The Meaning of 'Life'

NASA’s discovery — or invention? — of an earthly bacterium that consumes arsenic is a huge step toward understanding the diversity of life in the universe.
The space agency announced results of research it funded concerning an organism living in Mono Lake, Calif., during a news conference Dec. 2 at its headquarters in Washington, D.C. The presentation was streamed live on the Internet.

“We’ve discovered an organism that can substitute one element for another,” said Felisa Wolfe-Simon, the project’s lead researcher, with the U.S. Geological Survey, Menlo Park, Calif. She said the lake, close to Yosemite National Park, is high in arsenic yet teaming with life, from bacteria to brine shrimp to migratory birds.

[Felisa Wolfe-Simon works with mud from Mono Lake, Calif., in her project to grow microbes on arsenic. Image credit, Henry Bortman. Photo distributed by NASA]
Mono Lake is similar to the Great Salt Lake in that it has no outlet; salts carried into the lakes concentrate there, making the lakes saltier than the sea and toxic to ordinary lake-living lifeforms like frogs, fish and snails. But plankton, brine flies and brine shrimp provide a food base for migratory birds.

Wolfe-Simon’s group collected a bacterium from Mono Lake, strain GFAJ-1 of the common Gammaproteobacteria. They gave it plenty of nourishment but reduced the level of phosphorus while increasing that of arsenic.
They wanted to see if GFAJ-1 could incorporate the arsenic into its cell structure instead of phosphorus. Until now, biology students were taught that phosphorus is essential to life because it is a critical link in the DNA molecules present in every living cell. As the Public Broadcasting Service explains on one of its educational web pages, the DNA molecule that allows all life to reproduce resembles a long, spiraling ladder.
“It consists of just a few kinds of atoms: carbon, hydrogen, oxygen, nitrogen and phosphorus. Combinations of these atoms form the sugar-phosphate backbone of the DNA — the sides of the ladder, in other words,” says the PBS Internet site for A Science Odyssey.
Arsenic is toxic to most life. Wolfe-Simon said atoms of the elements are of similar physical size and that living cells have trouble differentiating between the two. Organisms have few defenses against the poison.
In the experiment, the bacterium continued to reproduce in a higher-arsenic environment. When the researchers removed all phosphorus and replaced it with a huge dose of arsenic, she said, “We found that not only did this microbe cope … but it grew and thrived.”
Where the bacterium had used phosphate, it now uses arsenic. The poison became an essential part of the DNA backbone and the cells are using it in their structure. For the first time, scientists coaxed a form of life into growing without one of the supposedly essential elements.
“We’ve cracked open the door to what’s possible for life elsewhere in the universe,” Wolfe-Simon said.
“Really, we have a new way of thinking, if this study holds up,” said Professor James Elser of Arizona State University, Tempe, speaking by phone during the news conference. Phosphorous is “well known to be extremely important” to all organisms, he added. When too much phosphorus leaks into the environment, for example by runoff from fields fertilized with phosphates, it can cause massive algal blooms in lakes.
Steve Benner of the Foundation for Applied Molecular Evolution, Gainesville, Fla., who said he brought onto the panel as the curmudgeon, warned against making too much of the discovery. The bacterium may simply confuse arsenic for phosphorus, he said. Arsenic is relatively unstable, and a form of life would “waste a lot of energy and a lot of time” trying to use it.
He conceded that an arsenic molecule might work well in a place like Titan, Saturn’s largest moon, because its temperature is so low as to slow chemical reactions.
Pamela Conrad, astrobiologist with NASA’s Goddard Space Flight Center, Greenbelt, Md., was more excited about the discovery. “I find this result delightful because it makes me have to expand my notion of what environmental constituents may encourage habitability,” she said. The implication is that we don’t know all there is to understand about what makes up a habitable environment.

[The bacterium strain GFAJ-1 grown on arsenic and without access to phosphorus. Image credit, Jodi Switzer Blum. Photo distributed by NASA]
Presumably, life could exist on a world loaded with arsenic but devoid of phosphorus. Stretching the idea further, could life can exist without a molecule similar to DNA?

Wolfe-Simon said some scientists believe life began at deep hydrothermal vents on the ocean floor. These hot volcanic vents spew many elements into the ocean. While phosphorus is locked up in rocks, dissolved arsenic released by the vents might have been more accessible to life.

What if the earliest forms of life developed DNA based on arsenic rather than phosphorus? Later, as life moving away from the vents, it may have substituted phosphorus for the arsenic.
In an August 2010 paper titled “Chemistry, Life and the Search for Aliens,” Benner wrote, “While ‘life’ may universally be a self-sustaining chemical system capable of Darwinian evolution, alien life may be quite different in its chemistry from the terran life we know here on Earth. In this case, it will be difficult to recognize, especially if it has not advanced beyond the single cell forms that have dominated most of the terran biosphere.”
The paper, in Proceedings of SPIE, asks if life could use sources of energy not available to earthly organisms. “Why not life in acidic or alkaline water? Can we change the elements that dominate terran biology? Replace carbon by silicon? Replace phosphorus by arsenic? Can we move further along the scale of weirdness, perhaps replacing the solvent water by something else?”
Now researchers have forced a form of life to replace phosphorus with arsenic, and biology textbooks will be rewritten. And we know the answer to one of Benner’s questions: Can living cells use arsenic instead of phosphorus? Yes.

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