Arsenic: Poison and Building Block for Life?
The press conference announced last week by NASA promised to reveal details about “an astrobiology finding that will impact the search for evidence of extraterrestrial life.”
The news, that researchers conducting tests in the harsh environment of Mono Lake in California had, according to NASA , “discovered the first known microorganism on Earth able to thrive and reproduce using the toxic chemical arsenic,” was indeed intriguing.
What wasn’t entirely clear in media accounts of the finding was what exactly it had to do with extraterrestrial life.
Indeed, Patrick Williamson, the Edward H. Harkness Professor of Biology at Amherst College, maintains that news reports from Lake Mono were missing some important context. Williamson recently sat down with Director of Public Affairs Peter Rooney to offer his take on the discovery’s significance. An edited transcript is below, or you can listen to portions of their conversation by clicking the play button below.
Edward H. Harkness Professor of Biology Patrick Williamson
What, exactly, did NASA fellow Felisa Wolfe-Simon and her colleagues discover at the bottom of Mono Lake in California?
Mono Lake is in the desert and among its characteristics is a very high concentration of arsenic, enough to poison most living things. So what Felisa Wolfe-Simon did to start the process was go into the lake, rummage around in the sediment and find bacteria that first could be cultured and then could be grown in a medium whose composition she can control.
That’s the discovery part. The creation part was that she apparently grew that bacteria in the presence of increasing concentrations of arsenic and decreasing levels of phosphate.
That’s essentially evolution in test tube, and she evolved herself a bacterium that could now live at staggering high concentrations of arsenic and vanishingly small concentrations of phosphorus.
That leads to a point that was mentioned in the various media accounts of this finding -- that a ‘rule' exists that life is based on six elements: carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulfur. When did this rule get articulated, and is this now the first instance where that rule has been broken?
It turns out that biology is not really big on laws. We don’t do laws. Evolution is much too inventive to get really dictatorial about laws, and this in particular is not so much a rule as an observation. Yes, if extraterrestrial life arrives in a flying saucer and shoots the top off the Empire State Building, that’s pretty good evidence in itself.
Most life probably won’t do that, so the signature for most life isn’t a ray gun; it’s a chemical signature, and that leads to the question ‘What is the chemical signature of life?’ And a close correlate of that question is ‘What does life consist of chemically?’
The trouble is we only have ready access to one set of living creatures and it turns out they’re evolved from a common ancestor and put together in the same way. And, not only are the same elements present, but they’re put together in the same ways, up to a fabulous level of complexity.
These elements together have properties that make them handy for putting together complex molecules, and it’s hard to imagine a chemical-based life that doesn’t evolve around complex molecules. That’s the sense in which there’s a rule – these six basic elements have chemical properties that make them capable of generating complex molecules.
As far as “life” is concerned, how are scientists defining it in this case? An organism that evolves, reproduces or passes information onto a next generation, or what?
Those are all tied together. Life in this case is something that reproduces hereditively – that is to say it grows, reproduces, and produces essentially copies of itself. Once you have those properties, you’re in business – now you’re alive.
How significant is this finding?
Although phosphorus isn’t the most abundant atom, it really turns out to be at the center of things that people are interested in. It turns up in places – like genes -- that are interesting to people. It turns out that genes require phosphorus. DNA is a phosphate- based polymer. Phosphorus also turns up in membranes. You can’t have a cell without a barrier that separates the cell from the rest of the world, and in all life on this planet that barrier is made of molecules that have phosphorus in them.
The nifty thing about this experiment is that it poses the question ‘How much of phosphorus can be replaced?’ This experiment has created a platform where that question can be asked. Now at least in principle there’s an organism where phosphorus has been eliminated from all but the most essential places of the cell.
That is an interesting question. Does its answer have anything to do with finding life elsewhere in the universe?
That’s why NASA is interested in this. The experiment begins to address the question we started with and that is ‘How are you going to know if you’ve stumbled across something that’s alive?’ What this experiment does is say ‘You aren’t going to know if you limit yourself to looking for complex molecules that contain the basic six elements, because you might run into life forms that don’t depend on those six basic elements.’
One of your courses this semester is Evolution and Intellectual Revolution. Does a finding like this do anything at all to force a reexamination of Darwin’s Theory or does it just reinforce and fit into it nicely?
Just a couple days ago in this class we began to discuss the original ancestor and what it is that we know about life when it was first beginning to get organized.
That’s a little later than this organism would apply to – by that time the role of phosphorus was already well established and the basic biochemical mechanisms were being sorted out. Had there been lots of arsenic around and not much phosphorus, this experiment suggests at that point it would have been possible to generate life forms that essentially depended upon arsenic instead of phosphorus.
That would have made phosphorus what the little old ladies would have served when unpleasant guests came to dinner.
Were there any other important aspects of this finding that you feel were missing from media accounts?
There are two that caught me scientifically. One was these are not organisms that don’t have phosphorus at all. The concentration of phosphorus in the artificial growth medium wasn’t zero. These organisms were still able to find phosphorus in their environment and they still contained phosphorus – just not very much of it.
The other thing I thought was interesting is that the bacterium apparently isn’t just tolerating the arsenic; it’s actually evolved to make use of an element that we ordinarily consider to be just a poison.