Arsenic supports life?

A strain of bacteria isolated from a salt lake in California can grow on arsenic, seemingly in lieu of phosphorus in its DNA and other major biomolecules.

Mono Lake, California Image: Image © 2010 Henry Bortman

The finding, published today (December 2) on the Science Express Web site, throws into doubt the long-held belief that phosphorus is absolutely essential to life, and broadens the range of environments in which scientists might expect to find extraterrestrial organisms.

"This is a surprise," said biochemist Barry Rosen of Florida International University, who was not involved in the research. "Not just for bacteria but for life in general, arsenic is one of the few elements that is considered to be only toxic and has no role in metabolism."

It's "pretty damn surprising," agreed ecologist James Elser of the Arizona State University, who also did not participate in the study. "I've spent my career studying phosphorus limitation, and how organisms use phosphorus, and how nucleic acids always have phosphorus in them, and now there's this exception. That's what's really weird."

Arsenic falls directly below phosphorus on the period table, and thus has many similar chemical properties. In contrast to relatively stable phosphorus-based molecules, however, arsenic compounds are extremely unstable. While phosphorus compounds take years, decades, or even millennia to break down, the rate of hydrolysis of arsenic compounds is usually measured in seconds or minutes.

In fact, its similarity to phosphorus and its instability partly explains why arsenic is so toxic. The body may not be able to distinguish between phosphate -- the most common form of phosphorus in organisms -- and its arsenic equivalent, arsenate. As a result, scientists suspect that arsenate can be incorporated into molecules and pathways that normally use phosphate, causing downstream processes to fail if the arsenate molecules are quick to break down or otherwise don't work properly.

But at least one organism seems to have tackled this problem. Sampling the sediment of Mono Lake in California, a salt lake with high dissolved arsenic concentrations, NASA astrobiologist Felisa Wolfe-Simon of the US Geological Survey and her colleagues identified a bacterium that can grow when cultured with arsenic, but only trace amounts of phosphorus. Under conditions of high arsenic, the bacteria didn't grow as well as when phosphorus was abundantly available, but they grew significantly more than when neither arsenic nor phosphorus was provided.

"That says, to me, that they really are using the arsenic," Rosen said.

To determine how the bacteria used the normally toxic element, the researchers provided the cultures with radiolabeled arsenic, and found it in parts of the cell containing proteins, metabolites, lipids and nucleic acids. Further analysis of the DNA suggested that the arsenic might simply be replacing the phosphorus in the backbone of the molecule atom-for-atom.

Scanning electron micrograph of the bacterial strain isolated from Mono Lake under high arsenic, low phosphorus conditions. Image courtesy of Science/AAAS

"The challenge then is to explain how it is conceivable [that] an organism is able to use [arsenic] in its genetic molecules," given that they fall apart so quickly, said astrobiologist Steven Benner of the Westheimer Institute at the Foundation for Applied Molecular Evolution, who was not involved in the research. One possibility, he suggested, is that interacting, as-yet unknown molecules stabilize the arsenic-based compounds, but first more research is needed to confirm how the arsenic is being incorporated into DNA and other molecules.

"They show that arsenic is in the DNA, but they don't show that it is participating in the backbone, replacing phosphate," Rosen said. "To be truly convincing, I'd like to see an actual molecule that has arsenic that is active and functional."

If arsenic is indeed serving as a surrogate for phosphorus under certain conditions, however, "the result will have sweeping consequences," said Benner, who served on a discussion panel today at a NASA news conference about the study. "It will overturn a century of information about the comparative behavior of phosphates and arsenates."

Another open question is whether or not these bacteria are using arsenic in their natural habitat of Mono Lake, said Elser, also a member of today's discussion panel. The experiments demonstrate that the microbes are capable of growing on arsenic, but these are contrived laboratory experiments. "The only way to answer that question is to get in a field situation and [use] radiolabeled arsenic under more realistic field conditions," he said.

Felisa Wolfe-Simon collects samples from Mono Lake. Image: Image ? 2010 Henry Bortman

The results raise some obvious questions about the chemical environments that might be able to support life, and expand the search for environments that contain extraterrestrial life. Additionally, the bizarre bacteria may provide some creative solutions to some critical problems. Because arsenic is a toxic and quite prevalent contaminant, for example, scientists are always looking for ways to remove it from the environment, Rosen said. "If we could devise organisms that could capture and accumulate the arsenic, it might be possible to use those for bioremediation."

Another potential application for an arsenic-loving microbe is in phosphorus recycling, Elser said. "Phosphorus for agriculture is going to start running out in a few decades," he said, and bacteria that use arsenic instead could help keep vital ecosystems running. (Click here to read last month's feature about Elser's work on the potential effects of a worldwide phosphorus shortage.)

"Those are just science fiction applications that now pop into mind now that we think there is an organism that might not really need phosphorus," Elser added, "which is just shocking for me to say."



F. Wolfe-Simon, et al., "A bacterium that can grow by using arsenic instead of phosphorus," Science Express, 10.1126/science.1197258, 2010.


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