No, NASA has not found life on another planet, but has found life here on Earth that is almost “alien” to our narrow, phosphate-based view of life. Scientists have discovered — or “trained,” actually — a type of bacteria that can live and grow almost entirely on a poison, arsenic, and incorporates it into its DNA. This “weird” form of life, which can use something other than phosphorus — what we think of as a basic building block of life — is quite different from what we think of as life on Earth. It doesn’t directly provide proof of a “shadow biosphere,” a second form of life that lives side-by-side with other life on our planet, but does suggest that the requirements for life’s beginnings and foundations may be more flexible than we thought. This means life elsewhere in the solar system and beyond could arise in a multitude of conditions.
“Our findings are a reminder that life-as-we-know-it could be much more flexible than we generally assume or can imagine,” said Felise Wolfe-Simon, lead author of a new paper in Science. “If something here on Earth can do something so unexpected, what else can life do that we haven’t seen yet?”
The salt-loving bacteria, strain GFAJ-1 of the Halomonadaceae family of Gammaproteobacteria,came from the toxic and briny Mono Lake, near Yosemite Park in California. The lake has no outlet, so over millennia has become one of the highest natural concentrations of arsenic on Earth.
Although the bacteria did not subsist entirely on arsenic in the lake, the researchers took the bacteria in the lab grew it in Petri dishes in which phosphate salt was gradually replaced by arsenic, until the bacteria could grow without needing phosphate, an essential building block for various macromolecules present in all cells, including nucleic acids, lipids and proteins.
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. Arsenic had completely replaced phosphate in the molecules of the bacteria, right down its DNA.
“Life as we know it requires particular chemical elements and excludes others,” said Ariel Anbar, a biogeochemist and astrobiologist from Arizona State University. “But are those the only options? How different could life be? One of the guiding principles in the search for life on other planets, and of our astrobiology program, is that we should ‘follow the elements. Felisa’s study teaches us that we ought to think harder about which elements to follow.”
Wolfe-Simon added, “We took what we do know about the ‘constants’ in biology, specifically that life requires the six elements CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur) in three components, namely DNA, proteins and fats, and used that as a basis to ask experimentally testable hypotheses even here on Earth.”
The idea that arsenic might be a substitute for phosphorus in life on Earth, was proposed by Wolfe-Simon and developed into a collaboration with Anbar and theoretical physicist and cosmologist Paul Davies. Their hypothesis was published in January 2009, in a paper titled “Did nature also choose arsenic?” in the International Journal of Astrobiology.
“We not only hypothesized that biochemical systems analogous to those known today could utilize arsenate in the equivalent biological role as phosphate,” said Wolfe-Simon “but also that such organisms could have evolved on the ancient Earth and might persist in unusual environments today.”
This new research is the first time that shows a microorganism is able to use a toxic chemical to sustain growth and life.
