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Science: Despite Claims, Microbe Needs Phosphorus After All
Two new reports show that a bacterium, known as GFAJ-1, requires small amounts of phosphate to grow—and that it cannot substitute arsenic for phosphorus to survive, as a 2010 report in Science had suggested.
The GFAJ-1 bacterium, which was discovered in the arsenic-rich sediments of California’s Mono Lake, became the center of a controversy last year after Felisa Wolfe-Simon of the NASA Astrobiology Institute and the U.S. Geological Survey and colleagues reported that the microorganism could incorporate arsenic into its DNA when phosphorus wasn't available.
Such a claim raised important questions about life’s basic requirements, since only six elements—carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus—make up the bulk of living matter. Arsenic, on the other hand, is typically toxic to living organisms although its chemical properties are similar to those of phosphorus.
[The full text of the original GFAJ-1 report by Wolfe-Simon and colleagues, along with an Editor’s Note and eight Technical Comments responding to the report, are available free to the public at the Science Web site.]
Tobias Erb of ETH Zurich and colleagues now show that, even though 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. The researchers found no evidence of biotic arsenic substitution for phosphorus in GFAJ-1 metabolites linked to central carbon metabolism. Phosphorus remained essential for the organism 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 of Princeton University 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. The researchers detected no arsenic covalently bound to the DNA of GFAJ-1. They also suggest that the bacterium is capable of tolerating high concentrations of the poison while simultaneously scavenging for low levels of phosphate to maintain its metabolism.
The new reports show that GFAJ-1 does not break the long-held rules of life, contrary to how the original researchers had interpreted their data.
In a press statement issued on their behalf, Science’s editors said “the scientific process is a naturally self-correcting one, as scientists attempt to replicate published results.
“Science is pleased to publish additional information on GFAJ-1, an extraordinarily resistant organism that should be of interest for further study, particularly related to arsenic-tolerance mechanisms.”
9 July 2012