Arsenic in DNA – maybe
News of surprising biochemistry: Thriving on Arsenic (NASA Astrobiology Magazine)
NASA microbiologist Felisa Wolfe-Simon has discovered bacteria that apparently can use arsenic in its DNA in place of phosphorus. Most biochemistry can be done with six elements: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur (CHONPS). Smaller amounts of a variety of other elements are also necessary to varying degrees depending on the organism, such as sodium, calcium, iron, and magnesium. Arsenic is similar enough to phosphorus (same column in the periodic table, Figure 1) that within these bacteria it may be able to play the same role.
From the Astrobiology Magazine article:
The recent discovery by Felisa Wolfe-Simon of an organism that can utilize arsenic in place of phosphorus, however, has demonstrated that life is still capable of surprising us in fundamental ways. The results of her research were published December 2 on Science Express and subsequently in the journal Science.
The organism in question is a bacterium, GFAJ-1, cultured by Wolfe-Simon from sediments she and her colleagues collected along the shore of Mono Lake, California. Mono Lake is hypersaline and highly alkaline. It also has one of the highest natural concentrations of arsenic in the world.
On the tree of life, according to the results of 16S rRNA sequencing, the rod-shaped GFAJ-1 nestles in among other salt-loving bacteria in the genus Halomonas. Many of these bacteria are known to be able to tolerate high levels of arsenic.
But Wolfe-Simon found that GFAJ-1 can go a step further. When starved of phosphorus, it can instead incorporate arsenic into its DNA, and continue growing as though nothing remarkable had happened.
“So far we’ve showed that it can do it in DNA, but it looks like it can do it in a whole lot of other biomolecules” as well, says Wolfe-Simon, a NASA research fellow in residence at the USGS in Menlo Park, California.
The article describes the methods used to purify the DNA, to ensure that the arsenic was truly incorporated into the structure of the DNA rather that being associated with other molecules. Not all, however, are convinced.
But Steven Benner, a distinguished fellow at the Foundation for Applied Molecular Evolution in Gainesville, FL, remains skeptical. If you “replace all the phosphates by arsenates,” in the backbone of DNA, he says, “every bond in that chain is going to hydrolyze [react with water and fall apart] with a half-life on the order of minutes, say 10 minutes.” So “if there is an arsenate equivalent of DNA in that bug, it has to be seriously stabilized” by some as-yet-unknown mechanism.
Benner suggests that perhaps the trace contaminants in the growth medium Wolf-Simon uses in her lab cultures are sufficient to supply the phosphorus needed for the cells’ DNA. He thinks it’s more likely that arsenic is being used elsewhere in the cells, in lipids for example. “Arsenate in lipids would be stable,” he says, and would “not fall apart in water.” What appears in Wolfe-Simon’s gel-purified extraction to be arsenate DNA, he says, may actually be DNA containing a standard phosphate-based backbone, but with arsenate associated with it in some unidentified way.
Microbiologists over the past few decades have discovered bacteria and archaea in increasingly hostile places, such as hot springs and deep in Earth’s crust. This has spurred on the hope that other worlds (e.g. Mars, Titan) also have places that would be suitable for bacterial life. The possibility of bacteria that can live with a chemical foundation other than CHONPS indicates that life might thrive in places where we otherwise would not have expected it to.
This discovery may not completely redefine life as we know it, but it does (if proven to be true) add one more bizarre thing that life can do.
Grace and Peace