Mono Lake (image from Wikipedia Commons)

One of my favorite science fiction movies is Contact, the Robert Zemeckis vehicle that spent some twenty years in developmental hell before finally being released in 1997.  Based on an idea from Carl Sagan (who wrote a book from the story only after the production of the movie got delayed), Contact tells the story of Ellie Arroway, a passionate astronomer who is determined to find proof of existence of extraterrestrial life by tirelessly combing the night sky with satellite receivers.  Not only does she succeed in finding and translating an alien signal, but she also gets chosen to be the one that meets the creature at the source.  The film got panned by a lot of science fiction fans who watched the entire two-and-a-half hours to catch a glimpse of the alien, only to find that it appears to Ellie in the form of – spoiler alert – her deceased father.  I have to admit that I too was a bit disappointed, but, nevertheless, I still really like this movie for two reasons.  First, because it explores the potential conflicts that arise if we ever do make contact with intelligent life from another planet.  Would they be friendly, or would they see us as a disposable obstacle in the quest for natural resources (an attitude that is, unfortunately, not foreign to our own species)?  And how would it affect the way we see ourselves, especially since we are so used to living on the most special planet in the universe?

The other reason I like Contact is because it takes the point of view that there are countless numbers of extraterrestrial civilizations that we just haven’t discovered yet, and that thought tickles the scientist in me.  Ask biologists or biochemists if they believe in the possibility of life beyond our planet, and they will probably say yes because they know that life here on earth is ultimately made of components that follow the basic rules of chemistry, and there is no reason to believe that life wouldn’t arise under similar conditions on other planets.  And considering that there are billions of planets in our galaxy alone (and hundreds of billions of galaxies in the universe), it is almost a given that life is out there. 

Just last fall, NASA announced that they had found the very first earth-like planet nestled in the constellation of Libra about 20 light years away from earth.   According to one astronomer, the planet, Gliese 581 g, has a 100% probability of harboring some kind of life.  It’s about 7 times closer to its sun than the earth is to our sun, but Gliese 581 g’s sun is a relatively weak red dwarf about a third the mass of our sun, and puts Gliese 581 g in the optimal zone to support life.  The discovery caused a lot of excitement.  Imaginations soared.  A couple of people even laid claim to the planet and began selling plots of alien land on eBay.  And then, only two months later, NASA made the announcement that many sci-fi fans had eagerly anticipated: NASA scientists had made a breakthrough in the search for extraterrestrial life.  Here’s what they said:

NASA will hold a news conference at 2 p.m. EST on Thursday, Dec. 2, to discuss an astrobiology finding that will impact the search for evidence of extraterrestrial life. Astrobiology is the study of the origin, evolution, distribution and future of life in the universe.

You can only imagine the buzz this new announcement generated over the Internet. Had life been found on Gliese 581 g?   A rogue radio signal, a cluster of streetlights, a cloud of swamp gas – any hint of life would forever change the way we look at the sky at night.

So you can also imagine the disappointment when, at the promised time, NASA made its announcement: A bacterium that can live off of arsenic had been discovered at the bottom of a California lake. 

No signs of extraterrestrial life, no whispers from a distant world.  Libra’s red dwarf, for now, remains a silent speck in the sky.  It was the biggest let-down for sci-fi fans since the end of Contact.  Why should NASA care if a bacterium lives on arsenic?  Or why should any of us care, for that matter? 

Well, here’s the answer: because it would raise the possibility that life could exist on planets unlike our own. 

All life as we know it is made of six “essential” elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.  All of the essential elements are made up of atoms that are relatively small compared to atoms of other elements, which is why they are clustered together in the top three rows of the periodic table, with the rest of the lighter elements:

The periodic table, with "essential" elements colored orange. Phosphorus (P) and arsenic (As) are outlined in red.

