Bathed in poison!

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Bathed in poison!
cropped periodic table showing arsenic, phosphorus, surrounding elements

Even people who can’t distinguish the periodic table from a dining table know arsenic is poisonous, although few realize why. Arsenic is chemically akin to phosphorus, one of life’s essential elements. But it’s not identical, and when arsenic substitutes for phosphorus, it produce a toxic compound instead of a protein or chunk of DNA.

Arsenic is just below phosphorus in the periodic table, which means they share many chemical similarities.

So we weren’t the only ones to be surprised by a study in today’s Science that identifies a bacterium that thrives on arsenic, at least in the lab, and incorporates this normally-poisonous element into proteins, fats and DNA.

Illustrations of man's face at top, woman kicking man in the rear, two old ladies standing at bottom

Image: Wikipedia

A more typical reaction to arsenic comes from the elderly poisoning victims in the macabre comedy “Arsenic and Old Lace.” In that play and movie, two dotty spinsters spiked elderberry wine with arsenic, strychnine and cyanide for a freelance euthanasia project.

The new study is the first to show that it is possible to substitute for one of the elite elements (carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur) that were thought to be found in all life, says Ariel Anbar, a professor of earth and space exploration at Arizona State University. “No one has previously shown that arsenic can be substituted, and I am not aware that anyone has found a substitution for any of the six essential elements. And that’s why this is a big deal.”

Arsenic was poison in this zany comedy. In Mono Lake, it is food for microbes.

Arsenic: It’s what’s for dinner

Felisa Wolfe-Simon, a former post-doctoral fellow with Anbar, gathered sediment and water from salty, alkaline, arsenic-rich Mono Lake in California and placed them in cultures intended to replicate Mono Lake water.

Dozens of white, rice-like organisms clustered on porous surface

Images courtesy of Science/AAAS
These bacteria, viewed under an electron microscope, metabolized arsenic as if it were phosphorus. Mouseover to see the same strain of bacteria growing with phosphorus but without arsenic.

“Over time we made serial dilutions, one in 10, one in 10,” always including a strain of lake microbes, says Wolfe-Simon.

Wolfe-Simon, who is now at the NASA Astrobiology Institute and U.S. Geological Survey, says the dilutions removed “essentially all” of the phosphorus. In some samples, she jacked up the arsenic roughly 2,000 times above the concentration in Mono Lake, which gets its arsenic from rocks and is already about 20,000 times above the Environmental Protection Agency standard for arsenic in drinking water.

“This is a huge amount of arsenic,” Wolfe-Simon says. “It’s surprising that they could grow, even with phosphorus, in that condition.”

Although the bacteria died in the absence of phosphorus and arsenic, they survived if only arsenic was available. “The arsenic seems to be substituting for phosphorus,” says Wolfe-Simon. “We have identified arsenic in cellular structures that are consistent with where we would expect to see phosphorus.”

Lake shore with tall white rock columns, snow-speckled mountains in the distance. Location of mono lake in CA on inset map

Photo: ©2010 Henry Bortman
Mono Lake, California, is salty, alkaline conditions, and toxic to many organisms. The lake is ideal for the study of extremophiles, microbes that live under bizarre temperature or chemistry.
Young woman in sun hat sitting on ground in desert setting sticking syringe in rock-like mud samples

Photo: ©2010 Henry Bortman
Felisa Wolfe-Simon takes samples from a sediment core at Mono Lake, California, in her search for microbes that can use arsenic as most microbes use phosphorus.

Stepping out of line

This elemental swaperoo could operate more broadly, since the elements in each column of the periodic table have chemical similarities. If one neighbor of phosphorus can sustain life without phosphorus, could the elements below carbon, nitrogen or oxygen do the same?

“This is not just about arsenic or Mono Lake,” says Wolfe-Simon. Life on Earth and the rest of the universe will be limited if it always requires six elements, but “If microbes can use arsenic as they can use phosphorus, that opens the door. What else can life do that is not yet known?”

In searching for life in the universe, NASA has focused on liquid water, another prerequisite for known life, but Anbar asserts that a search for the chemistry of life should stay broad. “Felisa’s results say we should think harder about which elements we should follow. We don’t want to be too influenced by the particular example of life on Earth. We want to push the boundaries.”

Call it “Arsenic in a new place.” Roll cameras!

– David Tenenbaum

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Terry Devitt, editor; S.V. Medaris, designer/illustrator; Jenny Seifert, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. Arsenic in drinking water.
  2. Chemistry of arsenic.
  3. More on arsenic-eating bacteria.
  4. Rewriting evolutionary history.
  5. Mono lake.
  6. Arsenic and old lace.
  7. A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus, Felisa Wolfe-Simon et al, Science, 3 December 2010.