Rumors have been swirling this week about a press conference NASA is starting right now. Some people have speculated that they’re going to announce evidence for life on another planet.
Well, not quite. Scientists have found a form of life that they claim bends the rules for life as we know it. But they didn’t need to go to another planet to find it. They just had to go to California.
The search for alien life has long been plagued by a philosophical question: what is life? Why is this so vexing? Well, let’s say that you’re hunting for change under your couch so that your four-year-old son can buy an ice cream cone from a truck that’s pulled up outside your house. Your son offers to help.
“What is change?” he asks.
“It’s…” You trail off, realizing that you’re about to get into a full-blown discussion of economics with a sugar-crazed four-year-old. So, instead, you open up your hand and show him a penny, a nickel, a dime. “It’s things like this.”
“Oh–okay!” your son says. He digs away happily. The two of you find lots of interesting things–paper clips, doll shoes, some sort of cracker–which you set aside in a little pile. But you’ve only found seventeen cents in change when the ice cream truck pulls away. Tears ensue.
As you’re tossing the pile of debris into the trash, you notice that there’s a dollar bill in the mix.
“Did you find this?” you ask.
“Yes,” your son sobs.
“Well, why didn’t you tell me?”
“It’s not change. Change is metal. That’s paper.”
Scientists have proposed hundreds of definitions for life, none of which has emerged as the winner. (For more on this quest, see “The Meaning of Life,” a cover story I wrote for SEED.) NASA, which would like to find life elsewhere in the universe, has taken a very practical approach to the question, simply asking what sort of definition of life should would be the best guide for their search. Traditionally, they’ve put a priority on life as we know it. All life on Earth uses DNA or RNA to encode genes; all life on Earth uses the same basic genetic code to translate genes into proteins; all life uses water as a solvent. One reason that NASA has put so much emphasis on looking for life on Mars is that it’s plausible that life as we know it might have existed on Mars back when the planet was warm and watery. And besides, how are we supposed to look for a form of life we’ve never seen before?
But in 2007 a National Academies of Science panel urged that we take a broader view of life, so that we wouldn’t miss the dollar bill in the couch. Other kinds of life were at least imaginable–such as organisms that used different backbones for their genes, or perhaps might swim through liquid methane like fish swim in water. (Here’s my write-up in the Times.) Some of the panelists–most notably, Steven Benner of the Foundation for Appllied Molecular Evolution–even endorsed a more radical notion. As I described in this feature for Discover, Benner and others speculate that maybe alien life is here on Earth.
A lot of evidence, for example, suggests that the first forms of life used RNA as both genes and enzymes. Later, double-stranded DNA evolved and DNA-based life wiped out RNA life. But perhaps RNA life still clings to existence in places where DNA-based life can’t drive them extinct. Benner suggests tiny pores in rocks that would be too small for bacteria.
No one has found RNA life yet, nor have they found any all-natural alien on Earth. But as I point out in Microcosm, there are definitely aliens among us.
They’re called E. coli.
Or, rather, they are laboratory stocks of E. coli that scientists have transformed so that they use new genetic codes or even use new nucleotides, the “letters” of DNA. No life that we know of has ever lived this way.
NASA’s press conference concerns another nearly-alien kind of life on our own planet. NASA has sponsored many expeditions to the toughest places on Earth for life to survive, from glaciers to deserts to acid-drenched mines. One of these expeditions was to Mono Lake, a practically toxic body of water, an extreme environment. It’s very salty, very alkaline, and is steeped in arsenic. The “weird life” report singled out arsenic-based life as one topic worth investigating, so Felisa Wolfe-Simon of the NASA Astrobiology Institute and her colleagues isolated a strain of bacteria and brought it back to the lab to study its growth.
