Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Thursday, December 2, 2010

Arsenic and old lace

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.


kurt9 said...

The bacteria is obviously pro-karyote. What makes Arsenic toxic to humans (and other life) is that it damages mitochondria. So, there isn't going to be any "Arsenic-based" Eukaryote life.

Do you think the NASA researchers inadvertently "created" this bacteria by slowly feeding it Arsenic and allowing accelerated evolution to do the job?

Caleb Scharf said...

I think it also does more than damage mitochondria - as arsenate replaces phostphate then many biochemical functions get disrupted. Prokaryotic life is (usually) equally susceptible to arsenic toxicity.

I think the experimental protocol will definitely need careful examination - if not accelerating evolution it could easily have selected out a particular mutation of GFAJ-1 that had this capability. Then it is not yet clear just how fully arsenic is incorporated into the cells - does it really replace the DNA backbone phosphates for example? Like all discoveries of this ilk I suspect some of the surprises will be muted with further work.

Caleb Scharf said...

You can catch me talking about this on the radio (WNYC) :http://tinyurl.com/3akqbev

Anonymous said...

Arsenic has been used in formulations with copper as a wood perservative. I believe the treated wood will last for decades but does eventually break down. One has to wonder about the types of organisms that can break down this kind of treated wood. Could it be we should be studying superfund sites for shadow biodiversity ?

Eniac said...

Arsenic is so toxic because it is so similar to phosphorus that it can not be separated from it in living systems, yet different enough to disrupt essential functions. There are two ways to become arsenic-tolerant: learn how to separate it (keep it out), or learn how to live with the difference in function (accept and integrate it).

The bacterium in question quite obviously did the latter, and I consider it very unlikely that it also learned to do the former, which is difficult to begin with and much more difficult in the face of the enormous artificial abundance ratio it was subjected to. This implies that there can be no niche left its biochemistry where phosphorus is not replaceable by arsenic, and the organisms thus ought to be able to live on arsenic alone, without a trace of phosphorus.

Eniac said...

kurt9: There is really no reason why this feat could not be accomplished by a Eukaryote. If you think so, you should be able to name a function of phosphorous specific to Eukaryotes that makes it more difficult to replace by arsenic than its much more fundemental functions as energy carrier, protein modification tag, and structural component of DNA and RNA. I don't think there is any function of phosphorus specific to Eukaryotes at all, much less one more sensitive to arsenic substitution.