Researchers Ronald Oremland and Felisa Wolfe-Simon examine a mud sample from Mono Lake. Wolfe-Simon called the discovery there "outrageous." (Henry Bortman, Science/AAAS / December 1, 2010)

 

Host – Robert Frederick

All life is thought to require six elements: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur, and uses these elements to make DNA, RNA, proteins, and fats.

 

Interviewee – Felisa Wolfe-Simon

Could you change C-H-O-N-P-S? Could you change one of the six dominant elements that we know of?

 

Host – Robert Frederick

Felisa Wolfe-Simon is a geobiochemist with the U.S. Geological Survey and NASA’s Astrobiology Institute. She and her team thought that arsenic – normally considered a toxic substance – could be a substitute for phosphorus, at least in a microbe.

 

Interviewee – Felisa Wolfe-Simon

Well, if you want to search for a microbe that might utilize arsenic instead of phosphorus, go somewhere in the environment where you are hedging your bets.

 

Host – Robert Frederick

The team went to California’s Mono Lake, which has naturally high levels of arsenic. They took samples of mud from the lake that would contain microbes that lived in the naturally high levels of arsenic. And then, back in the lab, they cultured the bacteria from the mud, sterilized other mud from the lake to make sure nothing was in it, and then introduced individual strains of bacteria back in, diluting it in such a way so that, over time, there were higher and higher concentrations of arsenic.

 

Interviewee – Felisa Wolfe-Simon

So, you do this 1 in 10, 1 in 10, 1 in 10 – so after, for example, the 1 in 1,000,000^th dilution from the original lake, losing all the memory of the original lake solution – we still had microbes growing and swimming and clearly very active at a physiological level.

 

Host – Robert Frederick

But it wasn’t just surviving high levels of arsenic. The team deprived the microbes of phosphorus, too.

 

Interviewee – Felisa Wolfe-Simon

The microbe we’ve discovered appears to be able to use arsenic if not given any phosphorus. So everywhere we expected phosphorus, we found arsenic.

 

Interviewee – Barry Rosen

This is a proof of principle.

 

Host – Robert Frederick

Barry Rosen is a biochemist at Florida International University and is not associated with the paper.

 

Interviewee – Barry Rosen

You know, they’re not demonstrating that there’s life hidden out there that uses arsenic in place of phosphate. That’s unlikely, at least on Earth. But what they’re demonstrating is that life can use arsenate. And so if they have to put selective pressure on the cells to do that, it’s really a demonstration that it's possible.

 

Host – Robert Frederick

In other words, because phosphorus is pretty much everywhere on Earth, including in Mono Lake, the bacterium Wolfe-Simon and her colleagues isolated may have evolved during the experiment to live using arsenic instead of phosphorus. Again study author Felisa Wolfe-Simon.

 

Interviewee – Felisa Wolfe-Simon

These cells will grow on phosphorus, and that’s something critical to remember. So, we’re very cautious in the paper to say that arsenic can substitute for phosphorus. We’re not claiming that this is some alien microbe or that its some other form of life from another planet. No. It’s something we can recognize. It’s on the Tree of Life. It’s just doing something a little differently.

 

Interviewee – Barry Rosen

Yes, I’m convinced that the arsenic is being incorporated.

 

Host – Robert Frederick

Again, Barry Rosen of Florida International University.

 

Interviewee – Barry Rosen

I think that the most persuasive observation is that the cells required either phosphate or arsenate to grow. So they aren’t just picking up extra phosphate from someplace, because otherwise they would grow in the absence of arsenate. It’s the kind of thing that really has considerable implications about the possibility of life on other planets and life that uses a different kind of chemistry from what we have on Earth.

 

Host – Robert Frederick

But study author Felisa Wolfe-Simon cautions that it’s not entirely clear that arsenic is taking the place of phosphorus in the functioning of the cell and in the DNA.

 

Interviewee – Felisa Wolfe-Simon

So in the paper we used what we would think of as every available type of technique to really show that the arsenic was really inside and being used. So we know they are growing on arsenic with no added phosphorus and they seem to grow well. We see that the arsenic is intracellular, and we see that the arsenic is associated with a band of genomic DNA and that arsenic in the cell appears to be in a structurally similar environment at length scales that would correspond or correlate to what phosphorus would look like in the backbone of DNA.

 

Interviewee – Barry Rosen

But as the saying goes, the devil is in the details. And as a biochemist, I’m obsessed with details.

 

Host – Robert Frederick

Again, Barry Rosen of Florida International University.

 

Interviewee – Barry Rosen

So I thought to be truly convincing the next step has to be to demonstrate that specific molecules, whether small molecules of intermediary metabolism—like glucose-6-phosphate or phospholipids—or larger molecules—like phosphoproteins—they really have to demonstrate that these molecules, purified from the cells, have arsenic in them, and that they are still active.

 

Host – Robert Frederick

And so used by the microbe. But, both Rosen and study author Wolfe-Simon say it could be years before they can sort that out. You can read Wolfe-Simon and colleagues’ paper, “A bacterium that can grow by using arsenic instead of phosphorus,” online, at sciencexpress.org.

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*Felisa Wolfe-Simon1,2, Jodi Switzer Blum2, Thomas R. Kulp2, Gwyneth W. Gordon3, Shelley E. Hoeft2, Jennifer Pett-Ridge4, John F. Stolz5, Samuel M. Webb6, Peter K. Weber4, Paul C. W. Davies1,7, Ariel D. Anbar1,3,8 and Ronald S. Oremland2

1NASA Astrobiology Institute, USA.
2U.S. Geological Survey, Menlo Park, CA, USA.
3School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA.
4Lawrence Livermore National Laboratory, Livermore, CA, USA.
5Department of Biological Sciences, Duquesne University, Pittsburgh, PA, USA.
6Stanford Synchrotron Radiation Lightsource, Menlo Park, CA, USA.
7BEYOND: Center for Fundamental Concepts in Science, Arizona State University, Tempe, AZ, USA.
8Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ, USA.