California arsenic dreaming

For at least as long as I've been reading it, speculative fiction has been proposing alternative biochemistries to the carbon based one which we know and love.

My first encounter with the idea was in one of Isaac Asimov's Wendell Urth mysteries, written when I was three years old though I didn't read it until I was ten. The talking stone, in which a race of "siliconies" live amongst the asteroids, posits a silicon based biochemical economy which, I later discovered, was popular with other writers and speculators too.

Another candidate is phosphorus which, like carbon, can (in combination with nitrogen) form a variety of complex molecules including long chains and phosphazene rings. But phosphorus also has a more conventional rôle in our own carbon based scheme of things, as one of the six main elements from which living matter is constructed. Amongst other things, phosphorus is a component in the cell's energy storage mechanisms, and in the binding together of DNA. Could phosphorus be replaced within our carbon ecology?

I am, as said recently, a mediocre chemist at best (and most chemists would describe me even less charitably than that), so I'm not in a position to have opinions on these speculations, but that doesn't stop me being interested. So, I was intrigued by an article in Science (organ of the American Association for the Advancement of Science) at the end of last year, positing a bacterium which had pulled exactly this trick: substituting arsenic for phosphorus.

As the authors point out, instances of biochemical substitution for trace elements are not unknown, examples being “tungsten for molybdenum and cadmium for zinc in some enzyme families and copper for iron as an oxygen-carrier in some arthropods and mollusks”. This would, however, be the first known example of substitution for one of the big six. Trace element substitutions rely on close similarity between the usual supects and their surrogates, and the authors argue for the same sort of similarity here:

“Arsenic ... is a chemical analog of phosphorus, which lies directly below [it] on the periodic table. Arsenic possesses a similar atomic radius, as well as near identical electronegativity to [phosphorus. The most common form of [phosphorus] in biology is phosphate ... which behaves similarly to arsenate over the range of biologically relevant pH and redox gradients. The physico-chemical similarity between [arsenate] and [phosphate] contributes to the biological toxicity of [arsenate] because metabolic pathways intended for [phosphate] cannot distinguish between the two molecules ... ... ... given the similarities of [arsenic] and [phosphorus] ... we hypothesized that [arsenate] could specifically substitute for [phosphate] in an organism possessing mechanisms to cope with the inherent instability of [arsenate] compounds.”

They then went on to identify a naturally occurring organism, a bacterium in a California lake, in which this substitution seemed in fact to have occurred.

My attraction to this is. of course, a potent amalgam of intellectual curiosity and childhood dreams. Alas, a number of comments in yesterday's issue (they are also available on the Science web site) dispute or question the findings. The authors respond; the debate continues...

As a nonchemist academic, I can only wait impartially for the final verdict. As a childhood dreamer, however, I surreptitiously cross my fingers and hope for that verdict to vindicate the original findings.

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• Isaac Asimov. "The talking stone" in Astounding science fiction. 1955, New York: Fantasy house.
• Felisa Wolfe-Simon, et al. 2010. "A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus" in Science [Online]. Available: http://www.sciencemag.org/content/early/2010/12/01/science.1197258.full.pdf [Accessed 2010-12-02].