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.