Thursday, December 2, 2010

Living on arsenic

There's a fascinating new paper in Science online from Wolfe-Simon et al. reporting their discovery of a strain of Gammaproteobacteria (called GFAJ-1) that can grow on arsenate instead of phosphate. Isn't that fascinating!?

I know what you're thinking: why do I care? Great question. You see, phosphate is biochemically really, really important. Phosphate makes up part of the backbone of DNA, so organisms need phosphate to keep their genes replicating properly. Phosphate is also found in RNA, which is involved in the expression of genes and the formation of proteins. Phosphate is also important for basic metabolism. The "energy unit of the cell" is ATP, adenosine triphosphate. Many unfavorable metabolic reactions can be accomplished by going through a high-energy intermediate bonded to phosphate. Oh yeah, cell membranes? Phospholipid bilayers with phosphate! Phosphate is one of the most important inorganic components of living things. It's kind of indispensable. It's not something you would imagine anything could live without.

Until now. Wolfe-Simon et al. isolated GFAJ-1 from arsenic-rich Mono Lake, California then grew it on media that lacked phosphate altogether but had arsenate instead. It grew. Further experiments demonstrated that the arsenate appears to have been incorporated into DNA, proteins, and other metabolites.

We should note that this is not a radically weird new bacterium. This is a member of the Gammaproteobacteria, which is a well-known group of bacteria. GFAJ-1 belongs to the family Halomonadaceae, which includes baceria known to take up and accumulate arsenic from the environment. So I guess if you were looking for a bacterium that could use arsenate for phosphate, a member of Halomonadaceae would be a good candidate. We should also note that the bacteria grew better on phosphate than on arsenate, presumably because arsenate bonds are much less stable than phosphate bonds.

Why would this be of interest to a creation/evolution blog? That's where the story gets ... imaginative. Here's an excerpt from a blog post over at MS-NBC:
If that could happen in the laboratory, why couldn't it happen naturally? ASU astrobiologist Paul Davies, another one of the paper's co-authors, has long held that "weird life" -- based on chemical building blocks unlike our own -- could exist right under our noses on Earth, or in extraterrestrial environments.

"This organism has dual capability," Davies said in today's announcement. "It can grow with either phosphorus or arsenic. That makes it very peculiar, though it falls short of being some form of truly 'alien' life belonging to a different tree of life with a separate origin. However, GFAJ-1 may be a pointer to even weirder organisms. The holy grail would be a microbe that contained no phosphorus at all."

Davies said GFAJ-1 was "surely the tip of a big iceberg" -- and Wolfe-Simon agreed.
Imaginative, yes? The relevance for creationism comes from the notion of weird life on this planet. "Weird life" was suggested in a previous paper by some of the authors of today's study, where they proposed:
We hypothesize that ancient biochemical systems, analogous to but distinct from those known today, could have utilized arsenate in the equivalent biological role as phosphate. Organisms utilizing such "weird life" biochemical pathways may have supported a "shadow biosphere" at the time of the origin and early evolution of life on Earth or on other planets. Such organisms may even persist on Earth today, undetected, in unusual niches.
That's a fanciful idea, and it seems that today's announcement of GFAJ-1, while extremely interesting, doesn't really validate this notion of weird, alternate metabolisms. All it seems to say is that some bacteria (that are pretty much like all the other bacteria we know about) can grow very poorly on arsenate instead of phosphate. More of a weird anomaly than a harbinger of discoveries to come.

Still, it's pretty neat stuff.

Wolfe-Simon et al. 2010. A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus. Science DOI 10.1126/science.1197258.
Wolfe-Simon et al. 2009. Did nature also choose arsenic?. International Journal of Astrobiology 8:69-74.

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