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News & Views item - December 2010 |
A Bacterium That May Be Able to Use Interchangeably Phosphorus and Arsenic. (December 3, 2010)
A team* of scientists in an paper published in Science online this Thursday (Science DOI: 10.1126/science.1197258) report they have found a bacterium growing in California's Mono Lake able to subsist on the heavy metal arsenic and that the organism can use arsenic to build the backbone of its genetic material, DNA.
While there has been considerable hype regarding the discovery and its effect on considering the means of extraterrestrial life forms the paper's authors are conservative in describing their findings.
Their abstract reads:
Life is mostly composed of the elements carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. Although these six elements make up nucleic acids, proteins, and lipids and thus the bulk of living matter, it is theoretically possible that some other elements in the periodic table could serve the same functions. Here, we describe a bacterium, strain GFAJ-1 of the Halomonadaceae, isolated from Mono Lake, California, which substitutes arsenic for phosphorus to sustain its growth. Our data show evidence for arsenate in macromolecules that normally contain phosphate, most notably nucleic acids and proteins. Exchange of one of the major bioelements may have profound evolutionary and geochemical significance.
Below is the transcript of the interview by Science's Robert Frederick
with Senior author
Felisa Wolfe-Simon
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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 1,000,000th 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.
*
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.