As I mentioned earlier, life as we know it uses DNA for its genes (except for some viruses that use RNA). DNA has a backbone made of two alternating units: sugar and phosphate. Phosphate is one phosphorus atom and four oxygen atoms. It just so happens that arsenic–despite being a poison–has a lot of chemical properties similar to phosophorus. In fact, one arsenic atom and four oxygen atoms combine to form a molecule called aresenate that behaves a lot like phosphate.
Wolfe-Simon and her colleagues reared the bacteria in their lab, initially feeding them a typical diet of essential nutrients, including phosphate. But then they gradually reduced the phosphate in their diet and replaced it with arsenate. Before long, as they report today in Science, the bacteria were growing nicely on an all-arsenate diet, without a speck of added phosphate. The scientists then probed the DNA of the bacteria and concluded that they were sticking the arsenate into the DNA in place of phosphate. Phosphate is also vital for other molecules, such as proteins, and the scientists found arsenate in them as well. In other words–arsenic-based life.
Or…maybe not. In Science, reporter Elizabeth Pennisi writes that some scientists are skeptical, seeing other explanations for the results. One possible alternative is that the bacteria are actually stuffing away the arsenic in shielded bubbles in huge amounts.
I got in touch with Benner, who also proved to be a skeptic. “I do not see any simple explanation for the reported results that is broadly consistent with other information well known to chemistry,” he says. He pointed out that phosphate compounds are incredibly durable in water, but arsenate compounds fall apart quickly. It was possible that arsenate was being stabilized by yet another molecule, but that was just speculation. Benner didn’t dismiss the experiment out of hand, though, saying that it would be straightforward to do more tests on the alleged arsenic-DNA molecules to see if that’s what they really are. “The result will have sweeping consequences,” he said.
If Wolfe-Simon can satisfy the critics, this will be research to watch. The Mono Lake bacteria probably don’t actually exist in an arsenic-based form in nature, since they grow much faster on phosophorus. They’re aliens, but aliens in the same way unnatural E. coli are, thanks to our intervention. But Wolfe-Simon’s results suggest that life based on arsenic is at least possible. It might even exist naturally in places on Earth where arsenic levels are very high and phosphorus is very scarce.
Such a discovery would indeed be huge news–although not as huge as a similar discovery on another planet. For now, we will have to content ourselves with arsenic-laced dreams.
(PS: You should be able to watch the press conference live starting at 2pm Thursday 12/2 here.)
Reference: Wolfe-Simon et al, “A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus” Science, 10.1126/science.1197258
[Image of Mono Lake by .Bala via Flickr, under Creative Commons License]
[Update: Fixed Wolfe-Simon's name. Now I am left with images of wolf salmon roaming in packs.]
[Update: Fellow Discover bloggers Ed Yong and Phil Plait are on the case, too.]
[Update: I've been adding in various corrections pointed out by astute readers. Importantly, the researchers raised the bacteria with no *added* phosphate. But the medium did have a little phosphate in it anyway. More about this on Monday!]
[Upate: Well, Monday became Tuesday, but better late than never: Here's my new article on the arsenic backlash at Slate.]
December 2nd, 2010 at 2:34 pm
It’s Wolfe-Simon, not Wolf-Salmon!
The Harvard chemist Frank Westheimer wrote a memorable paper explaining “why nature chose phosphates” which can be found here. It explains based on basic principles of chemistry why phosphates rather than arsenates or silicates have been chosen by life. It’s a must read for those who think alternative life-forms could be common.
[CZ: Thanks!]
December 2nd, 2010 at 4:01 pm
I think it would be exceedingly misleading to refer to “ordinary” life as ‘phosphorus-based’.
Seems to me the spin is out of hand on this. Even if these bacteria can sub-in arsenic for phosphorus when forced to, this would represent an interesting adaptaion of bacteria to high-arsenic environments, nothing more. We already have a large catalog of bacterial adaptations to other seemingly harsh chemical environments, and one more isn’t worth a hyped NASA presser. IMO.
