A bacterium that did not live on arsenic

Life as we know it does not include a bacterium that is able to live off arsenic, according to two papers published online by the journal Science .

In December 2010, a sensational discovery of a unique bacterium isolated from the toxic waters of Mono Lake in California was announced in the same journal. The bacterial strain, GFAJ-1, was substituting “arsenic for phosphorus to sustain its growth,” declared Felisa Wolfe-Simon, then at the NASA Astrobiology Institute in the U.S., and 11 other scientists in their paper.

Life forms on Earth rely on six elements to build their molecules — oxygen, carbon, hydrogen, nitrogen, sulphur and phosphorus. There was, it seemed, at least one organism capable of substituting arsenic, which is usually toxic, when phosphorus was not available. The GFAJ-1 bacterium was able to use arsenic in this manner in its DNA and proteins, according to Dr. Wolfe-Simon and her colleagues.

The implications were enormous. “The definition of life has just expanded,” remarked a senior official of the U.S. space agency, NASA, in a press release.

But many in the scientific community were unimpressed, arguing that extraordinary claims should be matched by similar levels of proof. The evidence that had been put forward for arsenic being incorporated into the bacterium’s DNA was seen as questionable. In May last year, Science published eight technical comments that raised several issues with the paper.

Now, two teams of scientists have independently studied the bacterium using much more stringent procedures and tests. One of them was led by Rosemary Redfield of the University of British Columbia in Canada, whose forthright critique of the original paper on her blog garnered a great deal of attention . The other was a group of Swiss scientists at ETH Zurich.

The GFAJ-1 bacterium “does not break the long-held rules of life, contrary to how Wolfe-Simon had interpreted her group’s data,” said Science in an editorial statement that accompanied the publication of the two papers.

The new research clearly showed that the bacterium could not substitute arsenic for phosphorus to survive. Instead, the two papers revealed that the medium used to growth the organism in the original experiments contained enough phosphate contamination to support its growth.

This bacterium was likely to be adept at scavenging phosphate under harsh conditions, which would help to explain why it could grow even when arsenic was present within the cells, statement noted.

But, as the journal also pointed out, the bacterium’s extraordinary resistance and its arsenic tolerance mechanisms would be of interest for further study.