Friday, December 03, 2010

Bacterium using Arsenic instead of Phosphorus discovered

Mono Lake in California is one of the so-called 'soda lakes', not filled with fizzy pop but with caustic alkali and high salt concentrations. What could possibly live in these harsh conditions? Well, microbes, of course. What's more, arsenic concentrations in this like are at levels which would be toxic to most organisms. If we're going to look for weird life forms, Mono Lake would be the place to go!

Mono Lake in California (Google Maps). View Larger Map

One such weird life form has just been discovered. It's a bacterium that uses arsenic in place of phosphorus in its biomolecules, including DNA. In intro biology we learn that all living things have DNA, and that DNA has a sugar-phosphate backbone along which the four bases (A, T, C, and G) are arranged. The phosphate groups in the backbone of DNA are supposedly universal, and it's hard to imagine how any organism could have a different chemistry for something so fundamental, but apparently such a creature does exist. In place of the phosphorus atoms in the phosphate groups, it has instead arsenic. Arsenic is an element that is in the same group as phosphorus, and so has the same chemical valence, and in theory can substitute for phosphorus because it can form similar chemical bonds. Until now such a scenario has seemed to be entirely in the realm of science fiction, but that appears now to be reality.

Using a combination of radioisotope labeling and spectroscopy, a team from NASA's Astrobiology Institute led by Felisa Wolfe-Simon (disclaimer: she was the TF for a class that I took two years ago) has shown that a strain of bacteria isolated from Mono Lake is capable of taking up arsenic, and further that the arsenic is localized in the biomolecules of the organism. This is an exciting discovery because it suggests that alternative chemistries of life are possible, and that organisms need not be limited to the canonical 'six fundamental elements of life': carbon, nitrogen, oxygen, phosphorus, sulfur, and hydrogen.

While this is a very important discovery, it is also crucial to emphasize what it is not, especially given the hype and publicity that this new piece of research has received. The fact that the work was done through the NASA Astrobiology Institute has led some to think that perhaps it has something to do with extraterrestrial life or new life forms. It is emphatically not the case: The bacterium is a member of the Halomonadaceae, a known family of the gamma-Proteobacteria. It is not part of the so-called 'shadow biosphere' (Wikipedia), which is the idea that alongside the canonical DNA/RNA/protein based life that we know of, there are 'organisms' based on fundamentally different chemistry (e.g. with some other form of information-coding molecules instead of DNA, or some other means of catalyzing biochemical reactions...). This discovery 'only' shows that one particular component of the highly-conserved basic biochemistry, namely the use of phosphorus in biomolecules like the nucleic acids, can be swapped out for an element with similar chemical properties. This is not a form of primitive life; if anything it represents an extreme form of specialization to an extreme habitat. Phosphorus is typically a highly limiting nutrient for life - this is why algae tend to form 'blooms' when phosphorus-containing fertilizers or detergents seep into water bodies - because the sudden influx of phosphorus allows a great burst of growth. In the case of this bacterium, called strain GFAJ-1, it is able to capitalize on the high arsenic concentration to replace some of its need for phosphorus.

Hence, not to detract from the excitement about this really cool piece of natural history, but these clarifications really needed to be made in the face of what was inevitably a lot of misunderstanding and misrepresentation in the popular press (with sensational headlines like "Alien life forms discovered?" and so on). What is exciting is certainly the fact that life like this has been discussed as a theoretical possibility for a long time, and is now finally known to be real.

Update (3 Dec 2010, 0043 EST):
Even the geek-cool comic strip xkcd has gotten into the act:

Source: xkcd by Randall Munroe

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