December 2nd, 2010 at 4:25 pm
[...] Artikel von Nature News, New York Times, Space.com, Discovery (mehr und mehr), Spaceflight Now, New Scientist, Wired, Scientific American, NPR, Discover, Centauri [...]
December 2nd, 2010 at 4:38 pm
[...] just how important this is, let’s turn to an analogy from one of my favorite science writers: Carl Zimmer. The search for alien life has long been plagued by a philosophical question: what is life? Why is [...]
December 2nd, 2010 at 5:36 pm
Often times I wonder if the definitions of life were all wrong. Why do the rhythms of life has to be driven by our colored view? Me thinks that NASA is looking to find some hyping to keep from losing funding
December 2nd, 2010 at 7:00 pm
“Phosphate is also vital for other molecules, such as proteins, and the scientists found phosphate in them as well. In other words–arsenic-based life.”
Shouldn’t that be
“Phosphate is also vital for other molecules, such as proteins, and the scientists found ARSENATE in them as well. In other words–arsenic-based life.”
[CZ: Indeed! Thanks.]
December 2nd, 2010 at 7:04 pm
[...] Of Arsenic and Aliens by Carl Zimmer. [...]
December 2nd, 2010 at 8:46 pm
Sveny does not understand DNA, how cute.
December 2nd, 2010 at 9:30 pm
[...] than storing) the arsenic. Carl Zimmer, another blogger for Discover magazine, writes in “Of Arsenic and Aliens” that NASA’s research hasn’t yet confirmed that Spirochaeta americana really [...]
December 3rd, 2010 at 12:02 am
You could have thrown in some foreign coins that were found under the couch…
December 3rd, 2010 at 2:12 am
[...] usual suspects have great, sober, and thought-provoking coverage of the new [...]
December 3rd, 2010 at 2:48 am
I’m not a scientist, just an avid armchair follower of most sciences, at least until I get to the Wonderland gate and find myself way over my head. I know that there as been speculation for decades about silicon-based life, but arsenic-based is a new one on me, unless I’ve simply forgotten about it ( easily possible, given my memory!).
It would be encouraging to learn that life based on another element, since it would greatly broaden the possibility of discovering *some* form of life elsewhere, even if merely bacterial life (as opposed to aliens with rays guns and space ships that slip back and forth across several dimensions). And just a day or two ago I read that astronomers are re-thinking the theoretical number of stars in the Universe, and now speculate there may be as many as three times more stars than they previously though — hence, that many more possibilities of finding habitable planets. Even if carbon-based life turns out to be the only kind possible, it’s hard to believe there isn’t *some* sort of life elsewhere, which I say partly because the 500 or so exoplanets we’ve discovered so far seem to pretty strongly indicate that there are a fair number of planets orbiting their stars in the Goldilocks Zone (though most aren’t truly Earth-like, to be sure, at least not so far).
Fascinating “maybe-a-breakthrough-discovery,” and great article!
Minerva Smelibut said on December 2nd, 2010 at 5:36 pm, in part, ” Me thinks that NASA is looking to find some hyping to keep from losing funding.”
C’mon, Minerva — surely not every single scientist on the planet sells his or her soul. What you wrote is akin to saying anyone who joins the Marines does so for the chance to visit faraway, perhaps even exotic places, meet fascinating people — then kill them.
@Richard Prins “foreign coins.” THAT brought a smile! Thanks!
December 3rd, 2010 at 4:10 am
[...] Ed Yong and Mr. Carl Zimmer have reviewed all the salient points, so I have nothing to add there. I have to say that I agree [...]
December 3rd, 2010 at 9:20 am
Several excellent points are made. Especially by Minerva Smelibut, who notices that NASA has figured out that some sensationalism is much like a squeaky wheel. Sadly, enough.
December 3rd, 2010 at 10:53 am
I think there are still some instances of “Wolf-Salmon” in the article itself…
December 3rd, 2010 at 10:55 am
Also, “phosophate” instead of “phosphate.”
