While researchers discovered the unusual bacteria here on Earth, they say it shows that life has possibilities beyond the major elements that have been considered essential.
"This organism has dual capability. It can grow with either phosphorous or arsenic. That makes it very peculiar, though it falls short of being some form of truly 'alien' life," commented Paul C. W. Davies of Arizona State University, a co-author of the report appearing in Thursday's online edition of the journal Science.
Six major elements have long been considered essential for life - carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.
But the researchers found that the bacteria, discovered in Mono Lake, Calif., is able to continue to grow after substituting arsenic for phosphorous.
"It makes you wonder what else is possible," said Ariel D. Anbar of Arizona State University, a co-author of the report.
The find is important in the search for life beyond Earth because researchers need to be able to recognize life, to know what life looks like, Anbar said.
The study focuses on a microbe found on Earth. However, the announcement of a news conference to discuss it, which did not disclose details of the find, generated widespread speculation on the Internet that the report would disclose the discovery of extraterrestrial life. It didn't.
The discovery "does show that in other planetary environments organisms might be able to use other elements to drive biochemistry and that the 'standard' set of elements we think are absolutely necessary for life might not be so fixed," commented Charles Cockell, professor at the Planetary and Space Sciences Research Institute, Open University, in Milton Keynes, United Kingdom. Cockell was not part of the research team.
"This work is novel because it shows the substitution of one element for another in fundamental biochemistry and biochemical structure," added Cockell.
It wasn't a chance discovery.
Felisa Wolfe-Simon of NASA's Astrobiology Institute, who led the study, targeted Mono Lake because it has high arsenic levels.
Arsenic and phosphorous are chemically similar, so she speculated that a microbe exposed to both might be able to substitute one for the other.
"Arsenic is toxic mainly because its chemical behavior is so similar to that of phosphorus. As a result, organisms have a hard time telling these elements apart. But arsenic is different enough that it doesn't work as well as phosphorus, so it gets in there and sort of gums up the works of our biochemical machinery," explained Anbar.
The researchers collected the bacteria known as GFAJ-1 and exposed it to increasing concentrations of arsenic, which it was able to adapt to and grow.
The microbe does grow better on phosphorous, but showing that it can live with arsenic instead raises the possibility that a life form using arsenic could occur naturally, either elsewhere on Earth or on another planet or moon where arsenic is more common.
Jamie S. Foster, an assistant professor of microbiology at the University of Florida, said the idea that arsenic could be substituted for phosphorous isn't new, but there has never been example where it was shown to work.
Arsenic was more common in the early times on Earth, she said, so researchers have speculated that early life forms might have used it.
"It does suggest that that there could be other ways to form life, not just how life formed on early Earth," said Foster, who was not part of Wolfe-Simon's research team.
The research was supported by NASA, the Department of Energy and the National Institutes of Health.