For those dogged souls still following the whole debacle of arsenic-eating bacteria, it seems that Science has published what should be close to the death knell for "arsenic life". I already mentioned the report by Rosy Redfield and there's another one by Tobias Erb's group at ETH. The title of the paper is "GFAJ-1 is an arsenate-resistant, phosphate-dependent organism".
It's worth reflecting on that title again; "arsenate-resistant, phosphate-dependent". Yes, that description applies to GFAJ-1. It also applies to me, Shamu the killer whale, E. coli 0157 and Francis Bacon. In fact it applies to all the normal life forms that we know. So basically the title says that GFAJ-1 is not much different in this respect from any other bacterium that you may happen to find in a thimbleful of mud scooped up from your backyard.
The paper goes on to analyze the behavior of the bacterium in the presence and absence of phosphorus and arsenate. The bacterium seems to survive in tiny concentrations of phosphate, a concentration that was interestingly deemed as an "impurity" in the original Wolfe-Simon studies. It also does not survive on arsenate but starts dividing as soon as trace amounts of phosphorus are added. The authors' conclusion is clear: "We conclude that cultures in the previous study might have grown on trace amounts of phosphate rather than arsenate". This is what several experts had suspected since the beginning. Their suspicion was based on life's extraordinarily resilience and its ability to zealously guard and use every single atom of precious growth nutrients.
The authors also analyze the composition of the biomolecules (nucleotides, sugars etc.) in GFAJ-1 in the presence and absence of arsenate. They find only phosphate incorporated in the organism's essential machinery. While this does not necessarily argue against the use of arsenate, it demonstrates that when given a choice GFAJ-1 clearly prefers phosphate.
That observation is however not as striking as the next one where they find some metabolites containing arsenate, specifically sugars with arsenate appended to them. The question then is, are these metabolites formed biogenically or abiotically? To try to distinguish between these possibilities, the authors ran mock experiments where they treated glucose medium with arsenates. The purported metabolites showed up in the products and their formation is also supported by simple thermodynamic arguments which favor the attachment of arsenates to sugars. Thus it seems that simple chemistry rather than complex biology is sufficient for explaining the small amounts of arsenated metabolites. The scientists further resort to careful experiments to rule out the existence of other arsenated biomolecules.
The sum total of these experiments says that GFAJ-1 can grow in the presence of phosphate, that it cannot grown in arsenate, and that it can grow in high concentrations of arsenate only when supplemented with limiting concentrations of phosphate. Taken together with the other paper by Rosy Redfield, this is as good a case against arsenic-based life that we can make right now.
The papers are good examples of the conservative yet decisive style that scientists are accustomed to pitching their results in. Unfortunately the original authors have not reacted as conservatively. If anything their responses are transparently shallow and unconvincing. When asked about the results, Felisa-Wolfe Simon said that:
"There is nothing in the data of these new papers that contradicts our published data."
That reply almost convinces me that denial is the most sincere form of self-deception.
A current collaborator of Wolfe-Simon had even more remarkable things to say:
“There are many reasons not to find things — I don’t find my keys some mornings,” he said. “That doesn’t mean they don’t exist. The absence of a finding is not definitive.”
To which I might add that there is a possibility that disgruntled unicorns with chemistry PhDs looking for jobs may well exist, since we haven't found any yet.
Update: Paul@Chembark nicely weighs in.
Update: Paul@Chembark nicely weighs in.