Elements in the same group (column) of the periodic table have similar properties, so it is theoretically possible that an element may be able to substitute for another from the same group, particularly if it is only one element away (up or down).  Some good examples of this are found in some species of microscopic diatoms which use silicon instead of carbon to make their exoskeletons.  Carbon (C) is in group 14 of the periodic table, and silicon (Si) is one element down, in the same group. 

Cells use phosphorus (P) to make DNA and fats, to store and transfer energy, and to modify sugars and proteins.  Arsenic (As), that old poison-of-choice for Victorian murderers, is in the same group (and only one element down) from phosphorus, so it has been hypothesized that arsenic could functionally substitute for phosphorus when arsenic is more abundant in the environment.  What’s more, the biologically active form of phosphorus is phosphate, which is phosphorus bound to four oxygen atoms, and arsenic can react with oxygen in same way to form arsenate:

Phosphate (left) and the larger arsenate (right)

(Nitrogen, also in the same group as phosphorus, can also react with oxygen to yield nitrate, but it can only stably do so with three oxygen atoms due to the small size of the nitrogen atom).

So how would you find examples of life on earth that can survive on arsenic instead of phosphorus?  Phosphorus is more abundant than arsenic on this planet – in fact, wherever you find arsenic, you’ll probably find a lot more phosphorus – so any form of life that could use arsenic would probably preferentially use phosphorus anyway.  So the best you can do is to look where there is a high level of arsenic, find something living there (most likely bacteria), and take away its phosphorus supply while still providing it with arsenic.  If something grows, it is likely using arsenate instead of phosphate to make DNA.

This is exactly what Felisa Wolfe-Simon did.  Wolfe-Simon, a microbial geobiologist and a research fellow for NASA and the US Geologic Survey in the San Francisco Bay area, has a keen interest in the interplay between ancient forms of life and the chemical elements in their environment, and how the exposure (and uptake) of certain elements affected biological evolution, particularly in life’s early stages.  She goes by the nickname “Iron Lisa” – a somewhat clever derivative of her first name (Fe [Iron] + Lisa).  Like many successful scientists, she has a definite creative side that serves to balance and fuel her scientific mind (she graduated from college with dual degrees in biology and oboe performance).  She pondered the arsenic question for years, summarizing the hypothesis in a 2009 article in the International Journal of Astrobiology, where she suggests that our planet may have once been home to alternate forms of life that utilize non-typical elements such as arsenic, and that some of these ancient organisms may still exist:

We hypothesize that ancient biochemical systems, analogous to but distinct from those known today, could have utilized arsenate in the equivalent biological role as phosphate.  Organisms utilizing such “weird life” biochemical pathways may have supported a “shadow biosphere” at the time of the origin and early evolution of life on Earth or on other planets.  Such organisms may even persist on Earth today, undetected, in unusual niches.

 Wolfe-Simon had the knowledge and the access to the technical expertise of colleagues to test this idea.  All she needed was a source of bacteria, preferably from some place that is naturally high in arsenic (an “unusual niche”).  She chose Mono Lake, a relatively small, shallow body of water just east of Yosemite National Park and practically in her backyard.  Because there is no outlet for the lake’s water, dissolved salts and minerals (including arsenic compounds) accumulate to make the water very alkaline and saline – at least twice as salty as the ocean.  But this lake is not barren.  Single-celled algae live in the warmer waters along its coast, and they form a critical link at the end of a surprisingly active if not typical food chain.  During the summer, the lake boils with trillions of the tiny brine shrimp Artemia monica, a species unique to this particular body of water.  Brine flies scuttle around its beaches and lay eggs in its water, using bubbles of air as mini scuba tanks as they crawl across the lake’s floor.  The fly larvae, once a staple for indigenous people of the area, are an important food source for thousands of species of migratory birds as they journey through the valley.  All this, in and around a lake that contains a human-lethal dose of arsenic in every 10 liters of its water.  If there were organisms that could incorporate arsenic into their biochemistry instead of phosphorus, there would be a decent chance that something could be found in Lake Mono.