[CZ: Thanks.]
December 3rd, 2010 at 12:08 pm
What I would like to see is a x-ray diffraction spectrum. By replacing P with As the double helix structure should be changed. That would be totally unambiguous.
December 3rd, 2010 at 12:28 pm
[...] driven by ASTOUNDING!! news and the failure to get even basic science stories right (with some very obvious and excellent exceptions), consider just what was behind the hype: excitement at the prospect of [...]
December 3rd, 2010 at 3:40 pm
[...] of all crappy public transport systems on the planet, it occurred to me that I was reading a terrific write-up by one the world’s leading science writers on the enfolding events surrounding a life form [...]
December 3rd, 2010 at 5:08 pm
? Directed at me? What’s it supposed to mean?
December 4th, 2010 at 12:20 am
Salt lakes are a fairly common Western United States phenomena. Many of these features cycle between being lakes, mushy brines, or hard salt flats depending on climate swings. But Mono Lake in particular has been very affected by water diversions by the City of Los Angeles starting abruptly in 1941. See: http://www.monolake.org/about/story.
Have non-Mono Lake bacteria been tested? How would a “normal” bacterial strain cope with increasing As?
At any rate, it seems to me that the fact the chemistry of the lake has not been consistent over time would be relevant.
December 4th, 2010 at 1:26 am
My naive non-Biologist questions are:
If I were raising something in captivity in my laboratory, I think I would attempt to feed it what I thought it wanted to eat. Why did the researchers start out with pure phosphate rather than arsenate or an exact match to the Mono Lake brine if these bacteria are supposed to be arsenic based life?
If I wanted to compare how the Mono Lake bacteria handled arsenate differently than normal strains, I think I might start both strains on phosphate and increase the arsenate. Can the non Mono Lake bacteria adapt? Or, at that point where the normal bacteria were near death, what was different with the way the Mono Lake bacteria were incorporating the As? Do normal bacteria end up with As distributed throughout? Wouldn’t this help answer the sequestration vs assimilation question raised above?
[CZ: Here is my non-biologist answer: These bacteria are not, despite some news reports this week, arsenic-based life. On Mono Lake, they are ordinary, phosphate-based life. But, perhaps due to their high tolerance for arsenic, they can swap arsenic for phosphate in an experiment--at least according to the scientists. As for your second question, arsenic is toxic to most organisms, including bacteria, and they have mechanisms to get rid of it from their system. Some have better mechanisms than others. And some bacteria at Mono Lake can even feed on arsenic: http://www.ncbi.nlm.nih.gov/pubmed/20511421. The bacteria in this paper were not dying off from arsenic poisoning--they were growing on pure arsenic and no phosphate (assuming there was no contamination).]
December 4th, 2010 at 4:15 am
[...] Ed Yong and Mr. Carl Zimmer have reviewed all the salient points, so I have nothing to add there. I have to say that I agree [...]
December 4th, 2010 at 1:19 pm
From a non living system perspective, I think that in high As, brine-y solutions that the relevant chemical equibria would be driven to extremes such that normal ideas as to what was or wasn’t stable “in water” would not apply. So substituting arsenic for phosphorus in chemical molecules under these conditions doesn’t strike me as necessarily all that impressive.
It seems to me that the amazing thing is that the bacteria stay alive under these conditions.
That was my thought behind my question: “at that point where the normal bacteria were (would be) near death, what was different with the way the Mono Lake bacteria were (would be) incorporating the As?”
December 4th, 2010 at 5:37 pm
Thanks for well-written and entertaining post. Your story about a four-year-old son, loose change, and scrambling to buy ice cream was perfect.
I would like to clarify one point you make about Mono Lake:
“One of these expeditions was to Mono Lake, a practically toxic body of water. It’s very salty, very alkaline, and is steeped in arsenic.”
