POISON-PROOF: A scanning electron micrograph shows the GFAJ-1 strain grown with arsenic.
Image: Science/AAAS
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Life as we know it is remarkably diverse and adaptive, permitting organisms to gain a toehold in some of the most outwardly inhospitable places on the planet. But it tends to rely on a tidy, predictable array of six nutrient elements, a modest alphabet of basic biology that leaves open the possibility of other combinations making up entirely different kinds of biological activity. Life as we know it, then, might not be all there is—for either terrestrial or extraterrestrial biology.
That possibility looks more promising in the light of a new study describing a bacterium isolated from California's Mono Lake that can use arsenic, which is usually poisonous to life, as one of its key nutrient elements. The microbe can even take up arsenic into its biomolecules, replacing phosphorus as a structural building block in DNA and possibly in energy-carrying molecules such as adenosine triphosphate (ATP) as well. The study appeared online December 2 in Science.
"This is a real breakthrough, a real surprise to me as well," says study co-author Ronald Oremland, a geomicrobiologist with the U.S. Geological Survey (USGS) in Menlo Park, Calif. "We have a new element in the group of six that, at least for this organism, can sustain life." The standard six nutrients are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.
Oremland had previously discovered bacteria in the hypersaline, arsenic-rich Mono Lake that use the usually toxic element in photosynthesis or respiration reactions, but no one had demonstrated the uptake of the element for internal use. He says that he kept running into fellow geomicrobiologist Felisa Wolfe-Simon, the new study's first author, at meetings, and that Wolfe-Simon raised a provocative question: What about arsenic, in the form of the arsenate ion, subbing for phosphate ions inside the cell? After all, arsenic is the downstairs neighbor to phosphorus on the periodic table of the elements, and phosphate and arsenate are chemical cousins. That similarity contributes to arsenic's toxicity—arsenate masquerades as the nutrient phosphate and thus gains access to the body's metabolic system.
Oremland was not initially convinced by Wolfe-Simon's idea. "I looked at her like she was a nutcase," he says. But Wolfe-Simon and her colleagues continued to develop the hypothesis, and earlier this year she joined Oremland at USGS on a NASA astrobiology fellowship.
To look for organisms that could use arsenic as a nutrient, the researchers inoculated sediments from Mono Lake into a growth medium, adding arsenic but not phosphorus. They isolated a strain of gammaproteobacteria called GFAJ-1 that grew in arsenate-rich conditions but did not grow when deprived of both arsenate and phosphate. "It grows better with phosphorus, but it grows just fine with arsenic," Oremland says.
"We kept on saying this can't be real, we must be missing something," he adds. But after a suite of high-tech analyses—x-ray spectroscopy, radioisotope tracers, mass spectroscopy—the researchers found that arsenate was indeed being incorporated into biomolecules, including the backbone of DNA, a slot usually occupied by phosphate.
The thoroughness of the analysis lends weight to the claim, says Dirk Schulze-Makuch, an astrobiologist at Washington State University in Pullman who was not involved in the research. "This is the first time that I've really seen good evidence that this has happened," he says. "You can't really look at one DNA molecule and say, okay, there's an arsenic, there's an arsenic." But the complementary tests can reveal the element's role within the microbes. "If you put this all together you can make a very convincing case," Schulze-Makuch says.
Just why the bacterium has a penchant for arsenic is not yet clear. Perhaps some life-forms evolved in an arsenic-rich environment and later migrated to a more typical region of Earth, where phosphorus is far more abundant. "Life might have been adapted for the use of arsenic and/or phosphorus," Oremland says. "Maybe that's one way to view this, but that's entirely conjecture."
Prior to the study's publication, speculation ran wild across Twitter and the blogosphere and in British newspapers after a NASA press release announced a December 2 news conference "to discuss an astrobiology finding that will impact the search for evidence of extraterrestrial life." One popular blog, kottke.org, stirred up a minor frenzy with the headline "Has NASA discovered extraterrestrial life?"
Some will undoubtedly be disappointed by the answer to that question and by the downright terrestrial nature of the new results. But the research nonetheless has implications for the myriad kinds of life that astrobiologists might someday find in the solar system or beyond. "This study really drives the point home of how adaptive life can be and that we should go out expecting the unexpected," Schulze-Makuch says. "If you look at other places, from the hydrocarbon lakes of Titan to the subsurface ocean of Europa to the deserts of Mars, we really should not underestimate the abilities of life to adapt to these places."
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26 Comments
Add CommentIt always amazes me when some Scientist grouping catches up with common sense and rational thinking. For years we have known that pure logic and rational thinking meant there would be millions of planets like Earth ... and now they discover them ! Now they find a life form that doesn't obey our parochial definition of how they should live ... AMAZING ! like we haven't been saying this for decades ...
Reply | Report Abuse | Link to thisIf I may paraphrase the late Carl Sagan: In the universe life is self-emergent.
Reply | Report Abuse | Link to this"pure logic and rational thinking" is only part of the way.
Reply | Report Abuse | Link to thisScientist need/required to test any and ALL any Thinking to be sure of what humans may think there "pure logic and rational thinking" is correct and bears out in reality.
