The dotty aunts in "Arsenic and Old Lace" may finally have met their match.
A microbe that thrives in the muck of eastern California's Mono Lake is able to use arsenic, which is poisonous to a wide range of creatures, as one of a half dozen fundamental building blocks in biologically critical molecules such as proteins and DNA.
If the results hold up to further scrutiny, they could improve the prospects for finding at least simple forms of extraterrestrial life on other planets or moons, including Saturn's moon Titan, astrobiologists say.
Astrobiologists have been using life on Earth as a basis for hunting for potential habitats beyond the third rock from the sun. But most of the attention has focused on what researchers offhandedly refer to as "life as we know it." Until now, biological canon has held that life on Earth is mainly built around a mix of six chemical elements: hydrogen, oxygen, carbon, sulfur, nitrogen, and phosphorus.
But a small number of scientists have been pushing the idea of "life as we don't know it," says Dirk Schulze-Makuch, an astrobiologist at Washington State University in Pullman, Wash., who counts himself as one of this small band.
"There could be different building blocks used, different types of elements, or different types of energy sources," he says.
And now, along comes a terrestrial form of life that can replace phosphorus with arsenic and thrive.
Revising the 'recipe' for life
Other bacteria in effect "breathe" arsenic or use it as a food source, converting it into something less harmful. But the newly discovered bacteria incorporate it into the very fabric of their most basic biological structures.
It's as though "the recipes to make the fundamental molecules of life are open for negotiation," adds James Elser, a biologist at Arizona State University who was not part of the group making the discovery.
Dr. Wolfe-Simon says the hunt for organisms that might be based on different groups of chemical elements began when she noticed that some of the elements organisms use in very small amounts – such as iron, zinc, and molybdenum – had ready substitutes. These substitutes are similar enough to the elements they would replace that many organisms can use them as stand-ins if the originals are unavailable.
That led her to ask if the same might not hold true for the six fundamental elements. It turns out that in many key traits, arsenic is similar to phosphorus, she and her colleagues explain. Mono Lake turned out to be a good place to hunt for organisms that might function with arsenic as a building block because among its other attributes, it contains relatively high concentrations of the element.
The team prepared Mono Lake mud in the lab, adding all the nutrients needed to encourage bacteria to grow, but varying the relative abundance of phosphorus and arsenic until the naturally occurring phosphorus was gone. The researchers watched to see if microbes would leave any survivors as the shift progressed. They did. And they thrived, albeit their growth was slower than bacteria exposed to higher phosphate levels.
But steeped as she was in conventional widsom, Wolfe-Simon says she didn't believe her results initially.
"I'm a biochemist. I said: This isn't right. I must have made a mistake," she recalled during a briefing Thursday at which she discussed the results.
But the team put the microbes through a series of detailed tests that demonstrated that arsenic indeed had replaced phosphorus in some key roles – for instance, as the "backbone" for the bacteria's ladder-like DNA, the molecule that contains an organism's genetic instructions.
A note of skepticism
For all the buzz the results have generated, however, it's too early to draw broad conclusions about arsenic as a one-for-one swap with phosphorus, says Steven Benner, a researcher at the Foundation for Applied Molecular Evolution in Gainesville, Fla.
Certain qualities of arsenic could make it a less-than-ideal replacement, he says. Arsenic's bonds to other elements in molecules, especially in the presence of water and at room-like temperatures, is relatively weak. So the molecules it inhabits tend to break apart easily – not terribly helpful if the arsenic is trying to hold DNA together.
It's possible that organisms may develop ways to in effect reinforce those weak links, he adds, but that remains an open issue.
However, in other environments, arsenic might well be a suitable stand-in for phosphorus, he acknowledges. For example, a hypothetical microbe on Titan – a frigid moon of Saturn dotted with lakes of hydrocarbon – might find arsenic a better replacement for phosphorus.
Whether or not the microbe proves to be a rare exception to the general rule, he adds, the microbes represent "an excellent system to support questions about how arsenic is tolerated and phosphorus is limited in organisms that are placed under environmental stress."
The results of the study by Wolfe-Simon, who is affiliated with the NASA Astrobiology Institute at the NASA-Ames Research Center in Mountain View, Calif., and her colleagues appears on Science Express, an on-line adjunct to the journal Science.