The saga of arsenic life appears to be finally coming to a close. Two papers out in Science this week put under the microscope the claim that the bacterium could incorporate arsenic into its DNA in place of phosphorus. And the two teams found no evidence that the bacteria could make use of arsenic.
When the arsenic life story kicked off back in December 2010, it was big news. (You can read our coverage of it here, here and here.) The discovery by a Nasa team led by Felisa Wolfe-Simon that a bacterium could make use of an exotic element not normally used by life cracked open the door an inch to the idea that there could be life on more planets than ever thought possible. After all, if we could find bacteria thriving in the arsenic laced lakes of California, then surely they could be eking out a living on inhospitable planets.
However, some researchers were less than impressed with the science and they took to social media channels to register their concerns with the paper, with Nasa and with Science for letting the paper get through. The paper quickly became a serious headache for the journal, and in June 2011 they took the unusual step of publishing eight short critical responses to the original paper that caused all the controversy, followed by a defence from Wolfe-Simon.
One of the most vocal critics of the arsenic life claim, Rosie Redfield, was soon blogging what she saw as the problems with the authors’ interpretation of the data. One of her biggest gripes was the way the authors inferred that the bacterial DNA contained arsenic – by examining the molecule’s arsenic to phosphorus ratio. She and others pointed out that if phosphate were being replaced by arsenate, the DNA ought to be extremely susceptible to hydrolysis. If arsenate containing DNA were stable, it would fly in the face of years of chemical data on how these compounds behaved.
Redfield is the lead author on the first paper, which examines whether arsenate DNA even exists. They grew the bacteria up in the same way as Wolfe-Simon’s group, isolated the DNA and washed it thoroughly. They picked up very little arsenic in the sample and conclude that the original result was all down to contamination. They also performed tests on the DNA immediately after it was isolated from the bacterium and then two months later to check for the expected hydrolysis of arsenate containing DNA and found none.
Another controversial point in the original arsenic life paper was that the bacterium was metabolising arsenate in levels of phosphate thought to be too low to allow it to grow. Alex Bradley, a microbiologist at Harvard University, US, pointed out, however, that the medium Wolfe-Simon’s team said contained too little phosphate for bacteria to survive on actually contained 300 times more phosphate than that found in the Sargasso Sea in the middle of the North Atlantic – a place where microbes thrive.
The second paper examined this by attempting to grow the bacterium in a truly phosphate free environment. They found no evidence that the bacterium can replace arsenate with phosphate. They did discover some arsenate-based compounds when growing up the bacteria, but concluded that this was the result of abiotic processes as the compounds disappeared with more stringent washing of the cells.
I guess the takeaway message here is that GFAJ-1 is just a hardy bacterium that can survive levels of arsenic toxic to most life. That and calling the bacterium that is meant to be the crowning achievement of your research career ‘Give Felisa A Job’ is unwise – unless you’re absolutely certain you’ve covered all your bases.