Don't think any other planets with liquid water have been confirmed yet. Liquid water is a universal solvent that allows chemicals to freely float, make contact each other and produce reactions. No other liquid can provide this capability.
Reply | Report Abuse | Link to thisEven these microbes were extracted from a lake.
Am I the only one who saw the movie "Evolution." This happens to be the entire basis for the movie. It doesn't seem that shocking though. The perfect mixture of temperature, pressure, gravity, space/time, water and X,(being arsenic in this case) will always create life. It is just a matter of bringing them all together. Its completely random chance, but all the factors coming together will create life with any element; I would hypothesize. The Universe is so large it has to occur somewhere...or sometime.
Reply | Report Abuse | Link to thisI'm quiet interested in the detail construction of the DNA of that kind of bacteria, but the article doesn't mention it. Maybe it is complicately different from the normal DNA. Another question is can the Arsenic-bacterium and phosphorus-bacterium copulate?
Reply | Report Abuse | Link to thisYeach, but that is just the hypothesis, fantastic hypothsis. Let's wait and see!
Reply | Report Abuse | Link to thisCopulate, I would assume not. To my knowledge sexually reproductive bacteria are an anomoly. Assuming that they reproduce asexually, like other bacteria, they would have no need for copulation. This would also imply a lack of genetic exchange and such a substantial difference in chemical structure as this in a dynamic environment will most likely allow one to overcome the other and adapt further, eventually braching off into its own species.
Reply | Report Abuse | Link to thisAs for the structure, the arsenic is simply (according to the current hypothesis of the researchers credited) replacing the phosphorous in the backbone of the DNA molecule and the structure remains the same otherwise. Both elements contain the same valence structure, but arsenate compounds are much more dissociative in water than phosphoric compounds are.
Mono lake is a hell hole for most life. It's more salt then water. One thing for sure this arsenic filled bug will kill any thing that trys to eat it. It's a good thing it's so isolated.
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Reply | Report Abuse | Link to thisTaerog I agree with what you say. My point is that it is not the shock and amazement that the media is trying to make it. It has been predicted for a long time, with no evidence to the contrary and now it has been confirmed.
JT - "No other liquid can provide this capability."
Reply | Report Abuse | Link to thisThe is absolutely no evidence to support this statement. This is exactly the kind of thinking that caused people to claim life couldn't live at temperatures above boiling point or on the bottom of the deep sea etc etc etc. Water is not a universal solvent and there is no chemical reason whatsoever why life in a different form could not form in other chemical environments.
I'm no scholar and certainly no chemist, but wikipedia's entry for water includes the following statements:
Reply | Report Abuse | Link to this"Water is a good solvent and is often referred to as the universal solvent. Substances that dissolve in water, e.g., salts, sugars, acids, alkalis, and some gases – especially oxygen, carbon dioxide (carbonation) are known as hydrophilic (water-loving) substances, while those that do not mix well with water (e.g., fats and oils), are known as hydrophobic (water-fearing) substances."
"All the major components in cells (proteins, DNA and polysaccharides) are... dissolved in water."
As I understand, many chemicals do not freely disperse in other fluids. As a result, they do not make frequent contact with each other and chemical reactions seldom occur.
So do you have some evidence for your assertion that "...there is no chemical reason whatsoever why life in a different form could not form in other chemical environments"?
Can you give some examples of life that does not require water?
Water is a polar substance, and because of the nature of polar substances, any polar substace (e.g. any salt) will dissolve in any polar liquid.
Reply | Report Abuse | Link to thisFor this reason, any other polar liquid qualifies as a liquid that can dissolve and not dissolve substances that need to be dissolved or not dissolved, respectively. What makes water special for life on Earth is 1. its abundance, 2. its polarity, 3. its a relatively small molecule, and 4. (highly related to 2 and 3) the temperatures at which it is a liquid. Therefore, any substance that meets qualifications 1, 2, 3, and is in a liquid state at the environment in which it resides (4) could, logically, support life.
I must admit though, that besides the rational aspect of my argument I do not have a specific example for another liquid. This does not mean there is not one, it just means we have not discovered it or I am not aware of it.
Thanks. On the basis of your assessment I'll concede that my initial statement was overly simplistic and definitive. Some may consider that anything is possible, but I think there are minimal requirements, as you mention.
Reply | Report Abuse | Link to thisLiquid methane could be a possible alternative to liquid H20. As is the idea behind possible methanogenic bacteria on Titan. "Life could develop strategies to overcome the low solubility of organics in liquid methane and use catalysts to accelerate biochemical reactions despite the low temperature" - POSSIBILITIES FOR METHANOGENIC LIFE IN LIQUID METHANE ON THE SURFACE OF TITAN by C.P. McKaya & H.D. Smith
Reply | Report Abuse | Link to thisPerhaps that might be possible, that's placing a lot of demands of life to overcome conditions not conducive to its fundamental requirements for chemical solubility enabling contact, interaction and reaction.
Reply | Report Abuse | Link to thisIf we find ourselves on Titan we should certainly look for life, but I wouldn't volunteer my tax money be spent going there to look for life. It's not likely there.