Arsenic exists in many forms, and not all are equally toxic to all life, nor are the type and/or concentrations in the water column such that inhibit an astonishingly productive ecosystem. Mono’s (pronounced Moe’-no) average annual primary productivity is 686 gC/m2-yr, and it often exceeds 1000 gC/m2-yr. The lake supports not only algae but trillions of brine shrimp and a massive population of alkali flies, in turn supporting millions of migratory and resident birds.
The lake’s microbial life has only recently received attention since about the time NASA first used the lake as a test bed for robotic Mars missions in 1995. The bacterium and viruses in the lake, in its seeps and springs, in its anoxic depths, and around brackish lagoons, present a great frontier of weird and unknown. It’s a micro-wilderness.
As Dr. Wolfe-Simon pointed out, the story is not about Mono Lake, but I did want to clarify that Mono is far from being a toxic body of water.
[CZ: Thanks! I'll revise that part of the post.]
December 4th, 2010 at 10:25 pm
[...] the nature of the bacteria found and the implications. There are a few good posts from Ed Yong, Carl Zimmer, and P.Z. Myers detailing exactly what was found and dispelling some of the hype and myths that [...]
December 4th, 2010 at 10:30 pm
Yeah; I have to pitch in with Benner, having read the paper but not the supplement: the element fractions depends on the extraction procedure, and it doesn’t seem to be checked but “standard”. For example, the DNA gel wash is supposed to remove small RNA (mostly) fragments, but what about proteins. (Which can be used for sequestering.)
@ Mehkong Kurt:
Perhaps in scifi; SiO2 being solid pretty much torpedoes the idea among scientists AFAIU.
December 5th, 2010 at 7:01 am
Tz ,tz,…
Wie bei Hempels unterm Sofa….
Georg
December 5th, 2010 at 12:24 pm
Barshe Miller is correct in pointing out that this story is not about Mono Lake. However, I think others besides myself reading this post would find this resource from the Mono Lake Committee to be of interest:
http://www.monobasinresearch.org/index.html
December 5th, 2010 at 2:15 pm
“SiO2 being solid pretty much torpedoes the idea among scientists AFAIU”
How does this rule out silicon-based life? (I say this as a complete non-biologist.) We had life on Earth long before any significant amount of atmospheric oxygen, so I would assume such life didn’t need much CO2 transport.
December 5th, 2010 at 4:12 pm
[...] Are there viruses of arsenic-utilizing bacteria? and The ‘Give Me a Job’ Microbe and Of Arsenic and Aliens and NASA’s real news: bacterium on Earth that lives off arsenic! and Close Encounters of the [...]
December 5th, 2010 at 4:23 pm
“Before long, as they report today in Science, the bacteria were growing nicely on an all-arsenate diet, without a speck of phosphate.”
This is wrong. The bacteria were always grown in phosphate — the “non-phosphate” media had 3 uM phosphate contamination, as the authors acknowledge in the text. This phosphate contamination is one reason why many scientists do not believe this paper at all.
The microbiologist Rosie Redfield has published a devastating critique of this paper:
http://rrresearch.blogspot.com/2010/12/arsenic-associated-bacteria-nasas.html
[CZ: Thanks--I will rein in the figurative language. And I highly recommend Redfield's post.]
December 6th, 2010 at 12:51 am
“The Mono Lake bacteria probably don’t actually exist in an arsenic-based form in nature, since they grow much faster on phosophorus.”
I can’t agree with this statement at all, to rebate it with an analogy, many facultative anaerobic microorganisms can grow faster with enough oxygen or other external electron acceptors but this doesn’t mean they don’t actually exist in anaerobic fermentative form in nature.
Likewise, GFAJ-1 could be adapted to thrive with either phosphorous or arsenic depending on the specific surrounding availability of both. An evolutionary adaptation to incorporate arsenic when phosphorous is scarce could better explain why these bacteria are capable to grow under phosphorous deprivation replacing it with arsenic.
