Researchers have discovered an organism that can use arsenic as a chemical building block for life.
A special strain of Halomonadaceae bacteria appears able to use arsenic — an element that is usually toxic to living things — instead of phosphorus, which is normally an essential ingredient in DNA, fats and proteins, according to a study published online Thursday in Science Express.
6 essential elements
Life on Earth typically requires the following elements to make DNA, proteins and fats:
"What I've presented to you today is a microbe doing something different than life as we knew it," Felisa Wolfe-Simon, the study's lead author, said at a news conference organized by NASA.
"We've cracked open the door to what's possible for life elsewhere in the universe.... What else might we find? What else might we want to look for?"
Pam Conrad, an astrobiologist with NASA's Goddard Space Flight Center in Greenbelt, Md., agreed the finding could expand the list of places in the universe where one might expect to find life: "The implication is that we still don't know everything there is to know about what would make a habitable environment on another planet. We have to increasingly broaden our perspective."
Wolfe-Simon, a NASA astrobiology research fellow with the U.S. Geological Survey, and her collaborators collected a strain of the bacteria, called GFAJ-1, from Mono Lake, a salty, alkaline lake with high arsenic levels about 300 kilometres east of San Francisco.
The lake also contains lots of phosphorus, an element that is essential to life because it is a key ingredient in molecules like DNA, fats and proteins.
Arsenic is located directly under phosphorus in the periodic table, meaning the two elements are chemically very similar. Nevertheless, arsenic is usually toxic to most creatures.
Wolfe-Simon proposed that some microbes might be able to adapt to using arsenic in place of phosphorus, and GFAJ-1 was a good candidate because it could naturally tolerate high arsenic levels.
She took the bacteria back to the lab to grow in a petri dish, gradually reducing the amount of phosphorus and increasing the amount of arsenic the organisms were exposed to.
Study co-author Ronald Oremland talks to Quirks & Quarks, Dec. 4 at noon on CBC Radio One
The bacteria continued to thrive, though they were larger and looked different from bacteria grown in the presence of phosphorus. For example, bacteria grown with high arsenic and low phosphorus levels had pouch-like structures inside them that might have caused the size gain.
Wolfe-Simon and her collaborators then ran chemical tests on the proteins, fats and DNA inside the cells and found arsenic in them all. The arsenic in the DNA was in the chemical form necessary to form bonds to carbon and oxygen, leading Wolfe-Simon to propose that the arsenic had taken the place of phosphorus in the DNA's chemical structure.
Paul Davies, a professor at Arizona State University who ran some of the chemical tests, said in a statement the bacteria's ability to use phosphorus and arsenic interchangeably makes it "very peculiar."
But he added: "It falls short of being some form of truly 'alien' life belonging to a different tree of life with a separate origin…. The holy grail would be a microbe that contained no phosphorus at all."
Alien life based on elements that are different from but similar to the essential elements for life on Earth appears frequently in popular science fiction. Life forms that use silicon instead of carbon or sulphur instead of oxygen have appeared in Star Wars, Star Trek, The X-files, Stargate SG-1 and Stargate Atlantis.
Meanwhile, some researchers interviewed in Science said they remained skeptical of the discovery and will be more convinced when the researchers directly show that the DNA, proteins and fats can still function when they are made with arsenic instead of phosphorus.
Steven Benner, a distinguished fellow at the Foundation for Applied Molecular Evolution in Gainesville, Fla., said at the NASA conference that he does believe the results showing the bacteria survive in high arsenic and low phosphorus. But he isn't yet convinced the arsenic is used to build the microbe's DNA, due to the results of experiments on chemical compounds similar to arsenic-based DNA.
"We know that they're relatively unstable," he said at the news conference, adding that they fall apart at room temperature in a matter of minutes.
However, he added that while arsenic-based DNA is believed to be chemically unstable at room temperature, it might be stable enough to be the basis of life in very cold environments such as Saturn's moon Titan, where the average temperature is about –180 Celsius.
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