An intriguing overview of Titan, thanks!
Reply | Report Abuse | Link to thisWell, based on your description and the possibility of liquid water I'll concede that there could be or have been some life Titan, and maybe on Mars and a few other moons, but that's all! For now, anyway...
My dear HowardB, There may indeed be planets out there like Earth, but never the same as Earth; no two planets will ever be exactly the same..very much the same natural phenomenum that no two humans are indentical in all respects.
Reply | Report Abuse | Link to thisJMilliken
Water is not the only liquid to provide the fluidity that increases the likelihood of life emerging.
Reply | Report Abuse | Link to thisJohn Milliken
Bacteria do share genetic information, they are very cooperative and pass, I believe, RNA between each other. This is how antibiotic resistance can spread through a colony, for example. I understood from the article that the species in question can survive using phosphorus *or* arsenic so I imagine that there is a swap over period when both phosphorus and arsenic are in use. Would that work? Am I right?
Reply | Report Abuse | Link to thisThe idea is that they are not swapping to arsenic from phosphorous but simply expanding their abilities. Because of the similarities between the two elements, it is possible for arsenic to replace a spot in a biomolecule that is usually held by phosphorous without impacting the function too much. The question is what mechanism is the cell using to keep the arsenate compounds from decomposing quickly in water and allowing it to stay alive. It does not surprise me that this particular trait would exist in a bacteria in a high salinity environment, as the hypotonic nature of the cell would allow for less dissociation by water.
Reply | Report Abuse | Link to thisCould an organism with this arsenic incorporating properties exist in a more pure water environment?
As for my blunder with my disregarding of bacterial conjugation, I do not believe that is the mechanism in question in this instance. It seems to me that if it only took a simple coding of RNA that could be transmitted from cell to cell to survive on similar elements to the ones generally accepted as needed, then there would be many more examples of elemental substition in biomolecules. I believe they are going to find a series of enzymes and proteins that the bacteria developed over time that allow for greater pumping of water across membranes and functionallity without it. Perhaps the final key was conjugated, or perhaps there was no final key at all. Perhaps I am wrong entirely and all that was needed was a relatively small number of genes and the bacteria conjugated them and grew forth from there. The real answer will lie in how the bacteria function in environments that are closer to the one of Mono Lake pre arsenic-enrichment and salinity spikes.
There may be other powerful polar solvents that we don't normally observe as a liquid, and therefore disregard. It's very hard to say that any molecule is completely unique, for we do not know of all the pressure and temperature conditions that exist on other planets that may favor other chemicals that can be used as a "universal" solvent(and anyway, water isn't a universal solvent, it's just a powerful polar solvent, as a universal solvent would dissolve everything, probably preventing life). And who can say that somewhere there is life based on a non-polar solvent that builds large polar isomers?
Reply | Report Abuse | Link to thisMy view is that there is life everywhere in the universe. It is all a matter of scale. Thank you for the enightening comments. I have really enjoyed reading them.
Reply | Report Abuse | Link to thisPlease, help me to understand what is so great in the substitution of arsenate for phosphate. This substitution is very old news -- just do a research by yourself. Also, arsenate is toxic to multicell organisms, not for unicell bacteria; and going back to substitution, that is just what happens in our metabolism: arsenate binds where phosphate should, and then all toxicity start.
Reply | Report Abuse | Link to thisI read the research article in "Science Express," and what they did was isolate GFAJ-1, the microorganism, and feed then with arsenate instead of phosphate. I like rice, feed me with beans and deprive me of rice, and I will instantaneously like beans.
Please, help me to understand what is so great in the substitution of arsenate for phosphate. This substitution is very old news -- just do a research by yourself. Also, arsenate is toxic to multicell organisms, not for unicell bacteria; and going back to substitution, that is just what happens in our metabolism: arsenate binds where phosphate should, and then all toxicity start.
Reply | Report Abuse | Link to thisI read the research article in "Science Express," and what they did was isolate GFAJ-1, the microorganism, and feed then with arsenate instead of phosphate. I like rice, feed me with beans and deprive me of rice, and I will instantaneously like beans.
I don't know where all the surprise is coming from. There is evidence that arsenic may actually be a dietary requirement even for humans. Check out the USDA Grand Forks Nutrition Research Center's view: "research indicates that arsenic is an essential element, but the specific biochemical reason for its importance in humans has not been found....present research is focused on the possibility that too little arsenic is associated with decreased DNA methylation and thus, increased cancer....Based on animal studies, a calculated arsenic requirement for humans is 12 to 25 micrograms per day." (http://www.ars.usda.gov/News/docs.htm?docid=10906.) Of course, arsenic is probably not being used in place of phosphorus in this case, but we really don't know (or if someone does, perhaps he or she could reply to this.)
Reply | Report Abuse | Link to thisI, too, am puzzled by the media take on this matter. My first impression was that some alien-like life extremophile had been found; only later did I find that the organism in question was merely the result of some laboratory tweaking, which I personally find more interesting an accomplishment than the discovery of a simple extremophile.