Regarding the bacteria found in Mono Lake, CA that scientists believe uses or can use arsenic in its DNA backbone where life as we know it uses phosphorus (according to their experiments depriving the microbes of phosphorus and providing much arsenic), have researchers conjectured and tested whether the energy currency molecule used is also arsenic-based instead of ATP?
I'd have to agree that it's highly improbably that the GFAJ-1 strain used nucleoside triarsenates as an energy source. There are three main lines of evidence pointed out in the Science reviews that were published after the initial publication of the paper.
- The incorporation of arsenate ions into NTAs is not a plug-and-play process, per se. Arsenate does not simply replace phosphate ions from molecules. In order for an NTA to form, it must arsenylate (like phosphorylate) a ribose molecule, then have the purine/pyrmidine ring form (and be attached to the ribose), and then two more arsenates have to be linked via ester bond to the other first arsenate. Mind you, this all happens in water. Arsenate esters are notoriously unstable in water, so it's difficult to imagine an NTA forming stably.
- The original team in Felisa Wolfe-Simon's lab didn't intentionally add phosphate ions to their growth media; instead, the approximate amount of phosphate impurity was measured and reported. The lab compared this level of phosphate with the minimal levels reported in E. coli from another paper and basically said, "There isn't enough phosphate in our growth media for our GFAJ-1 strain to grow solely on phosphorous." Well, that logic is a little non sequitur because GFAJ-1 is a halomonad (they said so themselves) and there is literature pointing to a species within Halomonadaceae that can grow on phosphate levels lower than that of the impurities reported.
- Although the Wolfe-Simon team added arsenate into their growth media, it's unlikely that any biologically-useful arsenic ended up in the cells at all. Under physiological electrochemical conditions, arsenate gets reduced to arsenite. Phosphate ions, on the other hand, are stable at physiological potentials.
All in all, there's a reason why life chose phosphorous instead of arsenic. Although both elements have ions with a propensity to make ester bonds, maybe arsenic just doesn't fit into our anthropocentric view of what life is.
This is a cool topic/question.
To answer your question. The hypothesis was based on the conjecture that there was so little phosphorus in the culture medium that phosphorous would have been replaced by arsenic in all its roles in the cell. IF they had found arsenate DNA, it would have been derived from NTAs (nucleotide tri-arsenates) or a hybrid Phosphorous/Arsenic analog of the compound as DNA polymerase consumes NTPs to create DNA. If there were only NTAs to drive DNA biosynthesis, then the cell's energy cycle would also have had to use ATA.
The primary evidence was that the mono lake strain grew in a fermentor (culture) with lots of arsenic (which is impressive) and very little phosphorous. how little? 3 micromolar. The investigators say that they did add a little phosphorus (3-5 micromolar), which, after some more careful accounting, appears to be enough to keep the bacteria growing at the observed rate without using arsenate nucleotides (submitted to Science).
This is not completely surprising as the original publication in 2010 of a preliminary finding in Science Express which only had x-ray abosorbtion fine edge spectroscopy work consistant with an arsenate like that found in a phosphorus backbone. Given that they did not produce a more direct reading of the compounds such as mass spec or an NMR experiment, this looked pretty iffy in the first place.
You an see why arsenic life was so improbable - a dozen (or more) vital pathways in the cell would have to adapt to use NTAs - pretty much all at once. If they had I suspect Mono lake would be full of those suckers.
Its sort of a bummer, for those of us who want to discover new forms of life, but you can't find what isn't there.
In my mind, this is definitively debunked in episode 32 of This Week in Microbiology. In short, no, GFAJ-1 does not use adenosine triarsenate as an energy currency, nor does not seem to incorporate any arsenic in its genetic material at all. One of the two well written answers above deserves the accepted status, but I thought I'd draw your attention to a scientific discussion behind this work.