Biologists have isolated a bacterium that can use a deadly chemical in place of one of life’s key building blocks, in a finding NASA says could have major implications for astrobiology and our understanding of life on Earth.
In the study, researchers examined a bacteria living in a very salty and arsenic-heavy lake in northeastern California, not far from Yosemite National Park. It is not a space alien, nor is it “new life” — it’s an existing bacteria that lives in a difficult environment and was deliberately manipulated in a lab.
But the results are interesting because nothing like this has ever been done before. All life as we know it depends on six key ingredients — carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus. This bacteria can switch from phosphorus to arsenic — usually a deadly toxin — and not only survive, but thrive. It can swap arsenic for phosphorus so completely that arsenic is incorporated into its DNA and other biomolecules like ATP, according to the study. This is a first, and it upends our assumptions about how life works.
What this means for astrobiology is pretty speculative, however. When looking for life in other worlds, especially promising places like Saturn’s moon Titan or in the Martian soil, scientists look for telltale signs of life as we know it. That means carbon-based life, respiration with oxygen and carbon dioxide, amino acids, and so on.
This finding tells us that we should ditch these assumptions and broaden our horizons. If a humble Earthling bacteria can live on a poisonous chemical, then who knows what might lurk elsewhere in the solar system? We’ll have to recalibrate our mass spectrometers.
"I find this result delightful because it may have to expand my notion of what environmental constituents might enable habitability," said Pamela Conrad, an astrobiologist at NASA's Goddard Space Flight Center and a principal investigator on the new Curiosity Mars rover, which will carry experiments designed to look for signs of life. "The implication is that we still don't know everything there is to know about what might make a habitable environment on another planet. We have to increasingly broaden our perspective."
In terms of its metabolism, the bacterium — a proteobacteria called GFAJ-1 — is actually not very interesting, according to Felisa Wolfe-Simon, a scientist with NASA's Astrobiology Institute and lead author of the paper released today. It is not a chemosynthetic bacteria, for instance, using chemicals instead of light to produce food. In that way, it's less exciting than well-studied extremophiles that live near superheated hydrothermal vents or the unforgiving sulfur lakes of Yellowstone National Park.
But it's interesting because it's a chemical mutant. In an arsenic-enriched environment in Wolfe-Simon's lab, its very DNA changed. It swapped arsenic for phosphorus in the nucleic acids that make up the backbone of DNA, and that's a revolutionary result, Elser said.
"Every living thing uses phosphorus to build its DNA," Elser said at a press conference Thursday. "The fact that I am sitting here today discussing the possibility that that is not true is quite shocking."
At the very least, that is interesting for our understanding of microbes, Wolfe-Simon said. Microorganisms are the oldest and most prevalent form of life, and this study shows that we know less about them than we thought. There may be many other species of microbes that can tolerate or thrive with arsenic, for instance. This is just the first time anyone has really ever tried to find one.
Wolfe-Simon said she had been thinking about chemical substitution for several years. Back in 2006, while she was a postdoctoral fellow at ASU, she proposed looking for life forms that can survive even substituting various chemicals for the building blocks of life. It's not a wild hypothesis — there are a few previous examples of trace metallic elements substituting for one another, including the switching of copper for iron as an oxygen carrier in some mollusks, for instance. The swapped elements share some chemical similarities, making the transition simpler.
Arsenic and phosphorus are also chemically analagous — arsenic is directly below phosphorus on your periodic table, and the elements have the same number of electrons in their outer shells, which makes them behave similarly. So swapping arsenic for phosphorus makes sense on paper. Wolfe-Simon wanted to find out if it worked in practice, and she went looking in a likely place — California's Mono Lake, which teems with life despite containing high levels of arsenic and a salinity level three times that of the oceans.
Wolfe-Simon and colleagues took core samples from the lake and brought GFAJ-1 into the lab. They simulated the lake environment and diluted the natural phosphorus until the mixture was rich in arsenic instead. The microbe thrived, and grew 1.5 times its previous size — its cells developed internal vacuole-like structures that account for some of that growth. Conrad, at NASA, said it makes sense that a life form's structure would change in response to its environment.
The team used various types of analysis to show the microbe accumulated arsenic in its DNA. It still contained some phosphorus, too, but not nearly enough to account for its growth, Wolfe-Simon said.
She is already working on an updated study to determine what the microbe will do when it can replicate with both arsenic and phosphorus as DNA ingredients. It will likely be the first of many studies to determine what the microbe can do — and how it can be used. It could help clean up arsenic-laden toxic waste, for instance. In a world with diminishing energy supplies (and dwindling phosphorus supplies) it could even conceivably lead to non-phosphorus-based sources of biofuel. But for now, that's largely science fiction, Elser said.
And, despite the hype over this little microbe, alien life remains firmly in that category as well.
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