NASA Life Discovery: New Bacteria Makes DNA With Arsenic

Mono Lake, California

by Richard A. Lovett

from National Geographic News

No, today's NASA announcement is not about proof of life on another world. A recent release hinting at "an astrobiology finding that will impact the search for evidence of extraterrestrial life" had bloggers abuzz the past few days with speculation that the space agency had discovered extraterrestrial life.The truth, however, is that scientists have found life on Earth that's perhaps the most "alien" organism yet seen.

A new species of bacteria found in California's Mono Lake is the first known life-form that uses arsenic to make its DNA and proteins, scientists announced today.

Dubbed the GFAJ-1 strain, the bacteria can substitute arsenic for phosphorus, one of the six main "building blocks" for most known life. The other key ingredients for life are carbon, hydrogen, oxygen, nitrogen, and sulfur. Arsenic is toxic to most known organisms, in part because it can mimic the chemical properties of phosphorus, allowing the poison to disrupt cellular activity.

The newfound bacteria, described online this week in the journal Science, not only tolerates high concentrations of arsenic, it actually incorporates the chemical into its cells, the study authors found."It's gone into all the vital bits and pieces," said study co-author Paul Davies, director of the BEYOND Center for Fundamental Concepts in Science at Arizona State University in Tempe.

While for now Earth is the only place we know that life exists, the discovery does hold implications for the search for life elsewhere in the universe, since it shows that organisms can exist in chemical environments biologists once wouldn't have imagined.

Did Life Arise Twice on Earth?
Astrobiologists found the arsenic-based bacteria while looking for a possible "second genesis" of life on Earth. The scientists were hoping to find evidence of a "shadow biosphere," sometimes called Life 2.0. Such a discovery would prove that, before life as we know it came to dominate the globe, the world had actually seen a separate, independent origin of life. "If life happened twice on one planet, it is sure to have happened on other planets around the universe," Davies said.Last year study leader Felisa Wolfe-Simon of NASA's Astrobiology Institute published a paper suggesting that one possible version of Life 2.0 would be a creature that chemically substitutes arsenic for phosphorus.

So Wolfe-Simon and colleagues took samples of bacteria from California's Mono Lake, a briny, arsenic-rich lake in a volcanic valley southeast of Yosemite National Park. The scientists cultured Mono Lake bacteria in Petri dishes, gradually increasing the amount of arsenic while reducing phosphorus. Chemical analyses with radioactive tracers showed that the GFAJ-1 strain bacteria was in fact using arsenic in its metabolism."Most [organisms] die, but these live on," study co-author Davies said. Despite their oddity, however, the bacteria are genetically too similar to ordinary life to truly be descendents of a second genesis."This is not Life 2.0," Davies said.

Bacteria a Truly Extreme Life-Form
Still, the GFAJ-1 strain might be called the most unusual of the extremophiles, bacteria that thrive under exceptionally harsh conditions, such as high heat, high salt, and low oxygen. Prior discoveries of such bacteria involved organisms that were otherwise "very ordinary," Chris McKay, an astrobiologist at NASA's Ames Research Center in Moffett Field, California, said in an email."The only thing 'extreme' about them was where they lived. Biochemically they were pretty normal," said McKay, who wasn't a member of the study team. The arsenic-based bacteria is "a very important find," McKay said. "It's the first example of what we can really call an extreme life-form in an extreme environment."

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Reference
Felisa Wolfe-Simon F, Blum JS, Kulp TR, Gordon GW, Hoeft SE, Pett-Ridge J, Stolz JF, Webb SM, Weber PK, Davies PCW, Anbarl AD, Oremland RS (2010) A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus. Science DOI: 10.1126/science.1197258

Abstract
Life is mostly composed of the elements carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. Although these six elements make up nucleic acids, proteins, and lipids and thus the bulk of living matter, it is theoretically possible that some other elements in the periodic table could serve the same functions. Here, we describe a bacterium, strain GFAJ-1 of the Halomonadaceae, isolated from Mono Lake, California, which substitutes arsenic for phosphorus to sustain its growth. Our data show evidence for arsenate in macromolecules that normally contain phosphate, most notably nucleic acids and proteins. Exchange of one of the major bioelements may have profound evolutionary and geochemical significance.