So Wolfe Simon and colleagues began the search for this hypothetical creature by collecting sediments from the lake and diluting the samples over and over in a nutrient broth free of phosphorus but with increasing amounts of arsenic.  Most of the living microorganisms died off; what survived was a single strain of bacteria that was still able to multiply in the presence of arsenic and, supposedly, in the absence of phosphorus.  Wolfe-Simon named the strain GFAJ-1 (“Give Felisa A Job”), and set about trying to determine if it was actively substituting arsenic for phosphorus in its DNA.  Her group used mass spectrometry to analyze the bacteria and its DNA after it had been growing in arsenic, and found that As was present in the protein and DNA fractions of the cells.  The group also measured X-ray absorption and concluded that the bonds that As was forming in the bacterial DNA were similar to those formed by P in typical DNA.  They submitted their findings to the high-profile journal Science, which published an online version of the article on December 2, 2010 – hence the proud announcement by NASA the very same day.

But Science did something a bit out of the ordinary for this particular paper: they waited to publish the hard-copy version of the paper until the scientific community had an ample amount of time to review and to respond to its findings.  When Science did eventually print the paper, it was accompanied by eight critiques plus a rebuttal by Wolfe-Simon.  Most of the critiques pointed out specific problems with the techniques used in the paper, such as questionable calculations of As:P ratios in the bacteria, a relatively superficial analysis of the bacterial DNA, incomplete purification of the DNA, and the fact that the bacteria themselves look a bit swollen and “unhealthy”  when cultured in the presence of arsenic:

GFAJ-1 grown in phosphorus (left) and arsenic (right). (Images from Wikipedia Commons)

But perhaps the biggest problem that Wolfe-Simon and colleagues must face is that arsenate is much more unstable than phosphate.  Phosphate esters – which are required for the incorporation of P into DNA – have half-lives of millions of years, while the analogous arsenate esters only exist for a millisecond before they fall apart.  So how could the cell use arsenate as a building material?  This is the main reason why many biochemists find the results of this study so hard to swallow, and why the burden of proof lies so heavily on Iron Lisa’s shoulders.  But to her credit, she has made GFAJ-1 freely available to other scientists with the hope that they will repeat and expand upon her own work.  We shall see if this little creature will find its way into microbial history.

While it may yet be possible (however unlikely) that life can arise in the absence of one or more of the essential elements, our own planet of earth serves as a shining model of how life can materialize when those elements are abundant and the weather is right.   So, while the questions surrounding GFAJ-1 remain unresolved, I think the focus for NASA should be more directed towards the other discovery of 2010: Gliese 581 g.  If a planet like Gliese 581 g was so easily found in our own neighborhood, think of how many other earthlike planets there could be in our galaxy alone!  On the other hand, the silence of the universe serves as a solid counter-argument, publicized memorably by physicist Enrico Firme in 1950: “If the universe is teeming with life, where is everybody?”  But perhaps the rare event is not life itself, but rather the emergence from that life a sentient species capable of travelling or communicating billions of miles across that vast, cold, empty vacuum of space.  Nonetheless, even if we were to find extraterrestrial life, and even if it were only a planet crawling with microbes, the discovery would send shockwaves through our culture and rearrange our views of our own little planet.  No longer would earth be an exceptional diamond in a barren universe.  Just like when Galileo announced the sun did not revolve around the earth, or when Darwin wrote that we are a mere twig on the tree of life, or when Newton revealed that an object is not at rest just because we perceive it to be so – science would strike another bittersweet blow, ever forcing us to shift our gaze from our narrow but comforting realm to a wider but much humbler understanding of ourselves.  Yet we will never stop searching – our curiosity is our bane – for those tiny hints that our world is not unique.  And we will keep finding them, whether they be twenty million light years away or on the salt-encrusted shore of a desert lake in California.