Arsenic is an insidious element. With 5 outer valence electrons the arsenic atom is chemically similar to the biologically critical element phosphorus, but only in crude terms. Life depends extensively on phosphorus - it helps form the molecular backbone of DNA, it is part of molecules like Adenosine triphosphate (ATP) that serves as a vital rechargeable chemical battery within all living cells, as well as many other biologically vital roles. Arsenic on the other hand can weasel its way in, waving its valence electrons in a come-hither fashion, and getting the best seat in the house. The problem is that once an organism takes in arsenic, replacing some of its phosphorus, it typically begins to malfunction - arsenic is is a fatter atom and biochemistry is a sensitive thing. There is good reason why arsenic has long been a poison of choice for nefarious human dealings.
As is often the case though, microbial life has an exception to the rule. There was a flurry of buzz earlier this week about an astrobiology press-release, rumors of alien organisms were rife. Now the embargo is lifted we can reveal what the fuss was about. Wolfe-Simon and colleagues in a paper in Science this week present some remarkable discoveries made by tinkering with a particularly hardy strain of bacterium (actually a proteobacterium, part of a very diverse group). The bacterium Halomonadaceae GFAJ-1 is a halophile (salt-lover), found in the alkaline and hyper-saline Mono Lake in California. Many such bacteria are known to be highly tolerant of toxins, such as arsenic, but now it turns out this one is far more than merely tolerant.
While carefully culturing organisms from Mono Lake in the lab Wolfe-Simon et al. gradually removed phosphates from the food supply, replacing them with arsenic compounds tagged with radio-isotopes to enable them to be tracked into the bacterial cells. One bacterium, GFAJ-1 grew faster than anyone else. Not only did GFAJ-1 survive it swapped out its phosphorus for arsenic - in cell structures, and most remarkably, in DNA itself. It appears to be nature's own Transformer. It's pretty amazing. How does this organism keep a viable bio-chemical system running with the same molecular structures but with the fat arsenic atoms replacing phosphorus? It remains to be seen whether strict one-to-one replacement is really occurring, it's also the case that phosphates are still required by this organism, albeit in small amounts.
The notion that life might use arsenic instead of phosphorus has cropped up before. In particular its been wafted around as a plausible case of 'shadow life', a parallel type of life with a separate but contemporaneous origin to phosphorus using life. That's certainly an intriguing idea, although how and why GFAJ-1 would have switched over to phosphorus yet still retained the capacity to use arsenic is unclear.
For astrobiology it definitely offers encouragement that some of the seemingly hostile chemical environments in our solar system - Martian perchlorates for tea anyone? - may be ok for some very specialized 'niche' lifestyles. However in the broader context I think there's a hitch for talk of arsenic based life on other planets. Phosphorus itself is made by hefty stars over about 15 solar masses and in terms of cosmic abundance it is relatively scarce compared to other major bio-chemically important elements. It is roughly 1000 times less abundant than carbon. Nonetheless, in terrestrial life it is much more concentrated, only a factor of 30 or so less abundant than the carbon in your ham sandwich. So there is a bit of a bottleneck for life in terms of having enough phosphorus around - it wouldn't take much of a deficit of phosphorus in a forming planetary system to leave things barren. Now, if we wanted to postulate planets of arsenic-based life the problem is even more acute since arsenic is 1000 times less abundant than phosphorus - it's really a trace by comparison. Despite some of the breathless discussions already going on; the chances of arsenic-based planet-wide biospheres....slim to nonexistent.