Scientists have discovered an arsenic eating bacterium, which opens up new areas of exploration for extraterrestrial life.
A report published today in the journal Science claims it's the first microorganism found that can sustain life using a 'toxic' chemical.
The research, led by NASA astrobiologist Dr Felisa Wolfe-Simon of the US Geological Survey, isolated a bacterium strain called GFAJ-1, which uses arsenic instead of phosphorus.
The samples of GFAJ-1, which is a member of the Halomonadaceae family of proteobacteria, were found in the toxic, briny waters of Mono Lake, California.
"We know that some microbes can breathe arsenic, but what we've found is a microbe doing something new - building parts of itself out of arsenic," says Wolfe-Simon.
Life is mostly composed of six elements; carbon, hydrogen, nitrogen, oxygen, sulphur and phosphorus.
These elements are complemented by trace amounts of other elements that serve critical cellular functions, such as enzyme co-factors.
Occassionally, trace elements can be substituted; tungsten for molybdenum, cadmium for zinc, and in some cases arthropods and molluscs copper substitutes for iron as an oxygen-carrier.
But there have been no prior reports of substitutions for any of the six major elements.
Although chemically similar to phosphate, arsenic is highly toxic to living organisms because it disrupts metabolic pathways.
The chemical similarity between the two contributes to the biological toxicity of arsenates (the salts or esters of arsenic) because metabolic pathways intended for phosphate can't distinguish between them.
Wolfe-Simon's team grew the bacteria in Petri dishes gradually replacing the usual phosphate salt with arsenic.
Eventually the bacteria could grow without needing phosphate at all. They didn't grow as quickly, but they did continue to grow.
Using radio-tracers, the team closely followed the path of arsenic in the bacteria; from the chemical's uptake to its incorporation into various cellular components.
They found it had completely replaced phosphate in the molecules of the bacteria, right down to its DNA.
Professor Malcolm Walter from the Australian Centre for Astrobiology at the University of New South Wales says it's an amazing discovery that changes the rules of how life can survive.
"We've all been taught since we were students that phosphorus is an important element in all life. It's an integral part of the genetic molecules DNA and RNA as well as proteins and other compounds," says Walter.
"Nobody ever imagined there could be life without phosphorus, but now apparently there is.
Walter says these bacteria still need liquid water, but it widens the places we can search.