Bacteria living in Mono Lake, in California, can live off arsenic, an element usually toxic to life.

ASHLAND: Astrobiologists have discovered a bacterium that can live entirely on arsenic - even incorporating toxic element into its DNA - which will impact the search for evidence of extraterrestrial life.

In a much-anticipated NASA press conference, researchers revealed the discovery, part of the Halomonadace ae family of proteobacteria, that were found to thrive on arsenic by chemically altering the element, according to Paul Davies, a physicist at Arizona State University in Tempe and a co-author of the study published in the journal Science.

The new finding opens up the likelihood, said Davies, that there exists an undiscovered shadow biosphere on Earth - that is, a second 'genesis' or lifeform not related to all known life.

Arsenic instead of phosphorous

Life as we know it thrives on phosphorous - an element essential in DNA and ATP, a molecule of energy transfer in cells. But arsenic, which is chemically similar to phosphorus, is poisonous for most life on Earth.

The bacteria found at the bottom of California's Mono Lake are able to swap arsenic for phosphorus. "These organisms didn't just adapt to tolerate [the arsenic]; they incorporated it into their vital machinery," Davies said.

No one had ever seriously searched for life that was different from the carbon-based, DNA-centred type about which we have long known - until recently.

Arsenic incorporated into DNA

Felisa Wolfe-Simon, a NASA astrobiologist, has been studying these life forms at Mono Lake, a lake known to be rich in arsenic.

According to Davies, Wolfe-Simon grew the bacteria in colonies using a growth medium that was entirely devoid of added phosphorus, the samples instead containing what would normally be lethal amounts of arsenic.

Using radio-tracers, the team studied the arsenic in the bacteria until it had replaced all of the phosphate, including that which had existed in the bacteria's DNA.

Did early life depend on arsenic?

Eventually, the bacteria grew without needing phosphate, an essential building block for all known life on earth, making it unlike anything scientists have ever seen.

"Phosphorus was hard to access on the early Earth because it was generally locked up in rocks and not very soluble," said Davies. "Arsenic, although less abundant overall, was more readily available in the oceans, where early life evolved."

Researchers have speculated that ancient life might have originally evolved with arsenic, leading them to believe that some arsenic life forms still existed in arsenic-rich environments, such as Mono Lake.

"New branch on the tree of life"

"This is a new branch on the tree of life," said Davies. "[But] not a new tree."

The finding, according to Davies, is an intriguing bridge in the gap between known life and that shadow biosphere which might not only exist on earth, but on other planets as well.

Added Chris McKay, an astrobiologist at NASA's Ames Research Center: "I think this is very important for astrobiology. It's not a second genesis of life but the first example of what we can really call an extreme life form in an extreme environment … There is the first example of a microorganism that has really changed itself at a fundamental biochemical level in order to live in an extreme environment. Very interesting indeed."