An Unusual Bacteria

Mono Lake in California is not your traditional lake; it is in what's known as an endorheic basin, meaning that there is no outflow of water from the lake to the ocean. As a result, Mono Lake has accumulated a lot of dissolved salts and is very alkaline; it is also naturally high in arsenic. Despite those harsh conditions, Mono Lake is thriving with life, and is also an important stop over for migratory birds.

A paper was published in December 2011(here is a link to the paper) about a potentially unique strain of bacteria found in Mono Lake. The bacteria, called GFAJ-1 (I read in an article a while back that it stands for "Give Felisa A Job," Felisa, being the lead researcher. I can't find the original article that says that though) not only survives in an enviornment with high arsenic levels, but is also thought to utilize arsenic into its DNA structure. You know, arsenic, the toxic heavy metal? Wait a second....

Transmission electron micrograph of the bacterium

 GFAJ-1.  Credit: Science/AAAS.

What the research team proposed is that this bacteria is using arsenic in its DNA instead of phosphorus. On a strictly structural basis, this would be possible for a few reasons- Both arsenic and phosphorus are in the same column of the periodic table and thus have the same number of valence electrons, meaning that they both can make the same number of bonds. Arsenic and phosphorus also have similar atomic radii. And it turns out that they both have almost the same electronegativity (arsenic is 2.18 and phosphorus is 2.19) which is just a fancy term for the affinity of an atom to attract electrons (and thus make bonds). However they have different ionization energies (the energy required to remove an electron), though in comparison to the other elements, the two are rather close.

The research team brought the bacteria back to the lab, and did some experiments growing the bacteria in a phosphorus limited environment (no added phosphorus, only what was originally present in the samples), and found that GFAJ-1 grew fine- that the arsenic was definitely not impeding the growth. The team also did experiments to find evidence that GFAJ-1 was actually incorporating the arsenic into its own biomolecules, by tracing radioactive arsenic. The results indicated that GFAJ-1 was actually using the arsenic in its DNA.

This finding rocked the scientific world, because it was a generally accepted fact that the elements necessary for life included carbon, hydrogen, oxygen, nitrogen, phosphorus,and sulfur. So to find that an organism was using arsenic instead of phosphorus was has huge implications for understanding different life forms, both here on Earth and in space. Perhaps there are other instances of elemental substitution that make life possible on other planets?

As with any radical idea, this finding has its critics. There is a lot of skepticism about how the samples were analyzed and the conclusions of the research team. It's an interesting find nonetheless, and I look forward to seeing what further research has to find on the idea!