Reports of “Alien DNA” are greatly exaggerated

The head­lines sound­ed real­ly excit­ing: Arsenic bac­te­ria with DNA com­plete­ly alien to what we know. This sound­ed like the stuff of sci­ence fic­tion! What does “com­plete­ly alien DNA” mean? Every liv­ing thing ever observed by sci­en­tists uses DNA to car­ry its genet­ic infor­ma­tion from one gen­er­a­tion to the next. (Yes, some virus­es use RNA instead of DNA, but virus­es don’t com­plete­ly qual­i­fy as liv­ing things.) Does this new “arsenic bac­teri­um” use dif­fer­ent base pairs or a dif­fer­ent genet­ic code than all of the oth­er liv­ing things on earth? If so, where did this bac­teri­um come from?

The first clue that this sto­ry is a gross exag­ger­a­tion comes in the admis­sion that this “arsenic bac­te­ria” species belongs to the Gammapro­teobac­te­ria, a class of gram-neg­a­tive bac­te­ria that includes many famil­iar species, includ­ing the E. coli in our bow­els. Why are these arsenic bac­te­ria “alien”? Actu­al­ly, they’re not. They just do some­thing that had nev­er been observed before. When grown in an envi­ron­ment that is poor in phos­pho­rus but rich in arsenic, these “arsenic bac­te­ria” use arsenic where they would ordi­nar­i­ly have used phos­pho­rus. The arsenic can even get incor­po­rat­ed into their DNA in the spots where phos­pho­rus would ordi­nar­i­ly go. How­ev­er, the DNA still uses the same base pairs and still codes for the same amino acids. Noth­ing impor­tant  real­ly changes. If you gave them some phos­pho­rus, they’d prob­a­bly go back to using that.

The find­ing that some bac­te­ria can use arsenic where they would nor­mal­ly use phos­pho­rus is inter­est­ing but not com­plete­ly unex­pect­ed, because arsenic is just below phos­pho­rus in the peri­od­ic table of the ele­ments. Arsenic and phos­pho­rus there­fore have sim­i­lar chem­i­cal prop­er­ties, which is part of the rea­son why arsenic is poi­so­nous to human beings.

Why is the dis­cov­ery of “arsenic bac­te­ria” impor­tant? One expert argues that the earth has a lim­it­ed sup­ply of con­cen­trat­ed deposits of phos­phates, and that these deposits are rapid­ly being deplet­ed. That’s true but com­plete­ly irrel­e­vant. We can’t sub­sti­tute arsenic for phos­pho­rus in agri­cul­ture or just about any­thing else, because arsenic is a dead­ly poi­son! Is it real­ly com­fort­ing to know that after the world’s human pop­u­la­tion has col­lapsed because of resource deple­tion, some bac­te­r­i­al pop­u­la­tions will go on with­out us?

In oth­er words, NASA did not dis­cov­er a new life form, or any alien DNA, or even any­thing tru­ly unex­pect­ed about bac­te­ria. The find­ing that some bac­te­ria can use arsenic in the place of phos­pho­rus under extreme con­di­tions is inter­est­ing to a bio­chemist or a micro­bi­ol­o­gist, but it doesn’t deserve the over­wrought, mis­lead­ing head­lines.