Alternatively, perhaps it is energetically more advantageous to directly incorporate arsenic into biomolecules than spend energy trying to get rid of it under conditions in which the combined effect of arsenic concentration and arsenic-phosphorous ratio is above certain threshold level.
December 7th, 2010 at 12:34 pm
[...] asked me to take a look at the scientific reactions emerging to last week’s big news about arsenic-based life. I got in touch with a dozen experts, and let’s just say, the results weren’t pretty. Check it [...]
December 7th, 2010 at 2:35 pm
[...] Are there viruses of arsenic-utilizing bacteria? and The ‘Give Me a Job’ Microbe and Of Arsenic and Aliens and NASA’s real news: bacterium on Earth that lives off arsenic! and Close Encounters of the [...]
December 8th, 2010 at 12:37 am
Two editorial notes:
“Benner suggests tiny pores in rocks that would be too big for bacteria.” Don’t you mean, “…too small for bacteria”?
You might want to correct spelling in this sentence: “One possibile alternative is that…”
[CZ: Thanks, thanks--fixed, fixed]
December 8th, 2010 at 5:04 am
[...] previous coverage of the Great Arsenic Announcement: The Loom: Of Arsenic And Aliens Bad Astronomy: NASA’s Real News: Bacteria on Earth That Lives Off Arsenic Bad Astronomy: [...]
December 8th, 2010 at 11:05 am
[...] by light years. While newspapers ran away with the story, it was left to bloggers like Ed Yong, Carl Zimmer, Lewis Dartnell and Phil Plait to put the story in its proper perspective. But more importantly it [...]
December 9th, 2010 at 11:42 am
for a great read, i recommend “Origins of Life” by Freeman Dyson, 1985 with a 2nd edition in 1999. then there’s James Lovelock’s great treatment in “Ages of Gaia” of the Oklo algal mat in Gabon that evolved its own nuclear reactor. life is OPPORTUNISTIC, and just because it’s harder to use As than P doesn’t make it impossible…call me an As-hole if you want to. i just hope there are bacteria out there that can use selenium instead of sulfur, to clean up all the Bureau of Reclamation’s water projects that have caused so many bird and amphibian deaths and deformities in the Pacific Flyway (yeah, also read “Death in the Marsh” by Tom Harris.)
December 9th, 2010 at 5:26 pm
[...] versteckt, damit es nicht mehr Schaden anrichtet? Außerdem seien Arsen-basierte Verbindungen in Wasser nicht sehr stabil, wie etwa Steven Benner anführt. Das Team um Wolfe-Simon nimmt daher folgendes an, wie Ars [...]
December 11th, 2010 at 8:27 am
Intelligent Bacteria: Cells are Incredibly Smart
For years I just sort of assumed that cells were self-reproducing blobs of protein. Maybe you did too. Turns out they’re way smarter than that. You will be amazed at this video. Dr. Bonnie Bassler from Princeton University presents a beautiful TED talk on how bacteria communicate with each other by forming words out of simple molecules.
She also explains…
How bacteria strategize together on how to ‘take down’ their host
Elegant systems of bioluminescence
Symbiotic relationships between organisms
Cells speak multiple languages
Enjoy this remarkable presentation. And a sincere thanks to Patrik Beno for sharing it with me.
Perry Marshall
http://www.cosmicfingerprints.com/blog/intelligent-bacteria/
This is really incredible. Where did the proposed bacterial molecular codes come from?
January 1st, 2011 at 12:00 am
[...] When life gives you arsenic, make arsenate-backboned DNA, non-alien Halomonadaceae!; The Loom: Of Arsenic and Aliens & What the critics said; Sandwalk: Arsenic and Bacteria; Byte Size Biology: A new life form? [...]
November 18th, 2011 at 5:51 pm
[...] Carl Zimmer. Dec 2 2010. Of arsenic and aliens. The Loom (Discover). http://blogs.discovermagazine.com/loom/2010/12/02/of-arsenic-and-aliens/ [...]