A new life form? Not so fastSo everybody is excited about the new GFAJ-1 bacterium that Felisa Wolfe-Simon and her colleagues have discovered. A common buzzphrase diffusing through the media and blogosphere is "NASA discovers a new life form". (Or, better yet alien life.) Big press conference, and I just finished going through the article that Wolfe-Simon and colleagues have published in Science. Great work. But is this really a new life form? Recap: A few years ago Wolfe-Simon and colleagues discovered that arsenic can be used as an electron donor and acceptor in certain bacteria which live in arsenic-rich conditions. That was really cool and interesting, because arsenic, usually a poison, is used by these bacteria to breathe, one of the most basic functions of life.
About ArsenicThe "poison of the aristocrats" is toxic to most life simply because arsenic resembles very closely one of the most basic atoms of life: phosphorus. Phosphorus is used in the cell membrane, in proteins, as a signalling molecule, as part of the energy "coinage" (ATP), and in the DNA backbone. Ingesting arsenic fools life's machinery into thinking that it is actually phosphorus and incorporate it. But that's when the machinery starts breaking down, because arsenic is not phosphorous, and it gums up the works. It's like putting cooking oil on your car instead of motor oil. Your car may run for a while, but pretty soon smoking and seizing will start, and your engine will die. But GFAJ-1 seems to be using arsenic as a replacement for phosphorous. Actually, it manages to grow in media that only has trace amounts of phosphate, and large concentrations of arsenate. (The "-ate" suffixsimply means the oxygenated form of both elements: PO3 for phosphate and AsO4 for arsenate.) GFAJ-1 goes forth and multiplies in arsenate-only conditions, but not in media that are devoid of arsenate and phosphate. It actually grows best with phosphate. So the growth rates induced by the nutrients looks like this: phosphate > arsenate > nothing. When growing GFAJ-1on arsenate, the scientists also measured the intracellular concentration of phosphate, and it was well below what was needed to sustain life. Does that mean GFAJ-1 uses arsenate instead of phosphate? To answer this question, the scientists used radiolabeled arsenate to answer that question. The radioactive arsenate was detected mostly in the DNA, also in proteins, but some was also found in the membrane. Also, the arsenate-growing bacteria had much less phosphate in them than is necessary to sustain life. So it seems that arsenate is being used by the cells in lieu of phosphate, but is arsenate truly being incorporated into biomolecules in the same manner as phosphate? The researchers checked that with DNA, looking at the structure using synchrotron X-ray studies. This technique let them look at the actual structure of the DNA, although the resolution is not as good as that of X-ray crystallography. They did find that arsenate was incorporated in the DNA backbone in the same manner of phosphate. And not only does GFAJ-1 survive, it thrives. A Following such a radical change from known biochemistry. Since the arsenate is in the DNA, it means that the whole DNA-replication and transcription machinery -- hundreds of proteins -- are all adapted to replicating and transcribing arsenate DNA (and very likely arsenate RNA too!) Does all this mean GFAJ-1 is a new form of life? New Life? The current thought is that all life on earth is descended from LUCA: the Last Universal Common Ancestor. LUCA had several traits that were incorporated into all life, such as lipid membranes, DNA as the genetic material, proteins as cellular machinery and also using phosphorous in several critical roles in life, including in the DNA backbone. So "new life" would mean that GFA-J1 is derived from a different common ancestor. If this is the case, than GFA-J1 is indeed a new life form, and the implications of this finding are mind-boggling: why stop at two ancestors? Why not three, five, or 1,000 different ancestors to life on earth, each producing its own biochemical progeny, with its own unique traits? After all, the reason we may not recognize biochemically-distinct life as life, is that we are not looking for it. All our tools are geared to detecting and analyzing life with the biochemistry we know. The fact that this team of scientists have managed to use tools to analyze such a deviation from known biochemistry is a huge accomplishment. Just look how long it took us to find this radical, yet oddly familiar, departure from conventional phosphate-based biochemistry. The question therefore is now: does substituting phosphorous by arsenic in the backbone mean that GFAJ-1 is derived from a different common ancestor than all other life that we know on earth? Unlikely. I would say that using arsenic as a phosphorus substitute is a very radical adaptation to phosphorus poor and arsenic-rich conditions. GFAJ-1 is still using the same biochemistry, with a heavy phosphorus adaptation. Obviously, many enzymes are adjusted to the arsenate lifestyle. Sequencing GFAJ-1's genome would probably be the next step, as this could provide us with leads as to how enzymes in GFAJ-1 can use arsenate and arsenate containing molecules. In brief: bacteria uses arsenic instead of phsophorus. Cool and exciting? Definitely. Is this huge? Yes. Extends our biochemical horizons? Certainly. New life? Unlikely
Felisa Wolfe-Simon, Jodi Switzer Blum, Thomas R. Kulp, Gwyneth W. Gordon, Shelley E. Hoeft, Jennifer Pett-Ridge, John F. Stolz, Samuel M. Webb, Peter K. Weber, Paul C. W. Davies, Ariel D. Anbar, & Ronald S. Oremland (2010). A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus Science : 10.1126/science.1197258