It is life, but not as we know it. The first life form on Earth to use deadly arsenic as one of its chemical building blocks has been discovered in a desert lake in California.
The find has astonished biologists who had believed that it was impossible for arsenic, normally considered a poison, to be used as building material for biological molecules such as DNA and proteins.
Scientists said last night that the discovery widens the scope for finding "weird" forms of extraterrestrial life on other planets that would have been considered – until now – to be too inhospitable for living organisms to evolve. For decades it has been accepted dogma in biology that all life on Earth, from the tiniest microbe to the biggest blue whale, relies on the same six chemical elements as the fundamental building blocks of biological molecules.
It was thought that the same principle must hold true for any extraterrestrial life forms living on other planets, which narrowed the search for habitable places where life could evolve.
Now, researchers in the United States, including Nasa-funded astrobiologists involved in the search for extraterrestrial life in space, have added the element arsenic to the list of six involved in the chemistry of life: oxygen, carbon, hydrogen, nitrogen, phosphorus and sulphur .
The arsenic-using microbe is known only by its code name, GFAJ-1, and was extracted from the muddy sediments of California's Mono Lake, which is naturally salty with high concentrations of arsenic.
Tests have demonstrated that the bacterium uses arsenic as a substitute for phosphorus, which is a neighbouring element on the Periodic Table and shares similar properties.
Although it grows better in a phosphorus-rich environment, the microbe appears happy to use arsenic when necessary in the form of arsenate, a molecule formed with oxygen that acts as a substitute for phosphate, the scientists said. Felisa Wolfe-Simon, of the Nasa Astrobiology Institute and US Geological Survey at Menlo Park, California, said: "Our findings are a reminder that life as we know it could be much more flexible than we generally assume or can imagine.
"Yet, this story isn't about arsenic or Mono Lake. If something here on Earth can do something so unexpected, what else can life do that we haven't seen yet?"
The study, published in the journal Science, reports that arsenic in the form of arsenate can make the appropriate chemical bonds with carbon and oxygen to act as a substitute for the phosphate molecule normally found in DNA and other biological molecules.
The scientists said that sophisticated analytical techniques have shown that the arsenic was incorporated into proteins, lipids, nucleic acids and other vital molecules found inside living cells. Arsenic was not simply an innocent bystander but an active element in the life-giving processes of the cell.
The discovery has vindicated a hypothesis that Dr Wolf-Simon put forward at a scientific meeting in 2006 when she suggested that primitive life forms in the early evolution of life on Earth may have used arsenic, and that similar microbes may still exist in some extreme environments.
"Such organisms could have evolved on the ancient Earth and might persist in unusual environments today," Dr Wolf-Simon said.