“Arsenic Life” Needs Phosphorus After All

A research team makes the trek to 10 Mile Beach, on the shore of arsenic-rich Mono Lake, to collect microbial samples. In the background, the eastern front of the Sierra Nevada mountains. Credit: ©2009 Henry Bortman

Two new reports detailed online Sunday, July 8, in the journal Science show that the bacterium known as GFAJ-1 requires small amounts of phosphate to grow — and that it cannot substitute arsenic for phosphorus to survive, as one previous report had suggested.

The GFAJ-1 bacterium, found in the arsenic-rich sediments of California’s Mono Lake, became the center of a controversy last year when researchers reported that the microorganism could incorporate arsenic into its DNA when phosphorus wasn’t available.

The new studies refute the December 2010 finding that, if confirmed, would have revolutionized how we think about life. "If true, such a finding would have important implications for our understanding of life’s basic requirements since all known forms of life on Earth use six elements: oxygen, carbon, hydrogen, nitrogen, phosphorus and sulfur," according to a statement released by the editors of Science.

If an organism on Earth were found to survive without one of these building blocks, it could mean that life is more adaptable than expected, increasing the odds of finding life on other planets. Felisa Wolfe-Simon, leader of the 2010 study, had found very low levels of phosphate within their study samples. However, her group concluded the contamination would’ve been insufficient to allow GFAJ-1 to grow.

South Tufa, Mono Lake, California September 22, 2011. Credit: ©2009 Henry Bortman

Tobias Erb and colleagues now show that, although GFAJ-1 is able to grow under high-arsenate and low-phosphate conditions, the bacterium needs to be supplied with some amount of phosphate in order to grow at all.

These researchers found no evidence of biotic arsenic substitution for phosphorus in GFAJ-1 metabolites linked to central carbon metabolism. Phosphorus remained essential for the organisms to grow, although GFAJ-1 was able to grow at much lower phosphate concentrations than other arsenate-resistant strains, they say.

A separate study by Marshall Reaves and colleagues confirms that GFAJ-1 is unable to grow in the absence of phosphate, and that arsenate alone is not enough to sustain the bacterium. These researchers detected no arsenic covalently bound to the DNA of GFAJ-1. They likewise suggest that the bacterium is capable of tolerating high concentrations of arsenic while simultaneously scavenging for low levels of phosphate to maintain its metabolism.

References: "Absence of Detectable Arsenate in DNA from Arsenate-Grown GFAJ-1 Cells," by M.L. Reaves; J.D. Rabinowitz; L. Kruglyak at Princeton University in Princeton, NJ; S. Sinha; R.J. Redfield at University of British Columbia in Vancouver, BC, Canada; L. Kruglyak at Howard Hughes Medical Institute in Princeton, NJ.

"GFAJ‐1 is an Arsenate‐Resistant, Phosphate‐Dependent Organism," by T.J. Erb; P. Kiefer; B. Hattendorf; D. Günther; J.A. Vorholt at ETH Zurich in Zurich Switzerland.