museum of Paleontology, a recipient of NSF funding).
NOVA's "Into the Abyss" (PBS Online): A mile and a half beneath the sea off the Pacific Northwest coast, a volcanic ridge has given birth to towering structures that spew toxic, superheated water. The structures, known as black smoker chimneys, are home to bizarre life forms that thrive far beyond the reach of the sun's light. Follow the daring attempt of an ambitious expedition to retrieve several of these black smokers from the seafloor.
Pacific Marine Environmental Laboratory's Vents Program (National Oceanic and Atmospheric Administration): Researching the effects of underwater hydrothermal venting systems.
Black Smoker Expedition (American Museum of Natural History):
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