One creature's poison is another one's meat

Today NASA held a press conference announcing the first demonstration that organisms could use arsenic in place of phosphorus in their cells. Not surprisingly, science fiction got there first . . .

Kirk told [Bones] about the tabekh sauce. Bones nodded at that and said, "Yes, I've heard of it. I don't think you'd want to try it, though."
"Why not?"
"One of the other ingredients is arsenic."
Kirk blinked.
"Apparently they like the bitter taste," Bones said. "Also, the arsenides are pretty important in their diet. Klingons can get into horrible arsenide deficiencies if they're not careful, especially in stressful situations –"

~ Doctor's Orders by Diane Duane (1990)

While Klingons need a bit of arsenic in their diet, the element is a deadly poison to humans and most other Earthly life forms. A big part of what makes arsenic toxic is that it has very similar - but not identical - chemical properties as phosphorus, one of the fundamental building blocks of life.

Phosphorus is a component of the phospholipid molecules that form cellular membranes, the backbone of DNA and, importantly adenosine triphosphate (ATP).  ATP is a molecule our cells use to store and transport energy. When arsenic is present in cells it interferes with the formation of ATP, which eventually leads to cell death.   For for a visual demonstration of how arsenic can substitute for phosphorus in biomolecules, see the video at the bottom of this page

GFAJ-1 bacteria grown on arsenic.

 Because  of the similarities between the properties of arsenic and phosphorus it's natural to speculate that organisms could evolve so that they could use arsenic in place of phosphorus.  Dr. Felisa Wolfe-Simon, a geobiologist with the US Geological Survey had been testing that idea by studying bacteria living  in California's Mono Lake, which has naturally high arsenic levels.

Wolfe-Simon's work is funded by the NASA Astrobiology Institute, and back in March she explained why finding arsenic-based life would be an important step in possibly understanding alien life.

Dr Wolfe-Simon has theorised that there may be life that chose an “evolutionary pathway” to utilise arsenic. If such microbes existed, it could suggest that life started on our planet not once but at least twice. In turn this would help to support the idea that life is much more likely to have started elsewhere in the galaxy.

It's a pretty interesting speculation about alternative biochemistry,  and I don't think it's too surprising to find the idea has already been played with in science fiction. 

The best example I can think of is science fiction author and microbiologist Joan Slonczewski's novel Brain Plague (Amazon.com affiliate link). The story is about sentient arsenic-dependent microorganisms called "micros" that live in colonies in human hosts.

"Oh Great One" the letters flashed green. "Our growing children need arsenic."

"Arsenic" Chrys looked up. "Isn't that what the slaves kill for?" On the street they called it "ace."

Doctor Sartorius extended an appendage. A claw snapped open, revealing a white pill. "Micros evolved on a plenet full of arsenic. They need it as an essential mineral."

"But ace is poison"

"It's a controlled substance," the doctor admitted. "But our dietary supplement traps the arsenic in special cagelike molecules that keep it out of your own cells. Only the micros can extract it."

~ Brain Plague by Joan Slonczewski (2000)

Slonczewski acknowledges fellow microbiologist Barry Rosen, who studies arsenic resistance in microbes, for "telling her about arsenic."

But until today, the idea was still speculation.  NASA held a press conference today to announce - after much wild speculation and hoopla - Wolfe-Simon's experiments had shown that the bacteria from Mono Lake could indeed use arsenic in place of phosphorus.

Wolfe-Simon and her team of researchers took samples of lake sediment containing bacteria (named GFAJ-1). The bacteria were placed in a medium in which all phosphorus has been removed, but arsenic was present - and they grew.  If arsenic was also removed from the medium, the bacteria stopped growing, so either phosphorus or arsenic are necessary. Their experiments further show that when phosphorus is not present, arsenic is incorporated into DNA, cellular membranes, proteins and other molecules where phosphorus is normally found.

It is important to note that the bacteria grow much better when phosphorus is present, which means that they aren't truly "arsenic based life". But the fact that the bacteria don't need phosphorus is a major finding that challenges the common wisdom as to what constitutes the necessary components for life. While carbon, oxygen, sulfur and nitrogen are still commonly required by all the living organisms that have been studied, now we know that an environment - on Earth or any other planet - without phosphorus may indeed support life if there is arsenic to take its place.

As Wolfe-Simon puts it:

"If something here on Earth can do something so unexpected, what else can life do that we haven't seen yet?"

Indeed!

For more about the science, check out posts by Athena Andreadis, Ed Yong, Sarah Goslee, PZ Myers, and Jef Akst.



Here is an overview of the study from Science magazine:

Edited to add: Carl Zimmer has an article in Slate about the technical criticisms that have appeared regarding the science in this paper: "Scientists see fatal flaws in the NASA study of arsenic-based life."

In particular, he points to University of British Columbia microbiologist Rosie Redfield's  scathing review of the article.  
Also see the responses by George Cody and Alex Bradley.

Technical Background:

Research of Felisa Wolfe-Simon

Wolfe-Simon F, et al. "Did nature also choose arsenic?" International Journal of Astrobiology 8(2):69-74 (2009) (pdf)

Wolfe-Simon F., et al "A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus" Science Express (2010) DOI: 10.1126/science.1197258 (abstract only without subscription).

NASA scientists have discovered a new extreme-loving microorganism in California's exotic Mono Lake  (2003) - note that this is a different organism, Spirochaeta americana. Mono Lake is full of weirdness!