Wednesday, July 11, 2012

Notorious Arsenic-Tolerant Bacterium Needs Phosphorus After All

GFAJ-1 bacteria can live in high concentrations of arsenic ? but do not incorporate the element in to their DNA. Image: Science/AAAS

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From Nature magazine

After 18 months of controversy, the official verdict is in: an arsenic-tolerant bacterium found in California?s Mono Lake cannot live without phosphorus.

In 2010, a group led by Felisa Wolfe-Simon, a microbiologist now at the Lawrence Berkeley National Laboratory in Berkeley, California, reported online in Science1 that the Halomonadaceae bacterium GFAJ-1 could include atoms of arsenic instead of phosphorus in crucial biochemicals such as DNA.

The bacteria were discovered thriving in the arsenic-rich sediment of the shallow saline Mono Lake, famed for its appearance on a picture-postcard insert to Pink Floyd?s 1975 album Wish You Were Here.

All known forms of life depend on at least six elements: hydrogen, carbon, nitrogen, oxygen, phosphorus and sulphur. Arsenic has some chemical similarities with phosphorus, but is usually toxic to life, so the suggestion that it could sustain life triggered a storm of questions, as well as criticism about how the find was revealed at an enthusiastic NASA press conference (see ?Microbe gets toxic response?).

As a result of the controversy, when Wolfe-Simon's paper appeared in print in Science last June, it was accompanied by eight technical comments2?9 from scientists responding to it.

Rosie Redfield, a microbiologist at the University of British Columbia in Vancouver, Canada, set about testing the finding. Earlier this year, she said that she could not reproduce Wolfe-Simon?s results in laboratory experiments (see ?Study challenges existence of arsenic-based life?).

Redfield is now a co-author of one of two papers that confirm that, although the bacteria can tolerate arsenic, they do depend on phosphorus. The papers were published10, 11 by Science on 8 July.

Toxicity tolerance
Redfield and her colleagues report10 that when GFAJ-1 bacteria were grown in a medium containing arsenic and a very small amount of phosphorus, their DNA contained no detectable arsenic compounds, such as arsenate (the arsenic analogue of phosphate). In the second paper, Julia Vorholt, a microbiologist at the Federal Institute of Technology in Zurich, Switzerland, and her colleagues report11 that the bacterium cannot grow in a phosphorus-free medium in the presence of arsenate. It can, however, grow in low-phosphate conditions in the presence of arsenate. GFAJ-1 ?is an arsenate-resistant, but still a phosphate-dependent bacterium?, the team writes.


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