One of the challenges of writing on deadline is that people are not waiting every moment of the day to answer your questions. My Slate piece on arsenic life was based on a dozen or so responses from an overwhelmingly skeptical group of experts. And now, an hour after my story went live, I got a reply from George Cody, a chemist at the Carnegie Institution who co-authored a major 2007 “weird life” report. Rather than let this thirteenth comment molder in my inbox, let me share it with you. It’s a bit technical but illuminating. I’ve condensed it for clarity (my clips marked by ellipses)–
I have been aware of the hypothesis of the possibility of substitution of arsenate for phosphate for some time…The issue that always comes up is the facility of hydrolysis of arseno ester bonds….The correct experiment to do would be mass spectrometry which would unambiguously determine whether an arsenate backbone was present or not in the DNA. I cannot accept this claim until such an experiment (easily done) is performed. ..
I recall a summer intern in my laboratory accidently culturing up a bacterial biofilm from a solution of concentrated fumarate, urea, and ammonium hydroxide in ultra-pure water (not surprisingly ammonia oxidizing bacteria); we were surprised but evidently the microorganisms were able to obtain the necessary nutrients, e.g. phosphate, from somewhere to grow to a point be being readily observed. Microorganisms can do quite a bit with a little. I recall a report in Nature by Benjamin Van Mooy (WHOI) where it was shown that certain marine organisms could use sulfate in their lipids when the availability of phosphate is very low. Actually, if arsenate had substituted for phosphate anywhere, I would have looked at the lipids first, again using mass-spectrometry.
Philosophically, if it turned out that an organism could use arsenate in place of phosphate, this would not in my opinion rewrite the rules of life as we know it; aside from the hydrolysis issue, arsenate is chemically very similar to phosphate. A careful chemist could likely synthesize DNA oligomers with an arsenate backbone. As I understand it this is precisely why arsenate is a poison. Ultimately, the idea of a shadow biosphere is interesting, but it would have to be demonstrated to be truly distinct from extant biochemistry, e.g. truly novel metabolic pathways, different bases for coding, different amino-acids or better still enzymes that were not based on amino-acids at all.
As the old adage goes “Extraordinary claims require…”
Likely what I have said mirrors what you have heard from others.
Indeed, it has.
Originally published December 7, 2010. Copyright 2010 Carl Zimmer.