Life as we know it is remarkably diverse and adaptive, permitting organisms to gain a toehold in some of the most outwardly inhospitable places on the planet. But it tends to rely on a tidy, predictable array of six nutrient elements, a modest alphabet of basic biology that leaves open the possibility of other combinations making up entirely different kinds of biological activity. Life as we know it, then, might not be all there is—for either terrestrial or extraterrestrial biology.

That possibility looks more promising in the light of a new study describing a bacterium isolated from California's Mono Lake that can use arsenic, which is usually poisonous to life, as one of its key nutrient elements. The microbe can even take up arsenic into its biomolecules, replacing phosphorus as a structural building block in DNA and possibly in energy-carrying molecules such as adenosine triphosphate (ATP) as well. The study appeared online December 2 in Science.

"This is a real breakthrough, a real surprise to me as well," says study co-author Ronald Oremland, a geomicrobiologist with the U.S. Geological Survey (USGS) in Menlo Park, Calif. "We have a new element in the group of six that, at least for this organism, can sustain life." The standard six nutrients are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.

Oremland had previously discovered bacteria in the hypersaline, arsenic-rich Mono Lake that use the usually toxic element in photosynthesis or respiration reactions, but no one had demonstrated the uptake of the element for internal use. He says that he kept running into fellow geomicrobiologist Felisa Wolfe-Simon, the new study's first author, at meetings, and that Wolfe-Simon raised a provocative question: What about arsenic, in the form of the arsenate ion, subbing for phosphate ions inside the cell? After all, arsenic is the downstairs neighbor to phosphorus on the periodic table of the elements, and phosphate and arsenate are chemical cousins. That similarity contributes to arsenic's toxicity—arsenate masquerades as the nutrient phosphate and thus gains access to the body's metabolic system.

Oremland was not initially convinced by Wolfe-Simon's idea. "I looked at her like she was a nutcase," he says. But Wolfe-Simon and her colleagues continued to develop the hypothesis, and earlier this year she joined Oremland at USGS on a NASA astrobiology fellowship.

To look for organisms that could use arsenic as a nutrient, the researchers inoculated sediments from Mono Lake into a growth medium, adding arsenic but not phosphorus. They isolated a strain of gammaproteobacteria called GFAJ-1 that grew in arsenate-rich conditions but did not grow when deprived of both arsenate and phosphate. "It grows better with phosphorus, but it grows just fine with arsenic," Oremland says.

"We kept on saying this can't be real, we must be missing something," he adds. But after a suite of high-tech analyses—x-ray spectroscopy, radioisotope tracers, mass spectroscopy—the researchers found that arsenate was indeed being incorporated into biomolecules, including the backbone of DNA, a slot usually occupied by phosphate.

The thoroughness of the analysis lends weight to the claim, says Dirk Schulze-Makuch, an astrobiologist at Washington State University in Pullman who was not involved in the research. "This is the first time that I've really seen good evidence that this has happened," he says. "You can't really look at one DNA molecule and say, okay, there's an arsenic, there's an arsenic." But the complementary tests can reveal the element's role within the microbes. "If you put this all together you can make a very convincing case," Schulze-Makuch says.

Just why the bacterium has a penchant for arsenic is not yet clear. Perhaps some life-forms evolved in an arsenic-rich environment and later migrated to a more typical region of Earth, where phosphorus is far more abundant. "Life might have been adapted for the use of arsenic and/or phosphorus," Oremland says. "Maybe that's one way to view this, but that's entirely conjecture."

Prior to the study's publication, speculation ran wild across Twitter and the blogosphere and in British newspapers after a NASA press release announced a December 2 news conference "to discuss an astrobiology finding that will impact the search for evidence of extraterrestrial life." One popular blog, kottke.org, stirred up a minor frenzy with the headline "Has NASA discovered extraterrestrial life?"

Some will undoubtedly be disappointed by the answer to that question and by the downright terrestrial nature of the new results. But the research nonetheless has implications for the myriad kinds of life that astrobiologists might someday find in the solar system or beyond. "This study really drives the point home of how adaptive life can be and that we should go out expecting the unexpected," Schulze-Makuch says. "If you look at other places, from the hydrocarbon lakes of Titan to the subsurface ocean of Europa to the deserts of Mars, we really should not underestimate the abilities of life to adapt to these places."