That announcement from NASA-funded scientists of the discovery of a bacterium that could incorporate arsenic into its DNA came in for quite a bit of criticism this week; in fact, it looks like the initial publicity about the discovery (“an arsenic-based life form!”) was mostly hype.

Carl Zimmer has a great look at what other scientists have written, with more technical details than you can shake a Petri dish at: Of arsenic and aliens: What the critics said.

For example, Shelley Copley explains one of her serious issues with the paper:

1) Figure S2 shows that the -As/+P cells have an As/C ratio of about 1.5 x 10-5, while +As/-P cells have an As/C ratio of about 3 x10-5. -As/+P cells have a P/C ratio of about 0.005, while +As/-P cells have a P/C ratio of about 0.002. These are not very big differences. Furthermore, these data suggest that the cells actually contain more P than As under both growth conditions. However, Table 1 shows that -As/+P cells contain 0.19% As and 0.02% P by dry weight. These data are not consistent with the data shown in Figure S2. (By the way, since the atomic weight of As is 2.4x that of P, the molar ratio is actually 4 rather than 10. But the data are still not consistent with Figure S2.)

The interpretation of these results is complicated by the fact that the cells have different sizes when grown under the two growth conditions, and especially because of the existence of large inclusions of unknown composition in vacuoles when cells are grown +As/-P. In any case, a demonstration that the cells contain As is not equivalent to a demonstration that arsenate has been incorporated into the DNA. The fact that As is found in cells that are growing in 40 mM AsO4- is not surprising.