NASA Finds Life on Earth; or,Arsenic and Old Lake
A NASA teaser about an announcement coming Dec 2 that will “will impact the search for evidence of extraterrestrial life” had watchers on edge. PhysOrg said, “Speculation that life has been discovered beyond Earth exploded on the Internet after NASA announced plans for a briefing involving scientists who study unusual life forms.” It wasn’t to be. The scientists announced the discovery of microbes in Mono Lake, California.
Felisa Wolf-Simon and team announced in Science Express1 that they had coaxed a microbe in the salty lake to take up arsenic instead of phosphate, an interesting result that, while not as sexy as extraterrestrial life, was “still ‘phenomenal’” according to Space.com. Henry Bortman at Space.com said the find is “potentially opening up a new pathway for life on Earth and other planets.” Science Daily tried to keep the hype going by stating that it “has changed the fundamental knowledge about what comprises all known life on Earth.” Some articles buried the experimental side of the story by pretending the scientists “discovered” microbes using arsenic: i.e., “Arsenic may be deadly to us, but now a microbe that can live and grow entirely off the poison has been discovered” (Live Science). Olivier Dessibourg used imagination to say on New Scientist, “We could be witnessing the first signs of a ‘shadow biosphere’ – a parallel form of life on Earth with a different biochemistry to all others,” but reporter David Shiga, also on New Scientist, called it “The ET discovery that wasn’t.”
Now that we know what it wasn’t, what was it? Elisabeth Pennisi explained in Science,2 “Meet GFAJ-1, a bacterial strain that researchers say can replace the phosphorus in its key biomolecules, including DNA, with the legendary poison arsenic.” Arsenic and phosphorus are very similar in chemical properties; phosphate, however, is more stable than arsenate and is used by living systems. Arsenic’s similarity to phosphorus is a main reason why it is toxic to life; it gets incorporated into proteins and DNA as a substitute, but causes breakdowns in metabolic processes.
The original paper in Science1 described how Wolf-Simon and her team isolated a microbe from Mono Lake waters and gradually reduced the phosphate available, while feeding the culture arsenate. After numerous rounds, some microbes continue to survive. The team checked thoroughly that the arsenate ions were being used in the cells’ DNA and proteins. Perhaps because the microbes are short-lived, some were able to avoid death. The authors acknowledged, however, that “it grew considerably better when provided with P” [phosphorus]. They were also unsure how it survived: “As for P. How arsenic insinuates itself into the structure of biomolecules is unclear, and the mechanisms by which such molecules operate are unknown.” The paper did not even say anything about astrobiology, the origin of life, or even evolution after the initial teaser in the abstract.
So despite the build-up of anticipation, these astrobiologists only had an experiment in artificial selection to announce. An extant life-form on our own planet could learn how to tolerate a toxin with a lot of loving care. It’s not even news; Wolf-Simon has been studying this for a long time (see Astrobiology Magazine), and the paper was submitted September 1. Whether her poison-tolerant microbe could survive in the wild was also highly doubtful.
Update 12/04/2010: What does the microbe’s acronym GFAJ-1 stand for? Live Science revealed it: it stands for “Give Felise a Job.”
1. Wolf-Simon et al, “A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus,” Science Express www.sciencexpress.org / 2 December 2010 / Page 1 / 10.1126/science.1197258.
2. Elisabeth Pennisi, “Biochemistry: What Poison? Bacterium Uses Arsenic to Build DNA and Other Molecules,” Science 3 December 2010: Vol. 330 no. 6009 p. 1302, DOI: 10.1126/science.330.6009.1302.
It didn’t help Astrobiology’s cause to call Wolf-Simon. Calling Wolf-Simon too often can make the public distrust scientism. This is not astrobiology; it’s intelligent design. Artificial selection is intelligent design. It may be a nefarious kind of I.D. – like giving poison to prisoners and seeing which ones die last. It doesn’t prove that the survivors will go out and live on the poison in the real world. While interesting that arsenic got incorporated into the actual biomolecules of the microbes, the authors admitted up front:
Arsenic (As) is a chemical analog of phosphorus (P), which lies directly below P on the periodic table. Arsenic possesses a similar atomic radius, as well as near identical electronegativity to P. The most common form of P in biology is phosphate (PO4 3-), which behaves similarly to arsenate (AsO43-) over the range of biologically relevant pH and redox gradients.
This means that arsenic can be a stand-in for phosphorus – almost:
The physico-chemical similarity between AsO43- and PO4 3- contributes to the biological toxicity of AsO43- because metabolic pathways intended for PO4 3- cannot distinguish between the two molecules and arsenate may be incorporated into some early steps in the pathways [(6) and refs therein]. However, it is thought that downstream metabolic processes are generally not compatible with As-incorporating molecules because of differences in the reactivities of P- and As-compounds. These downstream biochemical pathways may require the more chemically stable P-based metabolites; the lifetimes of more easily hydrolyzed As-bearing analogs are thought to be too short.
In you or me, the consequences would surely be fatal; but in microbes, whose lifetimes are short anyway, it is not all that surprising that some could survive. So this was a worthwhile and interesting experiment to find that they could, but it really has little to do with astrobiology or evolution.
If arsenic were such a good substitute, some cells would surely use it. Since it isn’t, the experiment should have provided a lesson on the narrow tolerances of life, not what the reporters are imagining: that “Deadly arsenic breathes life into organisms” (PhysOrg) or that “Life as we know it could be much more flexible than we generally assume or can imagine” as Felisa Wolf-Simon told New Scientist. Let’s not cry wolf or play Simple Simon with the media, OK?
Rob Sheldon on Uncommon Descent and Casey Luskin on Evolution News had additional comments on the story, claiming it is really bad news for Darwinists. Sheldon continued his take on The Procrustean with additional atomic details about the differences between phosphorus and arsenic.